compiler: implement analysis-local comptime-mutable memory #19414
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I don't fully understand the details of what this does, so I'll just ask directly: const x = comptime x: {
const X = struct {self: *const @This()};
var x: X = undefined;
x.self = &x;
break :x x;
};Update: Looking at the usage in my actual code again, I might not need this capability personally, |
|
You can of course construct such a value, but yes, making it the value of a global declaration will raise a compile error. However, that code isn't doing what you seem to think it is - the global Initializing a self-referential global is #131 (it's not the issue title, but resolving the issue will also allow direct self-reference). |
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This commit changes how we represent comptime-mutable memory (`comptime var`) in the compiler in order to implement the intended behavior that references to such memory can only exist at comptime. It does *not* clean up the representation of mutable values, improve the representation of comptime-known pointers, or fix the many bugs in the comptime pointer access code. These will be future enhancements. Comptime memory lives for the duration of a single Sema, and is not permitted to escape that one analysis, either by becoming runtime-known or by becoming comptime-known to other analyses. These restrictions mean that we can represent comptime allocations not via Decl, but with state local to Sema - specifically, the new `Sema.comptime_allocs` field. All comptime-mutable allocations, as well as any comptime-known const allocs containing references to such memory, live in here. This allows for relatively fast checking of whether a value references any comptime-mtuable memory, since we need only traverse values up to pointers: pointers to Decls can never reference comptime-mutable memory, and pointers into `Sema.comptime_allocs` always do. This change exposed some faulty pointer access logic in `Value.zig`. I've fixed the important cases, but there are some TODOs I've put in which are definitely possible to hit with sufficiently esoteric code. I plan to resolve these by auditing all direct accesses to pointers (most of them ought to use Sema to perform the pointer access!), but for now this is sufficient for all realistic code and to get tests passing. This change eliminates `Zcu.tmp_hack_arena`, instead using the Sema arena for comptime memory mutations, which is possible since comptime memory is now local to the current Sema. This change should allow `Decl` to store only an `InternPool.Index` rather than a full-blown `ty: Type, val: Value`. This commit does not perform this refactor.
This test has regressed due to a bug in the self-hosted COFF linker. Jakub recommended disabling it for now, since the COFF linker is being effectively rewritten, so there is little point in fixing the bug now.
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10/10 would merge again |
ianprime0509
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As of ziglang/zig#19414, a small workaround is necessary to avoid global variables containing references to comptime vars.
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As of ziglang/zig#19414, a small workaround is necessary to avoid global variables containing references to comptime vars.
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More release notes at https://ziggit.dev/t/comptime-mutable-memory-changes/3702 |
Techatrix
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As of ziglang/zig#19414, a small workaround is necessary to avoid global variables containing references to comptime vars.
mlugg
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We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
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Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
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Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
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Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
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Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
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Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
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Apr 12, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
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Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 13, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 14, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 14, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 14, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
added a commit
to mlugg/zig
that referenced
this pull request
Apr 15, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
mlugg
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this pull request
Apr 17, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
TUSF
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to TUSF/zig
that referenced
this pull request
May 9, 2024
We've got a big one here! This commit reworks how we represent pointers in the InternPool, and rewrites the logic for loading and storing from them at comptime. Firstly, the pointer representation. Previously, pointers were represented in a highly structured manner: pointers to fields, array elements, etc, were explicitly represented. This works well for simple cases, but is quite difficult to handle in the cases of unusual reinterpretations, pointer casts, offsets, etc. Therefore, pointers are now represented in a more "flat" manner. For types without well-defined layouts -- such as comptime-only types, automatic-layout aggregates, and so on -- we still use this "hierarchical" structure. However, for types with well-defined layouts, we use a byte offset associated with the pointer. This allows the comptime pointer access logic to deal with reinterpreted pointers far more gracefully, because the "base address" of a pointer -- for instance a `field` -- is a single value which pointer accesses cannot exceed since the parent has undefined layout. This strategy is also more useful to most backends -- see the updated logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do prefer a chain of field and elements accesses for lowering pointer values, such as SPIR-V, there is a helpful function in `Value` which creates a strategy to derive a pointer value using ideally only field and element accesses. This is actually more correct than the previous logic, since it correctly handles pointer casts which, after the dust has settled, end up referring exactly to an aggregate field or array element. In terms of the pointer access code, it has been rewritten from the ground up. The old logic had become rather a mess of special cases being added whenever bugs were hit, and was still riddled with bugs. The new logic was written to handle the "difficult" cases correctly, the most notable of which is restructuring of a comptime-only array (for instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`. Currently, the logic for loading and storing work somewhat differently, but a future change will likely improve the loading logic to bring it more in line with the store strategy. As far as I can tell, the rewrite has fixed all bugs exposed by ziglang#19414. As a part of this, the comptime bitcast logic has also been rewritten. Previously, bitcasts simply worked by serializing the entire value into an in-memory buffer, then deserializing it. This strategy has two key weaknesses: pointers, and undefined values. Representations of these values at comptime cannot be easily serialized/deserialized whilst preserving data, which means many bitcasts would become runtime-known if pointers were involved, or would turn `undefined` values into `0xAA`. The new logic works by "flattening" the datastructure to be cast into a sequence of bit-packed atomic values, and then "unflattening" it; using serialization when necessary, but with special handling for `undefined` values and for pointers which align in virtual memory. The resulting code is definitely slower -- more on this later -- but it is correct. The pointer access and bitcast logic required some helper functions and types which are not generally useful elsewhere, so I opted to split them into separate files `Sema/comptime_ptr_access.zig` and `Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small public APIs. Whilst working on this branch, I caught various unrelated bugs with transitive Sema errors, and with the handling of `undefined` values. These bugs have been fixed, and corresponding behavior test added. In terms of performance, I do anticipate that this commit will regress performance somewhat, because the new pointer access and bitcast logic is necessarily more complex. I have not yet taken performance measurements, but will do shortly, and post the results in this PR. If the performance regression is severe, I will do work to to optimize the new logic before merge. Resolves: ziglang#19452 Resolves: ziglang#19460
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This commit changes how we represent comptime-mutable memory (
comptime var) in the compiler in order to implement the intended behavior that references to such memory can only exist at comptime.It does not clean up the representation of mutable values, improve the representation of comptime-known pointers, or fix the many bugs in the comptime pointer access code. These will be future enhancements.
Comptime memory lives for the duration of a single Sema, and is not permitted to escape that one analysis, either by becoming runtime-known or by becoming comptime-known to other analyses. These restrictions mean that we can represent comptime allocations not via Decl, but with state local to Sema - specifically, the new
Sema.comptime_allocsfield. All comptime-mutable allocations, as well as any comptime-known const allocs containing references to such memory, live in here. This allows for relatively fast checking of whether a value references any comptime-mtuable memory, since we need only traverse values up to pointers: pointers to Decls can never reference comptime-mutable memory, and pointers intoSema.comptime_allocsalways do.This change exposed some faulty pointer access logic in
Value.zig. I've fixed the important cases, but there are some TODOs I've put in which are definitely possible to hit with sufficiently esoteric code. I plan to resolve these by auditing all direct accesses to pointers (most of them ought to use Sema to perform the pointer access!), but for now this is sufficient for all realistic code and to get tests passing.This change eliminates
Zcu.tmp_hack_arena, instead using the Sema arena for comptime memory mutations, which is possible since comptime memory is now local to the current Sema.This change should allow
Declto store only anInternPool.Indexrather than a full-blownty: Type, val: Value. This commit does not perform this refactor.