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proposal: runtime: make the ABI undefined as a step toward changing it #27539
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Can we also write pseudo-ops for assembly code so that assembly code can become ABI agnostic? E.g., |
Potentially. I worry that, while this could certainly be more flexible than what we have now, and is probably powerful enough to adapt to a register-based ABI, it may still tie us to specific design decisions. Some of this, I think, is just fundamental to assembly (or pseudo-assembly) language; for example, if a calling convention change requires more stack space (like passing dynamic allocation scope information), what do we do with the function's declared stack size? And usually assembly is written for performance reasons, but if we're trying to transparently adapt ABIs, that gives up some of the performance. |
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Do we ever want to have more than two ABIs? (By comparison, GCC on X86 has a large number of ABIs, through use of function attributes like |
I'm not sure. I don't think we should preclude that possibility (let's not call the pragma |
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What I'm wondering is, once we have three ABIs, how do we know which one assembler code uses? |
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I imagine we should have two ABIs, one "stable", and one "unspecified". The assembly code can only use the "stable" one, except for a small number of special functions like in the runtime. There could be as many variants of the "unspecified" ABI as we want, which the compiler can choose. If we really have more than two, I think the assembly code can only call functions with specified ABI. The specified ABIs should all be stable. |
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I'm less interested in assembly code calling other functions, which is less common, than in how assembly functions themselves are called. Where we have cases where assembly code is justified, perhaps it is also worth using the faster calling convention to call that code. (Or maybe it doesn't matter, but it seems worth considering.) |
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The other side of this question is what could we convince gollvm/gccgo to do. If Rust works with bespoke calling conventions, presumably there's a way to do this; should we give assembly the option of specifying precisely which registers to use, and declaring which ones it trashes? |
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That's fair, and I think having faster calls from Go to assembly is another thing that would be infeasible now, but this proposal could enable. I think we still want the stack and unstable ABIs for legacy and optimization reasons, but this would make it possible to introduce other stable ABIs. Implementation-wise, I imagine we would mark the assembly function's ABI in the symbol flags as well as on its Go declaration. To make ABI checking possible, we would introduce other variants of the CALL instruction that still assemble to a CALL, but mark the intended ABI of the call (perhaps via a relocation). These are probably the exact some mechanisms the compiler itself would use to implement multiple ABIs. |
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With respect to gollvm: LLVM definitely supports multiple calling conventions and has provisions for tagging each call and each function with the expected calling convention. With that said, there is (currently) a fixed set of conventions (e.g. you have to update LLVM if you want to add another one). Also worth noting that most of the aspects of carrying out the calling convention (for example, figuring out which params go in memory and which in registers) are expected to be handled by the front end (there is code in the gollvm bridge that handles these details). |
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Can this proposal also include that (gc supported) .syso object files be required to use the "stable stack" ABI as well? |
I guess I don't see how this could work in general. Part of the Go stack ABI is participating in Go's stack growth protocol. Other than functions that don't use any stack space, I don't see how you can write a function in "external assembly" code that correctly follows the stack ABI. If anything, it seems like functions in syso files should be required to use the system ABI (and cgo is the bridge to the system ABI). |
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@aclements I'm referring to the first part of https://github.com/golang/go/wiki/GcToolchainTricks
Using a stable stack ABI to unpack arguments and set return variables, without the cgo overhead. |
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I see. Since syso files are also used for code using the system ABI (for example, I believe Go's boringcrypto works this way), I don't think we can force ABI constraints one way or the other directly on syso files. However, for functions with Go declarations, the declaration itself could certainly be marked with an ABI and the compiler will generate the correct calling sequence, assuming such calls are allowed (e.g., if we had a marker for "system ABI", we probably wouldn't allow direct Go -> system ABI calls because those are complicated to bridge; that's what cgo is for). We wouldn't be able to check ABI compatibility between the caller and the callee in this case. We'd just have to take it on faith that the declaration was marked correctly. |
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@aclements If I understand you correctly then adding a function deceleration like this: for a compute .syso symbol would also save one writing the stubs for it otherwise when the declaration appearing without an annotation or used with (I thought Go boringcrypto was using regular cgo, with the C symbols resolved by the linker in the .syso object, while the race detector using a custom cgo mechanism with |
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The ABI declaration would not save you from writing the stub for it, since stub isn't serving to adapt the calling convention. But the stub would default to using the stack ABI, so it would continue to work as written, with perhaps the additional annotation on the Go declaration.
I'm not sure what point you're trying to make here. I was just saying that the function entry points in the boringcrypto syso file (and the race detector syso file) are using the system ABI. We can't just declare that all functions in syso files use the stack ABI. We don't know a priori what ABI they're using. |
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ok thanks for clarifying @aclements! |
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Is marking function as requiring stable ABI is better than marking it as requiring new ABI (when it is introduced)? For unmaintained projects there is no difference, because they won't have pragmas and wont update ABI, and eventually stop working, once ABI is changed. Maintained projects will have to change code anyway (at least by adding ABI pragma), so there is little reason not to switch to the new ABI, which presumably has some benefits. |
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For Go functions, we should definitely default to the unstable ABI and require marking those that require a stable ABI. We want to move everyone to the unstable ABI because that's what gets us un-stuck and the vast majority of code won't be able to tell the difference. For assembly functions, we have to default to the stack ABI since the vast majority of existing code will be able to tell the difference and currently assumes the stack ABI. I don't think we should even make it possible to declare assembly functions that use the unstable ABI or call functions from assembly that use the unstable ABI (though the runtime might need an escape hatch). We could perhaps migrate toward requiring all assembly functions to have an explicit ABI marking, but that seems like unnecessary churn. For Go function stubs, I'm not sure. Since those are usually for assembly functions, we could say that those default to the stack ABI. But that seems inconsistent with Go functions defaulting to the unstable ABI, and would require an explicit pragma to select the unstable ABI for cases that need that (such as linkname'd Go functions). So I think Go function stubs should default to the unstable ABI, but I'm less certain about that. |
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For Go calling assembly, maybe we can have function declarations (without body) default to stable ABI, unless otherwise specified. If the external function doesn't use the stack ABI, it doesn't work today. For assembly calling Go, maybe we can scan the assembly code, find all the func symbols, and assume them to have the stack ABI. Maybe we can have a tool to do that and insert the pragma on the Go side. (Assembly code calling Go function defined in another package? Oh no, don't do it...) |
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Defaulting to stack ABI in asm functions forever is extremely unfortunate. It encourages using more asm than necessary to avoid call overhead. It also causes weird situation were you know that better ABI is implemented, but you cannot use it in function that is so performance critical, that it is written in assembly. I'd personally prefer some option to use opt-in for new, versioned ABI, that may break in future releases. |
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To support existing code, in particular as you mentioned unmaintained code, we probably have to default to the stack ABI. But it is just default. Once a new ABI is specified, the assembly code will be able to use it. You just need to change the assembly code and add the pragma at same time. |
My concern with this is mostly that there are some uses for function declarations that aren't for calling assembly, such as linkname'ing Go functions across packages. In the case we'd still want to use the unstable ABI, so we'd need an annotation to opt-in to the unstable ABI. I'd rather not have such an annotation at all.
Yes, definitely. I think at first the vet tool should do this (checking ABI compatibility both ways), and once we're ready to change the unstable ABI, the linker should take responsibility for checking this. The vet tool can suggest exactly what to do to resolve the problem ("add pragma X here"). |
rsc
changed the title
On Power gccgo follows our normal ELF v2 PPC64 ABI, which means arguments and return values are passed in registers. gccgo doesn't recognize or know how to assemble Go assembler so calling them is not an issue. |
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Another motivation for un-defining the default ABI: we could free up a fixed register for storing the G or possibly even the stack bound, which would speed up function prologues and reduce function call overhead. |
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Here's a sketch of a new proposal that @rsc and I came up with for how to separate the stable and internal ABIs. This doesn't require any magic comments or user intervention whatsoever except in extremely unusual cases of breaking package boundaries with linkname or assembly (and even those mostly work). I have a mostly complete implementation of this (just ironing out some cgo weirdness because there's always cgo weirdness). The core idea is to use ABI wrappers to translate between ABIs where necessary. These wrappers live alongside the "native" implementation of a function, so that function can be called via any available ABI rather than being constrained to a single ABI. We repurpose the existing symbol version mechanism so these symbols can all have the same name without colliding, and the ABI is encoded into every symbol reference. The Go compiler always operates in terms of the internal ABI. Go functions use the internal ABI and calls from Go code use the internal ABI. Function values and method tables always use the internal ABI. If the Go compiler knows that the "native" implementation of a function uses another ABI, it can (and probably should) optimize call sites, but this is just an optimization. The Go assembler always generates stable ABI (we'll call this ABI0) function symbols and all symbol references are to ABI0. There's simply no way to write an internal ABI symbol. If we do add another stable ABI in the future, we can extend the symbol syntax to have a way to specify the ABI (probably put something in angle brackets). We add a mode to the Go assembler that scans the assembly and produces a "symbol ABIs" file containing the names of symbols defined and referenced via ABI0. This turns out to be pretty easy to do robustly, even accounting for Altogether, this keeps virtually all assembly code and linknamed symbols working as-is, which was a problem with my earlier proposal. I think there are only two cases that require user intervention:
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Change https://golang.org/cl/149483 mentions this issue: |
Because run.go doesn't pass the package being compiled to the compiler via the -p flag, it can't match up the main·f symbol from the assembler with the "func f" stub in Go, so it doesn't produce the correct assembly stub. Fix this by removing the package prefix from the assembly definition. Alternatively, we could make run.go pass -p to the compiler, but it's nicer to remove these package prefixes anyway. Should fix the linux-arm builder, which was broken by the introduction of function ABIs in CL 147160. Updates #27539. Change-Id: Id62b7701e1108a21a5ad48ffdb5dad4356c273a6 Reviewed-on: https://go-review.googlesource.com/c/149483 Run-TryBot: Austin Clements <austin@google.com> Reviewed-by: Keith Randall <khr@golang.org>
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Change https://golang.org/cl/149605 mentions this issue: |
When CL 147160 introduced function ABIs encoded as symbol versions in the linker, it became slightly more complicated to look up derived DWARF symbols. It fixed this by introducing a dwarfFuncSym function to hide this logic, but missed one derived lookup that was done in the object reader itself. As a result, we lost the isStmt tables from the compiler, so every PC was marked as a statement in the DWARF info. Fix this by moving this derived lookup out of the object reader and into the DWARF code and calling dwarfFuncSym to get the correctly versioned symbol. Should fix the linux-amd64-longtest builder. Updates #27539. Change-Id: If40d5ba28bab1918ac4ad18fbb5103666b6d978b Reviewed-on: https://go-review.googlesource.com/c/149605 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>
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Change https://golang.org/cl/149817 mentions this issue: |
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Change https://golang.org/cl/149818 mentions this issue: |
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Change https://golang.org/cl/150077 mentions this issue: |
Currently, both asm and compile have a -symabis flag, but in asm it's a boolean flag that means to generate a symbol ABIs file and in the compiler its a string flag giving the path of the symbol ABIs file to consume. I'm worried about this false symmetry biting us in the future, so rename asm's flag to -gensymabis. Updates #27539. Change-Id: I8b9c18a852d2838099718f8989813f19d82e7434 Reviewed-on: https://go-review.googlesource.com/c/149818 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
When CL 147160 introduced function ABIs encoded as symbol versions in the linker, it became slightly more complicated to look up derived DWARF symbols. It fixed this by introducing a dwarfFuncSym function to hide this logic, but missed one derived lookup that was done in the object reader itself. As a result, we lost the isStmt tables from the compiler, so every PC was marked as a statement in the DWARF info. Fix this by moving this derived lookup out of the object reader and into the DWARF code and calling dwarfFuncSym to get the correctly versioned symbol. Should fix the linux-amd64-longtest builder. Updates #27539. Change-Id: If40d5ba28bab1918ac4ad18fbb5103666b6d978b Reviewed-on: https://go-review.googlesource.com/c/149605 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>
Currently, both asm and compile have a -symabis flag, but in asm it's a boolean flag that means to generate a symbol ABIs file and in the compiler its a string flag giving the path of the symbol ABIs file to consume. I'm worried about this false symmetry biting us in the future, so rename asm's flag to -gensymabis. Updates #27539. Change-Id: I8b9c18a852d2838099718f8989813f19d82e7434 Reviewed-on: https://go-review.googlesource.com/c/149818 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
On some platforms, assembly in internal/syscall/unix references unexported runtime symbols. Catch these references so the compiler can generate the necessary ABI wrappers. Fixes #28769. Updates #27539. Change-Id: I118eebfb8b3d907b4c3562198e6afb49854f5827 Reviewed-on: https://go-review.googlesource.com/c/149817 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com> Reviewed-by: Clément Chigot <clement.chigot@atos.net> Reviewed-by: Keith Randall <khr@golang.org>
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Change https://golang.org/cl/157017 mentions this issue: |
Currently, obj.Ctxt's symbol table does not distinguish between ABI0 and ABIInternal symbols. This is *almost* okay, since a given symbol name in the final object file is only going to belong to one ABI or the other, but it requires that the compiler mark a Sym as being a function symbol before it retrieves its LSym. If it retrieves the LSym first, that LSym will be created as ABI0, and later marking the Sym as a function symbol won't change the LSym's ABI. Marking a Sym as a function symbol before looking up its LSym sounds easy, except Syms have a dual purpose: they are used just as interned strings (every function, variable, parameter, etc with the same textual name shares a Sym), and *also* to store state for whatever package global has that name. As a result, it's easy to slip up and look up an LSym when a Sym is serving as the name of a local variable, and then later mark it as a function when it's serving as the global with the name. In general, we were careful to avoid this, but #29610 demonstrates one case where we messed up. Because of on-demand importing from indexed export data, it's possible to compile a method wrapper for a type imported from another package before importing an init function from that package. If the argument of the method is named "init", the "init" LSym will be created as a data symbol when compiling the wrapper, before it gets marked as a function symbol. To fix this, we separate obj.Ctxt's symbol tables for ABI0 and ABIInternal symbols. This way, the compiler will simply get a different LSym once the Sym takes on its package-global meaning as a function. This fixes the above ordering issue, and means we no longer need to go out of our way to create the "init" function early and mark it as a function symbol. Fixes #29610. Updates #27539. Change-Id: Id9458b40017893d46ef9e4a3f9b47fc49e1ce8df Reviewed-on: https://go-review.googlesource.com/c/157017 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>
Updates golang/go#27539. Change-Id: I40e51d59847cdb2df9ad6e3b402c619329172f47 Reviewed-on: https://go-review.googlesource.com/c/150077 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
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I think this change broke The proposal "compatibility" section suggests some work-arounds, but I was not able to fix the error. I opened the issue go-qml/qml#190, but the package is abandoned so I'm not so confident someone will help there. Can someone please help me fix? |
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@neclepsio, thanks for the report. Unfortunately, reaching into the runtime implementation like this is really, really not supported. Given that go-qml is officially dead and there are other (maintained) Qt packages for Go, can you expand a bit on what you're trying to accomplish so I can better point you in the right direction? For example, on Linux it would be better to use |
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@aclements thank you for your reply. I'm just trying to upgrade to go 1.12 my package using go-qml. I don't understand what the assembly code does and what the cdata package is for. Unfortunately using other qt bindings would require to rewrite the ui part of my application from the ground up. Moreover, there are licensing issues with therecipe/qt. So it would be great to be able to keep go-qml on life support. |
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@aclements thank you for your suggestion to use |
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Glad you were able to get it working, @neclepsio. |
The are various changes we'd like to make (or explore making) to the Go ABI, such as changing to a register-based ABI (#18597), propagating register clobbers up the call graph to avoid unnecessary spills, and passing dynamic information about allocation scopes. However, every attempt we've made to modify the calling convention has encountered major hurdles with calling back and forth between Go and assembly code.
This proposal is not about a specific change to the ABI, but about how to get to a place where we have the freedom to change the ABI. It was in part inspired by how Rust handles this issue.
I propose:
This would involve supporting potentially two ABIs for the rest of time, but I believe that's actually a strength. This approach provides a stable ABI for the rare cases that need it, while simultaneously giving us as compiler writers the freedom to evolve the ABI in the vast majority of cases that don't need a stable ABI.
/cc @dr2chase @randall77 @ianlancetaylor @josharian