Get rid of reinterpret in its current form

[Keno]

I don't like the way reinterpret is currently implemented. It puns on the notion of an array. One problem we recently had to deal with is that alignment guarantees are different between different types (those are now disallowed). Another is that it prevents us from doing more strict TBAA on array element types. I think we should get rid of reinterpret completely and replace it with a ReinterpretArray type, who's getindex method performs the appropriate load from the original array. That way we never have an Array with incorrectly typed storage, but can retain the convenience (and once again allow reinterpret for types with mismatched alignment).

[vtjnash]

I think the issue we have is that we allow reinterpret to cast between types that a method like ReinterpretArray couldn't. For example, between structs and bytes (allowing it to observe padding bytes and alignment, for example).

However, do we still need that capability in general? In the parts of the IO system that used to use this, I've largely switched them to using Ref and doing unsafe_ byte IO (by casting to a Ptr{UInt8}) and recommending marking them noinline (for both the gc-root and as a TBAA barrier).

[Keno]

I'm fine with implementing ReinterpretArray's getindex by copying out enough elements to on-stack storage and then reinterpreting there. LLVM will fold any unnecessary copies easily enough. My only problem is with Array here.

[vtjnash]

Right, but how would you express that? Currently we don't allow any sort of struct-type-punning in reinterpret for values, just for Arrays (as a special case).

If copy the elements to stack, wouldn't that still imply that the source and destination TBAA are different, inhibiting optimization around it (since LLVM doesn't have a TBAA union type)?

[Keno]

The fact that source and destination TBAA can't be different is a limitation of the current memcpy implementation, not of the underlying semantics. I plan to fix that. That's an optimization however. The reinterpret changes would be breaking and would have to be done pre-1.0.

[lobingera]

So do you mean to change the implementation of reinterpret, or the syntax?

[Keno]

implementation. But the return type would change, so it's breaking.

[lobingera]

Then i'd like to see a better motivation than "I don't like the way reinterpret is currently implemented."

[Keno]

It's in the original post. It violates alignment guarantees (so you now get errors unless you know about alignment issues) and it prevents us from strengthening TBAA.

[iamed2]

I would love to see this. As a user I appreciate this interface.

[vtjnash]

Cross-ref additional issues with reinterpret: #16652

[StefanKarpinski]

What about this API:

• you can declare an alighnment for an array when you create it
• alignment can be computed from a given set of element types
• you can reinterpret to any element type for which an arrays alignment is valid

This requires a little forethought but it should be doable.

Another possible API for when that's not possible is to extract elements of a different type from an array without reinterpreting it, as if it had a different element type. That would potentially be slow of the alignment disagrees, but it wouldn't be wrong at least. We can have both as they're somewhat complementary.