[llvm-dev] cmpxchg on floats

[Nicolai Hähnle via llvm-dev]

On Fri, Aug 14, 2020 at 7:42 PM JF Bastien <jfbastien at apple.com> wrote:
> We (C, C++, and LLVM) are generally moving towards supporting FP as a first-class thing with all atomic operations †, including cmpxchg. It’s indeed *usually* specified as a bitwise comparison, not a floating-point one, although IIRC AMD has an FP cmpxchg.

We do, but this should arguably be a different instruction because it
behaves differently from bitwise compare.

Cheers,
Nicolai


> Similarly, some of the operations are allowed to have separate FP state (say, atomic add won’t necessarily affect the scalar FP execution’s exception state, might have a different rounding mode, etc).
>
> I’m OK with MLIR matching the current state of LLVM, but I want to point out that the docs might be incorrect, or are expected to change in the future.
>
> † See http://wg21.link/p0020 as well as r360421 D58251 D50491 D26266 D26266 r264845 D15471
>
> On Aug 14, 2020, at 6:15 AM, Nicolai Hähnle via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> We've relaxed `atomicrmw xchg` to support floating point types but not
> cmpxchg -- the cmpxchg comparison behavior is not a floating point
> comparison, so that would be potentially misleading. I'd say adding
> the assertion is a good idea.
>
> Cheers,
> Nicolai
>
> On Thu, Aug 13, 2020 at 10:59 PM Chris Lattner via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
>
> Does the code generator support this?  If not, we should add an assertion to the verifier.  If so, we can consider relaxing langref or making it more specific about floats.
>
>
> I agree with this. Generally it’s a bit tricky because, some of these are tied to the atomic expand pass, where a backend says what it supports.
>
>
> -Chris
>
> On Aug 13, 2020, at 6:35 AM, Alex Zinenko via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> Hi LLVM-dev,
>
> when working on MLIR-to-LLVM-IR conversion, I noticed that it is possible to programmatically construct a cmpxchg instruction operating on floats (or actually any type since there is no assertion on pointer element type here https://github.com/llvm/llvm-project/blob/9c2e708f0dc547d386ea528450a33ef4bd2a750b/llvm/lib/IR/Instructions.cpp#L1501), but LangRef specifies that only integers and pointers are accepted (https://llvm.org/docs/LangRef.html#cmpxchg-instruction). Does somebody rely on other types being accepted in cmpxchg or should we add an assertion for the element type to match the LangRef?
>
> --
> Alex
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