llvm dev - May 2021 - Reading FP Environment status from IR

Hi All,

We would like to prevent certain optimizations (like hoisting an fdiv
instruction) when the application has set the floating point environment
flags using APIs like
feraiseexcept
<https://en.cppreference.com/w/cpp/numeric/fenv/feraiseexcept>
 or
if it uses  #pragma STDC FENV_ACCESS
<https://en.cppreference.com/w/cpp/preprocessor/impl>.

Is there any way to know this at LLVM-IR level?

Regards,
Raghesh
------------------------------
Raghesh Aloor
AMD India Pvt. Ltd.
Bengaluru.
------------------------------
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We have intrinsic versions of the floating point instructions to suppress
optimizations that are unsafe when exceptions are enabled.
https://llvm.org/docs/LangRef.html#constrained-floating-point-intrinsics
Currently I think it prevents nearly all optimizations.

Clang should emit these for some targets like X86 when it sees #pragma STDC
FENV_ACCESS. Clang can only do this for targets that have implemented
proper support for the constrained intrinsics in the backend.

~Craig


On Thu, May 6, 2021 at 2:26 AM raghesh via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> Hi All,
>
> We would like to prevent certain optimizations (like hoisting an fdiv
> instruction) when the application has set the floating point environment
> flags using APIs like
> feraiseexcept
> <https://en.cppreference.com/w/cpp/numeric/fenv/feraiseexcept>  or if
it
> uses  #pragma STDC FENV_ACCESS
> <https://en.cppreference.com/w/cpp/preprocessor/impl>.
>
> Is there any way to know this at LLVM-IR level?
>
> Regards,
> Raghesh
> ------------------------------
> Raghesh Aloor
> AMD India Pvt. Ltd.
> Bengaluru.
> ------------------------------
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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