[cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?

[Richard Smith via cfe-dev]

On 31 August 2017 at 11:09, Kaylor, Andrew via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> There are still a few things missing from the optimizer to get it
> completely robust, but I think there is enough in place for front end work
> to begin.  As I think I’ve demonstrated in my recent attempt to contribute
> a clang patch I’m not skilled enough with the front end to be the person to
> pull this off without an excessive amount of oversight, but as Erich
> indicated we do have some good front end people here who have this on their
> TODO list.  It’s just not at the top of the TODO list yet.
>
>
>
> If anyone is interested in the details of the LLVM side of things, there
> are constrained FP intrinisics (still marked as experimental at this point)
> documented in the language reference.  The initial patch can be seen here:
>
>
>
> https://reviews.llvm.org/D27028
>
>
>
> I’ve since added another group of intrinsics to handle the libm-equivalent
> intrinsics, and more recently Wei Ding contributed an fma intrinsic.
>
>
>
> The idea is that the front end will emit the constrained intrinsics in
> place of equivalent general FP operations or intrinsics in scopes where
> FENV_ACCESS is enabled.  This will prevent the optimizer from making
> optimizations that assume default fenv settings (which is what we want the
> optimizer to do in all other cases).  Eventually, we’ll want to go back and
> teach specific optimizations to understand the intrinsics so that where
> possible optimizations can be performed in a manner consistent with dynamic
> rounding modes and strict exception handling.
>

How do you deal with the hoisting-into-fenv_access problem? Eg:

double f(double a, double b, double c) {
  {
#pragma STDC FENV_ACCESS ON
    feenableexcept(FE_OVERFLOW);
    double d = a * b;
    fedisableexcept(FE_OVERFLOW);
  }
  return c * d;
}

What stops llvm from hoisting the second fmul up to before the
fedisableexcept?


> -Andy
>
>
>
> *From:* Hal Finkel [mailto:hfinkel at anl.gov]
> *Sent:* Thursday, August 31, 2017 10:45 AM
> *To:* Richard Smith <richard at metafoo.co.uk>; Marcus Johnson <
> bumblebritches57 at gmail.com>
> *Cc:* Clang Dev <cfe-dev at lists.llvm.org>; Kaylor, Andrew <
> andrew.kaylor at intel.com>
> *Subject:* Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
>
>
>
>
>
> On 08/31/2017 12:10 PM, Richard Smith via cfe-dev wrote:
>
> Because no-one has implemented it. Patches would be welcome, but will need
> to start with a design and implementation of the requisite llvm extensions.
>
>
> Yes. This is what Andrew Kaylor has been working on (cc'd).
>
>  -Hal
>
>
>
>
> On 31 Aug 2017 10:06, "Marcus Johnson via cfe-dev" <cfe-dev at lists.llvm.org>
> wrote:
>
> ^^^^^^
>
>
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>
> --
>
> Hal Finkel
>
> Lead, Compiler Technology and Programming Languages
>
> Leadership Computing Facility
>
> Argonne National Laboratory
>
>
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