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

[Richard Smith via cfe-dev]

I think that's also not enough; you'd get the same problem after inlining,
and across modules with LTO. You would need to also prevent any
interprocedural code motion across a FENV_ACCESS / non-FENV_ACCESS boundary.

And even that doesn't seem to be enough. Suppose that some scalar
optimization pass finds a clever way to converts some integer operation
into a floating-point operation, such that it can prove that the FP values
never overflow (I believe Chandler has an example of this that comes up in
some real crypto code). Now suppose there's a case where the integer
operands are undef, but that the code in question is bypassed in that case.
If the FP operations get hoisted, and you happen to have FP exceptions
enabled, you have a potential miscompile.

Fundamentally, it seems to me that feenableexcept is unsound in the current
LLVM IR model of floating point, if we assume that fadd, fmul, fsub etc do
not have side-effects.

On 31 August 2017 at 14:20, Kaylor, Andrew via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> If that’s the case, we may need to use the constrained intrinsics for all
> FP operations when FENV_ACCESS is enabled anywhere in a function.
>
>
>
> *From:* Richard Smith [mailto:richard at metafoo.co.uk]
> *Sent:* Thursday, August 31, 2017 2:18 PM
> *To:* Kaylor, Andrew <andrew.kaylor at intel.com>
> *Cc:* Clang Dev <cfe-dev at lists.llvm.org>; Marcus Johnson <
> bumblebritches57 at gmail.com>; wei.ding2 at amd.com
>
> *Subject:* Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
>
>
>
> On 31 August 2017 at 14:14, Kaylor, Andrew via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> I believe that we will rely on fedisableexcept() being marked as having
> unmodeled side-effects to prevent a hoist like that.
>
>
>
> fadd can be hoisted past *anything*, can't it?
>
>
>
> *From:* Richard Smith [mailto:richard at metafoo.co.uk]
> *Sent:* Thursday, August 31, 2017 2:09 PM
> *To:* Kaylor, Andrew <andrew.kaylor at intel.com>
> *Cc:* Marcus Johnson <bumblebritches57 at gmail.com>; Clang Dev <
> cfe-dev at lists.llvm.org>; wei.ding2 at amd.com
>
>
> *Subject:* Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
>
>
>
> 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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