[cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
Hal Finkel via cfe-dev
cfe-dev at lists.llvm.org
Thu Aug 31 14:40:50 PDT 2017
On 08/31/2017 04:31 PM, Richard Smith via cfe-dev wrote:
> 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.
Or we prevent inlining.
>
> 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.
Good point. However, that's not a new problem, and we currently deal
with this by respecting the noimplicitfloat attribute (and I think we'd
definitely need to use that attribute if we allow fooling with the FP
environment).
-Hal
>
> 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 <mailto: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
> <mailto:richard at metafoo.co.uk>]
> *Sent:* Thursday, August 31, 2017 2:18 PM
> *To:* Kaylor, Andrew <andrew.kaylor at intel.com
> <mailto:andrew.kaylor at intel.com>>
> *Cc:* Clang Dev <cfe-dev at lists.llvm.org
> <mailto:cfe-dev at lists.llvm.org>>; Marcus Johnson
> <bumblebritches57 at gmail.com <mailto:bumblebritches57 at gmail.com>>;
> wei.ding2 at amd.com <mailto: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 <mailto: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
> <mailto:richard at metafoo.co.uk>]
> *Sent:* Thursday, August 31, 2017 2:09 PM
> *To:* Kaylor, Andrew <andrew.kaylor at intel.com
> <mailto:andrew.kaylor at intel.com>>
> *Cc:* Marcus Johnson <bumblebritches57 at gmail.com
> <mailto:bumblebritches57 at gmail.com>>; Clang Dev
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>;
> wei.ding2 at amd.com <mailto: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 <mailto: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
> <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
> <mailto:hfinkel at anl.gov>]
> *Sent:* Thursday, August 31, 2017 10:45 AM
> *To:* Richard Smith <richard at metafoo.co.uk
> <mailto:richard at metafoo.co.uk>>; Marcus Johnson
> <bumblebritches57 at gmail.com
> <mailto:bumblebritches57 at gmail.com>>
> *Cc:* Clang Dev <cfe-dev at lists.llvm.org
> <mailto:cfe-dev at lists.llvm.org>>; Kaylor, Andrew
> <andrew.kaylor at intel.com <mailto: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
> <mailto: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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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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