[cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
Hal Finkel via cfe-dev
cfe-dev at lists.llvm.org
Thu Aug 31 14:46:56 PDT 2017
On 08/31/2017 04:40 PM, Hal Finkel via cfe-dev wrote:
>
>
> 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.
I agree with this: we can't mix them. If a function allows FP
environment access anywhere, then we'll need to use the intrinsics
everywhere. We'll also need to use noimplicitfloat. We'll need to
disallow inlining as well where there's a mismatch (we could, for
example, add an "fp_env_access" attribute, and disallow inlining when
there's a caller/callee attribute mismatch).
-Hal
>>
>> 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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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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