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

Thu Aug 31 18:40:33 PDT 2017

On 31 August 2017 at 18:17, Hal Finkel via cfe-dev <cfe-dev at lists.llvm.org>
wrote:

>
> On 08/31/2017 05:41 PM, Richard Smith wrote:
>
> If everyone's happy with that approach, I agree we're ready to start
> looking at the frontend side of things :-) I think there are a few open
> questions:
>
> * Is it ever worth outlining an FENV_ACCESS block from a function to
> minimize the scope in which optimizations are restricted? (Presumably in
> the long term, all the LLVM optimizations that can operate on FP operations
> will do the same thing to intrinsic calls that are in "never traps, rounds
> to nearest" mode, so it may well not be worthwhile.)
>
>
> Outlining is probably a reasonable idea if there are floating-point
> computations outside of the FENV_ACCESS blocks in the functions. The
> tradeoffs here could be tricky, however, and I'd suggest waiting until we
> find some motivating cases. Even long term, I don't expect parity (just
> because if we're going to teach all of the transformations to treat them
> identically, we might as well extend the semantics of the IR operations
> themselves, and it's not clear to me that will even be worthwhile).
>

Hmm. I suppose there is still a semantic difference between, say, a "never
trap, round to nearest mul" intrinsic and fmul -- the former cannot be
reordered past a function call (because it might change the rounding mode),
but the latter can. So perhaps there is some argument for outlining even in
the hypothetical long term when LLVM can optimize the intrinsics well.

> * What impact does FENV_ACCESS have on constant expression evaluation,
> particularly in C++11 onwards where some FP operations are required to be
> evaluated during compilation?
> * What happens if FENV_ACCESS is enabled at the end of a module?
>
>
> Would you prefer that it stay within the module? I think I'd prefer that.
>

Yes, with an enabled-by-default warning if a module ends with a non-default
setting (on the basis that we want a modules build and a non-modules build
using the same header to have the same semantics).

Thanks again,
> Hal
>
>
> * How does FENV_ACCESS interact with namespaces? Does it only last until
> the `}` (like at block scope) or not?
>
> I've taken some of these to the C++ committee to see if they have opinions
> about how this feature should work in C++.
>
> On 31 August 2017 at 15:28, Kaylor, Andrew via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> This sounds promising.
>>
>>
>>
>> I would not that when I added the strictfp attribute, I intended that the
>> front end would attach this attribute to the callsite of all function calls
>> within a scope that required strict floating point semantics.  It was
>> supposed to be a way of preventing calls to libm functions from being
>> optimized as LibFunc calls without the front end needing to know which
>> functions could be processed that way.
>>
>>
>>
>> I think it is a natural extension to use this attribute on functions that
>> contain code requiring strict floating point handling, but the
>> documentation will need to be updated.
>>
>>
>>
>> *From:* Hal Finkel [mailto:hfinkel at anl.gov]
>> *Sent:* Thursday, August 31, 2017 3:08 PM
>> *To:* Richard Smith <richard at metafoo.co.uk>
>> *Cc:* Kaylor, Andrew <andrew.kaylor at intel.com>; 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 08/31/2017 05:02 PM, Richard Smith wrote:
>>
>> On 31 August 2017 at 14:40, Hal Finkel via cfe-dev <
>> cfe-dev at lists.llvm.org> 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.
>>
>>
>>
>> Sure, I was considering that to be a form of interprocedural code motion
>> :)
>>
>> 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).
>>
>>
>>
>> OK, so the idea would be that we'd lower a function containing
>> FENV_ACCESS (or possibly an outlined block of such a function) with
>> intrinsics for all FP operations, specially-annotated libm function calls,
>> and noimplicitfloat and strictfp attributes to prevent generation of new FP
>> operations and inlining into non-strictfp functions. Right? (And we could
>> imagine a verifier check that ensures that you don't have pure FP
>> operations inside a strictfp function.)
>>
>>
>> Yes, exactly.
>>
>>
>>
>>
>> Given the function annotations, do we need special intrinsics at all, or
>> could we instead require that passes check whether the enclosing function
>> is marked strictfp before optimizing, in the same way that some
>> optimizations must be gated by a check for noimplicitfloat?
>>
>>
>> That's another possible design. We decided that the intrinsics were less
>> intrusive. The problems is that it's not just FP-specific optimizations
>> that would need to check the attribute, it is also other optimizations
>> doing other kinds of code motion and value propagation. Having IR-level
>> operations that are side-effect-free, except when some special function
>> attribute is present, seems undesirable.
>>
>>  -Hal
>>
>>
>>
>>
>>  -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> 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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>> --
>>
>> Hal Finkel
>>
>> Lead, Compiler Technology and Programming Languages
>>
>> Leadership Computing Facility
>>
>> Argonne National Laboratory
>>
>> _______________________________________________ cfe-dev mailing list
>> cfe-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/
>> mailman/listinfo/cfe-dev
>>
>> --
>>
>> Hal Finkel
>>
>> Lead, Compiler Technology and Programming Languages
>>
>> Leadership Computing Facility
>>
>> Argonne National Laboratory
>>
>> _______________________________________________ cfe-dev mailing list
>> cfe-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/
>> mailman/listinfo/cfe-dev
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
> _______________________________________________
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