[cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
Hal Finkel via cfe-dev
cfe-dev at lists.llvm.org
Thu Aug 31 15:03:17 PDT 2017
On 08/31/2017 04:55 PM, Richard Smith via cfe-dev wrote:
> On 31 August 2017 at 14:48, Kaylor, Andrew via cfe-dev
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
> I had considered the inlining issue, and updating the inliner to
> handle this was on my list of things yet to be done. I’m not sure
> I understand what you’re saying about LTO.
>
>
> The point about LTO was that you can't solve the inlining problem by
> using intrinsics throughout an entire module if FENV_ACCESS is used
> anywhere, because you might see code from outside the module.
>
> There are some points about the standard specification that seem a
> bit unclear to me, specifically with regard to how things work if
> you call an FENV_ACCESS-off function from within an FENV_ACCESS-on
> scope. I believe that when I talked with our front end guys here
> about that our conclusion was that doing that sort of thing is
> undefined behavior. I’m not sure if this is related to your LTO
> concern or not.
>
>
> I don't think that observation helps much, since you can call
> FENV_ACCESS-on code from FENV_ACCESS-off code, which is equally
> problematic. It's also not true that the above case results in UB: you
> only get UB if you're in a non-default FP mode when you enter the
> FENV_ACCESS-off code, or if that code attempts to inspect or alter the
> FP environment -- so the hoisting problem still seems to exist.
>
> In any event, you definitely raise some good questions that I
> don’t have answers for. I’ll have to give this some more thought.
>
> I just want to add that one of the primary design goals in what
> I’ve done so far was to not doing anything that would inhibit
> optimizations or require extra work on the part of the
> optimization passes in the case where we don’t care about
> FENV_ACCESS. That requirement makes attaching side-effects to the
> FP operations quite difficult.
>
>
> Indeed; FWIW that approach seems like the right one to me. We need a
> very strong barrier between anything that might use feenableexcept and
> anything that might use LLVM's "pure" FP operations, and I don't
> believe we have such a thing in LLVM IR yet. So I don't think we're
> ready for frontend work on FENV_ACCESS, since we don't yet know how it
> will be represented in IR.
To be clear, we've had several extensive discussions about this, on and
off list, and Andy has started adding the corresponding intrinsics into
the IR. There was a presumption about a lack of mixing, however, and we
do need to work out how to prevent mixing the native IR operations with
the intrinsics (although, perhaps we just did that).
-Hal
> -Andy
>
> *From:*Richard Smith [mailto:richard at metafoo.co.uk
> <mailto:richard at metafoo.co.uk>]
> *Sent:* Thursday, August 31, 2017 2:32 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?
>
> 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 <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:
>
> ^^^^^^
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> <mailto:cfe-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>
> _______________________________________________
>
> cfe-dev mailing list
>
> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> <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
> <mailto:cfe-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>
> _______________________________________________ cfe-dev
> mailing list cfe-dev at lists.llvm.org
> <mailto:cfe-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>
> _______________________________________________ cfe-dev
> mailing list cfe-dev at lists.llvm.org
> <mailto:cfe-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>
> _______________________________________________ cfe-dev mailing
> list cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>
> _______________________________________________
> 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20170831/c67404dc/attachment.html>
More information about the cfe-dev
mailing list