[llvm-dev] [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
John McCall via llvm-dev
llvm-dev at lists.llvm.org
Tue Jan 9 22:55:30 PST 2018
> On Jan 9, 2018, at 3:50 PM, Kaylor, Andrew <andrew.kaylor at intel.com> wrote:
> >The standard argument against trying to introduce "scope-like" mechanisms to LLVM IR is inlining;
> >unless you're going to prevent functions that use stricter/laxer FP rules from being inlined >into
> >each other (which sounds disastrous), you're going to need to communicate strictness on an
> >instruction-by-instruction basis. If the backend wants to handle that by using the strictest
> >rule that it sees in use anywhere in the function because pattern-matching is otherwise too
> >error-prone, ok, that's its right; but the IR really should be per-instruction.
> I added a function level attribute, strictfp, which is meant to help with this. I don’t believe the inlining handling of the attribute is implemented yet, but what I’m thinking is that we would never inline a function that had the strictfp attribute and if we inlined a non-strictfp function into a strictfp function, we would transform any FP operations into their constrained equivalents at that time. In the short term, we’d probably just block both types of inlining.
> It may sound disastrous, but I think there’s an understanding that using strict FP semantics is going to significantly inhibit optimizations. In the short term, that’s actually the purpose of the constrained intrinsics -- to disable all optimizations until we can teach the optimizer to do things correctly. The plan is that once this is all implemented to produce correct results, we’ll go back and try to re-enable as many optimizations as possible, which may eventually include doing something more intelligent with inlining.
> With regard to your “instruction level” comments, my intention is that the use of the intrinsics will impose the necessary restrictions at the instruction level. Optimizations (other than inlining) should never need to check the function level attribute. But if we mixed “raw” FP operations and constrained intrinsics within a single function there would be no way to prevent motion of the “raw” operations across the intrinsics.
Is that a problem? Semantics are guaranteed only for strictfp operations, i.e. ones that use the intrinsics. Raw operations can get reordered across intrinsics and change semantics, but that seems allowable, right?
> The reason I brought up the scope level nature of the pragma was just to suggest that it might be a property that we could take advantage of to handle the transition from IR to MIR. I haven’t come up with a way to bake the strict FP information into the instructions across the ISel boundary, but I think it might be possible to temporarily add it to a block and then have an early machine code pass that used this information in some way once the MIR was all in place. I’m open to the possibility that that was a bad idea.
> From: rjmccall at apple.com [mailto:rjmccall at apple.com]
> Sent: Tuesday, January 09, 2018 11:12 AM
> To: Kaylor, Andrew <andrew.kaylor at intel.com>
> Cc: Ulrich Weigand <Ulrich.Weigand at de.ibm.com>; kpn at neutralgood.org; bumblebritches57 at gmail.com; bob.huemmer at sas.com; llvm-dev <llvm-dev at lists.llvm.org>; Richard Smith <richard at metafoo.co.uk>; cfe-dev at lists.llvm.org
> Subject: Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
> On Jan 9, 2018, at 1:53 PM, Kaylor, Andrew via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
> I think we’re going to need to create a new mechanism to communicate strict FP modes to the backend. I think we need to avoid doing anything that will require re-inventing or duplicating all of the pattern matching that goes on in instruction selection (which is the reason we’re currently dropping that information). I’m out of my depth on this transition, but I think maybe we could handle it with some kind of attribute on the MBB.
> In C/C++, at least, it’s my understanding that the pragmas always apply at the scope-level (as opposed to having the possibility of being instruction-specific), and we’ve previously agreed that our implementation will really need to apply the rules across entire functions in the sense that if any part of a function uses the constrained intrinsics all FP operations in the function will need to use them (though different metadata arguments may be used in different scopes). So I think that opens our options a bit.
> Regarding constant folding, I think you are correct that it isn’t happening anywhere in the backends at the moment. There is some constant folding done during instruction selection, but the existing mechanism prevents that. My concern is that given LLVM’s development model, if there is nothing in place to prevent constant folding and no consensus that it shouldn’t be allowed then we should probably believe that someone will eventually do it.
> The standard argument against trying to introduce "scope-like" mechanisms to LLVM IR is inlining; unless you're going to prevent functions that use stricter/laxer FP rules from being inlined into each other (which sounds disastrous), you're going to need to communicate strictness on an instruction-by-instruction basis. If the backend wants to handle that by using the strictest rule that it sees in use anywhere in the function because pattern-matching is otherwise too error-prone, ok, that's its right; but the IR really should be per-instruction.
> From: Ulrich Weigand [mailto:Ulrich.Weigand at de.ibm.com <mailto:Ulrich.Weigand at de.ibm.com>]
> Sent: Tuesday, January 09, 2018 9:59 AM
> To: Kaylor, Andrew <andrew.kaylor at intel.com <mailto:andrew.kaylor at intel.com>>; kpn at neutralgood.org <mailto:kpn at neutralgood.org>
> Cc: Hal Finkel <hfinkel at anl.gov <mailto:hfinkel at anl.gov>>; Richard Smith <richard at metafoo.co.uk <mailto:richard at metafoo.co.uk>>; bob.huemmer at sas.com <mailto:bob.huemmer at sas.com>; bumblebritches57 at gmail.com <mailto:bumblebritches57 at gmail.com>; wei.ding2 at amd.com <mailto:wei.ding2 at amd.com>; cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>; llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> Subject: Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?
> Andrew Kaylor wrote:
> >In general, the current "strict FP" handling stops at instruction
> >selection. At the MachineIR level we don't currently have a mechanism
> >to prevent inappropriate optimizations based on floating point
> >constraints, or indeed to convey such constraints to the backend.
> >Implicit register use modeling may provide some restriction on some
> >architectures, but this is definitely lacking for X86 targets. On the
> >other hand, I'm not aware of any specific current problems, so in many
> >cases we may "get lucky" and have the correct thing happen by chance.
> >Obviously that's not a viable long term solution. I have a rough plan
> >for adding improved register modeling to the X86 backend, which should
> >take care of instruction scheduling issues, but we'd still need a
> >mechanism to prevent constant folding optimizations and such.
> Given that Kevin intends to target SystemZ, I'll be happy to work on the SystemZ back-end support for this feature. I agree that we should be using implicit control register dependencies, which will at least prevent moving floating-point operations across instructions that e.g. change rounding modes. However, the main property we need to model is that floating-point operations may *trap*. I guess this can be done using UnmodeledSideEffects, but I'm not quite clear on how to make this dependent on whether or not a "strict" operation is requested (without duplicating all the instruction patterns ...).
> Once we do use something like UnmodeledSideEffects, I think MachineIR passes should handle everything correctly; in the end, the requirements are not really different from those of other trapping instructions. B.t.w. I don't think anybody does constant folding on floating-point constants at the MachineIR level anyway ... have you seen this anywhere?
> Mit freundlichen Gruessen / Best Regards
> Ulrich Weigand
> Dr. Ulrich Weigand | Phone: +49-7031/16-3727
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