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

[Kaylor, Andrew via llvm-dev]

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.

-Andy

From: Ulrich Weigand [mailto:Ulrich.Weigand at de.ibm.com]
Sent: Tuesday, January 09, 2018 9:59 AM
To: Kaylor, Andrew <andrew.kaylor at intel.com>; kpn at neutralgood.org
Cc: Hal Finkel <hfinkel at anl.gov>; Richard Smith <richard at metafoo.co.uk>; bob.huemmer at sas.com; bumblebritches57 at gmail.com; wei.ding2 at amd.com; cfe-dev at lists.llvm.org; llvm-dev <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

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