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

Kaylor, Andrew via cfe-dev cfe-dev at lists.llvm.org
Mon Jan 8 11:15:56 PST 2018

Hi Kevin,

Thanks for reaching out about this, and thanks especially for offering to help. I've had some other priorities that have prevented me from making progress on this recently.

As far as I know, there is no support at all in clang for handling the FENV_ACCESS pragma. I have a sample patch somewhere that I created to demonstrate how the front end would create the constrained intrinsics instead of normal FP operations, but correctly implementing support for the pragma requires more front end and language expertise than I possess. I believe Clark Nelson, who does have such expertise, has this on his long term TODO list but I don't know anything about the actual timeframe when the work will begin.

On the LLVM side of things there are a few significant holes. As you've noticed, the FP to integer conversions operations still need intrinsics, as do fcmp, fptrunc, and fpext. There are probably others that I'm overlooking. The FP to SI conversion has an additional wrinkle that needs to be worked out in that the default lowering of this conversion to machine instructions is not exception safe.

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.

As for what you could begin work on, it should be a fairly straight-forward task to implement the intrinsics for fptosi, fptoui, fcmp, fptrunc, and fpext. That would be a gentle introduction. Beyond that, it would be very helpful to have some pathfinding work done to solidify exactly what the remaining shortcomings are. I have a patch somewhere (stale by now, but I could refresh it pretty easily) that unconditionally converts all FP operations to the equivalent constrained intrinsics. You could use that to do testing and find out what's broken.


-----Original Message-----
From: Kevin P. Neal [mailto:kpn at neutralgood.org] 
Sent: Monday, January 08, 2018 6:41 AM
To: Hal Finkel via cfe-dev <cfe-dev at lists.llvm.org>
Cc: Richard Smith <richard at metafoo.co.uk>; Kaylor, Andrew <andrew.kaylor at intel.com>; Marcus Johnson <bumblebritches57 at gmail.com>; wei.ding2 at amd.com; Bob Huemmer <bob.huemmer at sas.com>
Subject: Re: [cfe-dev] Why is #pragma STDC FENV_ACCESS not supported?

On Thu, Aug 31, 2017 at 05:03:17PM -0500, Hal Finkel via cfe-dev wrote:
>    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

What's the current status of this work? My employeer very much needs this work done sooner rather than later, and I've been tasked with helping make it happen.

What, exactly, still needs to be done to complete this work? I've seen some of the discussions about it, and I've seen the documentation on the new llvm constrained floating point intrinsics. But clang I don't think supports them yet, fptosi is not on the list anyway, and I'm not sure what else is needed. So I'm asking, what all is needed and what can I work on to move this forward?

Is there any work in progress code that anyone would be willing to share?
For example, any code using the new intrinsics? Andy?

The specific case we're running into today is that we have code being reordered in ways that trigger traps when handling a NaN. This code:

#include <math.h>

int foo(double d) {
   int x = (!isnan(d) ? (int)d : 45);
   return x;

... becomes this:

define signext i32 @foo(double) local_unnamed_addr #0 !dbg !10 {
  tail call void @llvm.dbg.value(metadata double %0, i64 0, metadata !15, metadata !17), !dbg !18
  %2 = tail call signext i32 @__isnan(double %0) #3, !dbg !19
  %3 = icmp eq i32 %2, 0, !dbg !19
  %4 = fptosi double %0 to i32, !dbg !20
  %5 = select i1 %3, i32 %4, i32 45, !dbg !19
  tail call void @llvm.dbg.value(metadata i32 %5, i64 0, metadata !16, metadata !17), !dbg !21
  ret i32 %5, !dbg !22

So the fptosi gets moved _above_ the select and the trap happens. This in code that was written to avoid a trap in exactly this case.

We're compiling with clang 5.0.0 "-g -O1" targeting SystemZ.
Kevin P. Neal                                http://www.pobox.com/~kpn/
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