[PATCH] propagate IR-level fast-math-flags to DAG nodes, disabled by default

[Sanjay Patel]

Thanks, Nick.

This is the same bug (convert repeated divisions by the same divisor to
multiplication by the reciprocal) that should be solved by D9893 (
http://reviews.llvm.org/D9893 ). After I apply that patch, the output asm
looks correct to me.

On Fri, Jun 19, 2015 at 5:53 PM, Nick Lewycky <nlewycky at google.com> wrote:

> On 17 June 2015 at 13:57, Sanjay Patel <spatel at rotateright.com> wrote:
>
>>
>>
>> On Wed, Jun 17, 2015 at 12:59 AM, Nick Lewycky <nicholas at mxc.ca> wrote:
>>
>>> Sanjay Patel wrote:
>>>
>>>> Please let me know if I can still take you up on this offer:
>>>> "Could you add FMF under a flag and then let me test it before turning
>>>> it on? I can test for compiler crashes in 24 hours, or for miscompiles over
>>>> a weekend."
>>>>
>>>
>>> Right! Yes, I can do that.
>>>
>>
>> Great! Let me know when you see bugs crawling out from under the rug. :)
>>
>
> Just finished the first creduce.
>
> Testcase:
>
> int a, b, c;
> void fn1() {
>   b = c / (1.0 + c);
>   a = 1 / (1.0 + c);
> }
>
> Build with "clang -O2 -mllvm -enable-fmf-dag -ffast-math" to get
>
>  fatal error: error in backend: Cannot select: 0x2a8a760: f64 =
>       ConstantFP<1.000000e+00> [ID=7]
> In function: _Z3fn1v
>
> and here it is as .ll:
>
> target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-grtev4-linux-gnu"
>
> @a = global i32 0, align 4
> @b = global i32 0, align 4
> @c = global i32 0, align 4
>
> ; Function Attrs: nounwind uwtable
> define void @_Z3fn1v() #0 {
>   %1 = load i32, i32* @c, align 4, !tbaa !1
>   %2 = sitofp i32 %1 to double
>   %3 = fadd fast double %2, 1.000000e+00
>   %4 = fdiv fast double %2, %3
>   %5 = fptosi double %4 to i32
>   store i32 %5, i32* @b, align 4, !tbaa !1
>   %6 = fdiv fast double 1.000000e+00, %3
>   %7 = fptosi double %6 to i32
>   store i32 %7, i32* @a, align 4, !tbaa !1
>   ret void
> }
>
> attributes #0 = { nounwind uwtable "disable-tail-calls"="false"
> "less-precise-fpmad"="false" "no-frame-pointer-elim"="false"
> "no-infs-fp-math"="true" "no-nans-fp-math"="true"
> "stack-protector-buffer-size"="8" "target-cpu"="x86-64"
> "target-features"="+sse,+sse2" "unsafe-fp-math"="true"
> "use-soft-float"="false" }
>
> !llvm.ident = !{!0}
>
> !0 = !{!"clang version google3-trunk (trunk r240125)"}
> !1 = !{!2, !2, i64 0}
> !2 = !{!"int", !3, i64 0}
> !3 = !{!"omnipotent char", !4, i64 0}
> !4 = !{!"Simple C/C++ TBAA"}
>
>
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