[llvm-dev] default behavior or

[Cameron McInally via llvm-dev]

On Wed, May 27, 2020 at 1:36 AM kamlesh kumar via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi Devs,
> going by this link https://llvm.org/docs/LangRef.html#floatenv
> it says that floating point operation does not have side effects by defaults.
> but when compile a test case
> i.e.
> cat a.c
> float foo(float a, float b)
> {
> return a+b;
> }
>
> $clang a.c -O2 -S -emit-llvm
> emit ir like:
> $cat a.ll
> ---------------------------------------
> ; ModuleID = 'a.c'
> source_filename = "a.c"
> target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
>
> ; Function Attrs: norecurse nounwind readnone uwtable
> define dso_local float @foo(float %a, float %b) local_unnamed_addr #0 {
> entry:
>   %add = fadd float %a, %b
>   ret float %add
> }
>
> attributes #0 = { norecurse nounwind readnone uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="none" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
> ---------------------
> and the setting "no-trapping-math"="false" should mean -ftrapping-math but it does not and it also contradict with documentation.
>
> I think clang should emit this "no-trapping-math"="true" by default if it goes with this assumption  https://llvm.org/docs/LangRef.html#floatenv ?
> what is default behavior of clang -fno-trapping-math or -ftrapping-math?
> also clang default behavior does not match gcc?
> https://godbolt.org/z/B3YvxE

Hi Kamlesh,

I haven't looked in a few months, but IINM, -trapping-math is a no-op
in LLVM. But yes, what you found is confusing and should be sorted
out.

The front end currently generates experimental constrained intrinsics
if -ftrapping-math is present on the command line. If you want to
generate trap-safe code, you could try that option or
-ffp-model/-ffp-exception-behavior depending on your needs.

https://clang.llvm.org/docs/UsersManual.html#cmdoption-ffp-model

-Cam