[llvm-dev] default behavior or (original) (raw)
kamlesh kumar via llvm-dev llvm-dev at lists.llvm.org
Tue May 26 22:36:04 PDT 2020
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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
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