llvm dev - May 2020 - By default clang does not emit trap insn

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

./kamlesh
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On Wed, May 27, 2020 at 11:06 AM kamlesh kumar <kamleshbhalui at
gmail.com>
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
>
> ./kamlesh
>
>
>
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I believe support for trapping math is experimental/work in progress:
https://reviews.llvm.org/D62731

On Tue, May 26, 2020 at 10:36 PM 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
>
> ./kamlesh
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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looks like experimental/work in progress support:
https://reviews.llvm.org/D62731

On Tue, May 26, 2020 at 10:39 PM kamlesh kumar via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
>
> On Wed, May 27, 2020 at 11:06 AM kamlesh kumar <kamleshbhalui at
gmail.com>
> 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
>>
>> ./kamlesh
>>
>>
>> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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