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

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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cced the author.

On Wed, May 27, 2020 at 11:41 AM David Blaikie <dblaikie at gmail.com>
wrote:
> 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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Here is problematic code:
https://github.com/llvm/llvm-project/blob/master/clang/lib/Driver/ToolChains/Clang.cpp#L2836
if `TrappingMath` is false it should set "-fno-trapping-math",but it
isn't
because there is another else if.

it should be changed to

  if (TrappingMath) {
    // FP Exception Behavior is also set to strict
    assert(FPExceptionBehavior.equals("strict"));
    CmdArgs.push_back("-ftrapping-math");
  } else
    CmdArgs.push_back("-fno-trapping-math");

any thoughts?

./kamlesh


On Wed, May 27, 2020 at 12:29 PM kamlesh kumar <kamleshbhalui at
gmail.com>
wrote:
> cced the author.
>
> On Wed, May 27, 2020 at 11:41 AM David Blaikie <dblaikie at
gmail.com> wrote:
>
>> 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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