[X86][clang] 128bit floating-point operations in x86 machines

[CoTinker]

testcase:

#include <fenv.h>
#include <stdlib.h>
#include <stdio.h>

int main (void)
{
  volatile __float128 a = 0x0.fffp-16382q, b = 0x0.fffp0q, c;
  c = a / b;
  if (fetestexcept (FE_UNDERFLOW | FE_INEXACT))
    printf("Fail1\n");
  if (c != 0x1p-16382q)
    printf("Fail2\n");
  exit (0);
}

compile command:

% clang test.c -o test -lm -m32
% ./test
Fail1
Fail2

compile in x86_64 will return normally, but x86 output Fail.
https://godbolt.org/z/vhdsMhd8d

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Longsheng Mou (CoTinker)

testcase:

#include <fenv.h>
#include <stdlib.h>
#include <stdio.h>

int main (void)
{
  volatile __float128 a = 0x0.fffp-16382q, b = 0x0.fffp0q, c;
  c = a / b;
  if (fetestexcept (FE_UNDERFLOW | FE_INEXACT))
    printf("Fail1\n");
  if (c != 0x1p-16382q)
    printf("Fail2\n");
  exit (0);
}

compile command:

% clang test.c -o test -lm -m32
% ./test
Fail1
Fail2

compile in x86_64 will return normally, but x86 output Fail.
https://godbolt.org/z/vhdsMhd8d

[arsenm]

Technically you need to enable FENV_ACCESS for this program

[CoTinker]

Technically you need to enable FENV_ACCESS for this program

#pragma STDC FENV_ACCESS ON
It seems not work.

[CoTinker]

CC @RKSimon @phoebewang

[hstk30-hw]

@arsenm Hi, enable FENV_ACCESS still fail. Does FENV_ACCESS supported in m32 ?
https://godbolt.org/z/MsYTe1cPc

[hstk30-hw]

CC @brad0

[hstk30-hw]

https://godbolt.org/z/4KbaWcvsG

I see the asm, clang x86 with m32 abi is not compatible with libgcc.

[hstk30-hw]

CC @hvdijk

[arsenm]

@arsenm Hi, enable FENV_ACCESS still fail. Does FENV_ACCESS supported in m32 ? https://godbolt.org/z/MsYTe1cPc

I think it should work the same as x64

[hstk30-hw]

In fact, not work! I guess float128 (long double) not align to 16 in x86-32.
I'm reading the x86 isel dag->dag pass, maybe lowering mir gets mistake in the step.

[hstk30-hw]

Do you have any idea about this problem?
It seems a legacy.
https://lists.llvm.org/pipermail/llvm-dev/2016-December/108033.html
Seems no body care about x86-32? @arsenm