[PATCH v2] math: Fix x86_64 build for -Os (BZ 33367)

[H.J. Lu]

On Tue, Sep 9, 2025 at 5:21 AM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 09/09/25 01:11, H.J. Lu wrote:
> > On Mon, Sep 8, 2025 at 1:47 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >>
> >> On Mon, Sep 8, 2025 at 1:11 PM Adhemerval Zanella Netto
> >> <adhemerval.zanella@linaro.org> wrote:
> >>>
> >>>
> >>>
> >>> On 08/09/25 17:09, H.J. Lu wrote:
> >>>> On Mon, Sep 8, 2025 at 12:56 PM Adhemerval Zanella
> >>>> <adhemerval.zanella@linaro.org> wrote:
> >>>>>
> >>>>> The compiler might not inline the trunc function call for
> >>>>> USE_TRUNC_BUILTIN [1].
> >>>>>
> >>>>> This patch adds an optimized __trunc/__truncf for x86 used
> >>>>> on modf ifunc variant to avoid the trunc libcall.
> >>>>>
> >>>>> Checked on x86_64, x86_64-v2, x86_64-v3, and x86_64-v4. Used -O2 and
> >>>>> -Os options. Performed a full make check on x86_64 with both
> >>>>>  optimizations.
> >>>>>
> >>>>> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121861
> >>>>> ---
> >>>>>  sysdeps/x86/fpu/math_private.h                | 27 +++++++++++++++++++
> >>>>>  sysdeps/x86_64/fpu/multiarch/s_modf-avx.c     |  4 +++
> >>>>>  sysdeps/x86_64/fpu/multiarch/s_modf-sse4_1.c  |  4 +++
> >>>>>  sysdeps/x86_64/fpu/multiarch/s_modf.c         |  3 ++-
> >>>>>  sysdeps/x86_64/fpu/multiarch/s_modff-avx.c    |  4 +++
> >>>>>  sysdeps/x86_64/fpu/multiarch/s_modff-sse4_1.c |  4 +++
> >>>>>  sysdeps/x86_64/fpu/multiarch/s_modff.c        |  3 ++-
> >>>>>  7 files changed, 47 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/sysdeps/x86/fpu/math_private.h b/sysdeps/x86/fpu/math_private.h
> >>>>> index 132f011809..d30d580cea 100644
> >>>>> --- a/sysdeps/x86/fpu/math_private.h
> >>>>> +++ b/sysdeps/x86/fpu/math_private.h
> >>>>> @@ -19,6 +19,7 @@
> >>>>>  #ifndef X86_MATH_PRIVATE_H
> >>>>>  #define X86_MATH_PRIVATE_H 1
> >>>>>
> >>>>> +#include <math.h>
> >>>>>  #include_next <math_private.h>
> >>>>>
> >>>>>  __extern_always_inline long double
> >>>>> @@ -29,4 +30,30 @@ __NTH (__ieee754_atan2l (long double y, long double x))
> >>>>>    return ret;
> >>>>>  }
> >>>>>
> >>>>> +__extern_always_inline double
> >>>>> +__trunc (double x)
> >>>>> +{
> >>>>> +#ifdef __AVX__
> >>>>> +  asm ("vroundsd $11, %1, %1, %0" : "=v" (x) : "v" (x));
> >>>>> +#elif defined __SSE4_1__
> >>>>> +  asm ("roundsd $11, %1, %0" : "=x" (x) : "x" (x));
> >>>>> +#else
> >>>>> +  x = trunc (x);
> >>>>> +#endif
> >>>>> +  return x;
> >>>>> +}
> >>>>
> >>>> I will fix GCC 16.   Please make sure that inline asm isn't used for
> >>>> GCC 16 or above.
> >>>
> >>> Would it make a lot of difference on modf implementation to add this extra
> >>> switch?
> >>
> >> I think an extra
> >>
> >> #if GCC_COMPAT(16, 1)
> >>
> >> is simple enough.  But it can wait.
> >
> > GCC 16 is fixed.  But -fno-trapping-math doesn't work well due to
> >
> > https://sourceware.org/bugzilla/show_bug.cgi?id=33405
> >
> > since -fsignaling-nans takes precedence over -fno-trapping-math.
> >
> > We should add a configure test to check if the GCC bug is fixed.
>
> I am not sure which direction to use, both your patch enable -fno-trapping-math [1]
> or this one works.  Your changes adds some generic Makefile rules, and the extra
> use-builtins-trunc* which eventually we will need to remove since it is already
> fixed upstream; the inline version at least is more clear of what tried to
> accomplish and do not mess with the math flags.

Your patch works better.  But it needs some adjustments as I have suggested.

-- 
H.J.