Re: [PATCH v2 03/10] i386: Use generic fmod

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Mar 28, 2024 at 8:57 AM Adhemerval Zanella Netto
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 28/03/24 12:55, H.J. Lu wrote:
> > On Thu, Mar 28, 2024 at 8:48 AM Adhemerval Zanella Netto
> > <adhemerval.zanella@linaro.org> wrote:
> >>
> >>
> >>
> >> On 28/03/24 12:42, H.J. Lu wrote:
> >>> On Thu, Mar 28, 2024 at 8:14 AM Adhemerval Zanella Netto
> >>> <adhemerval.zanella@linaro.org> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 28/03/24 11:51, H.J. Lu wrote:
> >>>>> On Thu, Mar 28, 2024 at 7:11 AM Adhemerval Zanella Netto
> >>>>> <adhemerval.zanella@linaro.org> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 27/03/24 18:38, H.J. Lu wrote:
> >>>>>>> On Wed, Mar 27, 2024 at 1:37 PM Adhemerval Zanella Netto
> >>>>>>> <adhemerval.zanella@linaro.org> wrote:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> On 27/03/24 16:55, H.J. Lu wrote:
> >>>>>>>>> On Wed, Mar 27, 2024 at 12:40 PM Adhemerval Zanella
> >>>>>>>>> <adhemerval.zanella@linaro.org> wrote:
> >>>>>>>>>>
> >>>>>>>>>> The benchtest results shows a slight improvement (Ryzen 5900, gcc
> >>>>>>>>>> 13.2.1):
> >>>>>>>>>>
> >>>>>>>>>> * sysdeps/i386/fpu/e_fmod.S:
> >>>>>>>>>>   "fmod": {
> >>>>>>>>>>    "subnormals": {
> >>>>>>>>>>     "duration": 3.68855e+09,
> >>>>>>>>>>     "iterations": 2.12608e+08,
> >>>>>>>>>>     "max": 62.012,
> >>>>>>>>>>     "min": 16.798,
> >>>>>>>>>>     "mean": 17.349
> >>>>>>>>>>    },
> >>>>>>>>>>    "normal": {
> >>>>>>>>>>     "duration": 3.88459e+09,
> >>>>>>>>>>     "iterations": 7.168e+06,
> >>>>>>>>>>     "max": 2879.12,
> >>>>>>>>>>     "min": 16.909,
> >>>>>>>>>>     "mean": 541.934
> >>>>>>>>>>    },
> >>>>>>>>>>    "close-exponents": {
> >>>>>>>>>>     "duration": 3.692e+09,
> >>>>>>>>>>     "iterations": 1.96608e+08,
> >>>>>>>>>>     "max": 66.452,
> >>>>>>>>>>     "min": 16.835,
> >>>>>>>>>>     "mean": 18.7785
> >>>>>>>>>>    }
> >>>>>>>>>>   }
> >>>>>>>>>>
> >>>>>>>>>> * generic
> >>>>>>>>>>   "fmod": {
> >>>>>>>>>>    "subnormals": {
> >>>>>>>>>>     "duration": 3.68645e+09,
> >>>>>>>>>>     "iterations": 2.2848e+08,
> >>>>>>>>>>     "max": 66.896,
> >>>>>>>>>>     "min": 15.91,
> >>>>>>>>>>     "mean": 16.1347
> >>>>>>>>>>    },
> >>>>>>>>>>    "normal": {
> >>>>>>>>>>     "duration": 4.1455e+09,
> >>>>>>>>>>     "iterations": 8.192e+06,
> >>>>>>>>>>     "max": 3376.18,
> >>>>>>>>>>     "min": 15.873,
> >>>>>>>>>>     "mean": 506.043
> >>>>>>>>>>    },
> >>>>>>>>>>    "close-exponents": {
> >>>>>>>>>>     "duration": 3.70197e+09,
> >>>>>>>>>>     "iterations": 2.08896e+08,
> >>>>>>>>>>     "max": 69.597,
> >>>>>>>>>>     "min": 15.947,
> >>>>>>>>>>     "mean": 17.7216
> >>>>>>>>>>    }
> >>>>>>>>>>   }
> >>>>>>>>>> ---
> >>>>>>>>>>  sysdeps/i386/fpu/Versions                 |  4 ++++
> >>>>>>>>>>  sysdeps/i386/fpu/e_fmod.S                 | 18 ------------------
> >>>>>>>>>>  sysdeps/i386/fpu/e_fmod.c                 |  2 ++
> >>>>>>>>>>  sysdeps/i386/fpu/math_err.c               |  1 -
> >>>>>>>>>>  sysdeps/i386/fpu/w_fmod_compat.c          | 15 ---------------
> >>>>>>>>>>  sysdeps/ieee754/dbl-64/e_fmod.c           |  5 ++++-
> >>>>>>>>>>  sysdeps/mach/hurd/i386/libm.abilist       |  1 +
> >>>>>>>>>>  sysdeps/unix/sysv/linux/i386/libm.abilist |  1 +
> >>>>>>>>>>  8 files changed, 12 insertions(+), 35 deletions(-)
> >>>>>>>>>>  delete mode 100644 sysdeps/i386/fpu/e_fmod.S
> >>>>>>>>>>  create mode 100644 sysdeps/i386/fpu/e_fmod.c
> >>>>>>>>>>  delete mode 100644 sysdeps/i386/fpu/math_err.c
> >>>>>>>>>>  delete mode 100644 sysdeps/i386/fpu/w_fmod_compat.c
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/sysdeps/i386/fpu/Versions b/sysdeps/i386/fpu/Versions
> >>>>>>>>>> index a2eec371f1..d37bc1eae6 100644
> >>>>>>>>>> --- a/sysdeps/i386/fpu/Versions
> >>>>>>>>>> +++ b/sysdeps/i386/fpu/Versions
> >>>>>>>>>> @@ -3,4 +3,8 @@ libm {
> >>>>>>>>>>      # functions used in inline functions or macros
> >>>>>>>>>>      __expl; __expm1l;
> >>>>>>>>>>    }
> >>>>>>>>>> +  GLIBC_2.40 {
> >>>>>>>>>> +    # No SVID compatible error handling.
> >>>>>>>>>> +    fmod;
> >>>>>>>>>> +  }
> >>>>>>>>>
> >>>>>>>>> This changes the ABI.  I assume that it fixes a real bug.   Is there a bug
> >>>>>>>>> report open for this?
> >>>>>>>>>
> >>>>>>>>
> >>>>>>>> The new version is the way to provide the system without the SVID compat
> >>>>>>>> support, which we for all ABIs but i386 on 2.38. For instance:
> >>>>>>>>
> >>>>>>>> find . -iname libm.abilist | xargs grep -w fmod
> >>>>>>>> ./sysdeps/unix/sysv/linux/sparc/sparc32/libm.abilist:GLIBC_2.0 fmod F
> >>>>>>>> ./sysdeps/unix/sysv/linux/sparc/sparc32/libm.abilist:GLIBC_2.38 fmod F
> >>>>>>>> [...]
> >>>>>>>>
> >>>>>>>> For i386 specifically, the old SVID symbol will be kept as fmod@GLIBC_2.0.
> >>>>>>>>
> >>>>>>>
> >>>>>>> Does it fix a run-time test which fails without the fix?
> >>>>>>>
> >>>>>>
> >>>>>> Not really, but it is one less assembly implementation in favor a generic one
> >>>>>> (which also shows a slight improvement on recent chips) and it sync i386
> >>>>>> with generic code (so less possible issues, such as the static lib in this
> >>>>>> patchset).
> >>>>>
> >>>>> Why do we need a new symbol?
> >>>>
> >>>> Because the new fmod@GLIBC_2.40 for i386 won't have the SVID handling,
> >>>> similar to what has been done for other architectures with
> >>>> 16439f419b270184ec501c531bf20d83b6745fb0;
> >>>
> >>> Does it change i386 fmod behavior? If yes, we need a testcase to verify it.
> >>> If not, why is it needed?
> >>>
> >>
> >> It is not strictly required, but it makes i386 has one less assembly optimization
> >> that do not follow the rest of the code and it optimizes it slight because. Since
> >> we do actually have check for SVID, the default math tests already check the
> >> required symbol semantic.
> >
> > fmod@GLIBC_2.40 is added because of the SVID handling.  But there is no
> > user visible behavior change.  Is this correct?
>
> The user visible is the missing SVID handling (which I think noone actually uses
> it).  That's the main reason we need the compat dance and this extra complexity.
> Maybe one day we just can drop this for good...

If we want to provide the SVID compatibility, 2 testcases are needed:

1.  A testcase to show that the new implementation is incompatible with SVID.
2.  A testcase to show that the compat symbol provides the SVID compatibility.


-- 
H.J.