[PATCH 8/8] math: Use math_opt_barrier on underflow/overflow

[Adhemerval Zanella Netto]

On 17/12/25 21:05, H.J. Lu wrote:
> On Thu, Dec 18, 2025 at 1:56 AM Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>>
>> This avoids compiler to move the operation before the 'iy' compare.
>>
>> It fixes math/test-float128-pow regreesions when building with clang:
>>
>> Failure: pow (-0x1.000002p+0, 0xf.ffffffffffff8p+1020): Exception "Underflow" set
>> Failure: pow (-0x1.000002p+0, 0xf.ffffffffffffbffffffffffffcp+1020): Exception "Underflow" set
>> Failure: pow (-0x1.000002p+0, 0xf.fffffffffffffffffffffffffff8p+16380): Exception "Underflow" set
>> Failure: pow (-0x1.000002p+0, 0xf.fffffffffffffffp+16380): Exception "Underflow" set
>> Failure: pow (-0x1.000002p+0, 0xf.fffffp+124): Exception "Underflow" set
>> Failure: pow (-0x1.00000ep+0, 0xf.ffffffffffff8p+1020): Exception "Underflow" set
>> Failure: pow (-0x1.00000ep+0, 0xf.ffffffffffffbffffffffffffcp+1020): Exception "Underflow" set
>> Failure: pow (-0x1.00000ep+0, 0xf.fffffffffffffffffffffffffff8p+16380): Exception "Underflow" set
>> Failure: pow (-0x1.00000ep+0, 0xf.fffffffffffffffp+16380): Exception "Underflow" set
>> Failure: pow (-0x2p+0, -0xf.ffffffffffff8p+1020): Exception "Overflow" set
>> Failure: pow (-0x2p+0, -0xf.ffffffffffffbffffffffffffcp+1020): Exception "Overflow" set
>> Failure: pow (-0x2p+0, -0xf.fffffffffffffffffffffffffff8p+16380): Exception "Overflow" set
>> Failure: pow (-0x2p+0, -0xf.fffffffffffffffp+16380): Exception "Overflow" set
>> Failure: pow (-0x2p+0, -0xf.fffffp+124): Exception "Overflow" set
>> Failure: pow (-0x2p+0, 0xf.ffffffffffff8p+1020): Exception "Underflow" set
>> Failure: pow (-0x2p+0, 0xf.ffffffffffffbffffffffffffcp+1020): Exception "Underflow" set
>> Failure: pow (-0x2p+0, 0xf.fffffffffffffffffffffffffff8p+16380): Exception "Underflow" set
>> Failure: pow (-0x2p+0, 0xf.fffffffffffffffp+16380): Exception "Underflow" set
>> Failure: pow (-0x2p+0, 0xf.fffffp+124): Exception "Underflow" set
>> [...]
>>
>> Checked on x86_64-linux-gnu and aarch64-linux-gnu with gcc-15 and
>> clang-18.
>> ---
>>  sysdeps/ieee754/ldbl-128/e_powl.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/sysdeps/ieee754/ldbl-128/e_powl.c b/sysdeps/ieee754/ldbl-128/e_powl.c
>> index 4e20616705..9ff5a43573 100644
>> --- a/sysdeps/ieee754/ldbl-128/e_powl.c
>> +++ b/sysdeps/ieee754/ldbl-128/e_powl.c
>> @@ -279,9 +279,11 @@ __ieee754_powl (_Float128 x, _Float128 y)
>>        if (iy > 0x407d654b)
>>         {
>>           if (ix <= 0x3ffeffff)
>> -           return (hy < 0) ? huge * huge : tiny * tiny;
>> +           return (hy < 0) ? math_opt_barrier (huge * huge)
>> +             : math_opt_barrier (tiny * tiny);
>>           if (ix >= 0x3fff0000)
>> -           return (hy > 0) ? huge * huge : tiny * tiny;
>> +           return (hy > 0) ? math_opt_barrier (huge * huge)
>> +             : math_opt_barrier (tiny * tiny);
>>         }
> 
> Does it generate less optimized codes for GCC?

I do not think this really matter here, these are for corner cases where
the function underflow/overflow.  And I think this are really a issue
with clang with -frounding-math, since it can not assume that the 
multiplication will not generate underflow/overflow exceptions (so it can
move out the comparison).

In any case, I think this is a quite common technique that we use on other
math implementations to either force overflow/underflow exception or to avoid
some compiler issues (like 2fe5e2af0995a6e6ee2c761e55e7596a3220d07c). 

> 
>>        /* over/underflow if x is not close to one */
>>        if (ix < 0x3ffeffff)
>> --
>> 2.43.0
>>
> 
>