[PATCH v2 4/5] math: Use math_opt_barrier on ldbl-128 powl underflow/overflow handling

[H.J. Lu]

On Sat, Dec 20, 2025 at 2:54 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 [1]:
>
> 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.
>
> [1] https://github.com/llvm/llvm-project/issues/173080
> ---
>  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);
>         }
>        /* over/underflow if x is not close to one */
>        if (ix < 0x3ffeffff)
> --
> 2.43.0
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.