math: Fix potential underflow on ldbl-128 erfl

The multiplication operation is required only if the branch is taken,
and the compiler might not optimize it away.

It fixes the following issues when the testcase is built with clang:

FAIL: math/test-ldouble-erf
Failure: erf (-0x4p-16384): Exception "Underflow" set
Failure: erf (0x1p-10000): Exception "Underflow" set
Failure: erf (0x3.8b7f12369ded54f38760a41abb84p-16384): Exception "Underflow" set
Failure: erf (0x3.8b7f12369ded54f38760a41abb88p-16384): Exception "Underflow" set
Failure: erf (0x3.8b7f12369ded54f4p-16384): Exception "Underflow" set
Failure: erf (0x3.8b7f12369ded54f8p-16384): Exception "Underflow" set
Failure: erf (0x3.8b7f12369ded5518p-16384): Exception "Underflow" set
Failure: erf (0x3.8b7f12369ded551cp-16384): Exception "Underflow" set
Failure: erf (0x3.8b7f12369ded552p-16384): Exception "Underflow" set
Failure: erf (0x4p-16384): Exception "Underflow" set
Failure: erf_downward (-0x4p-16384): Exception "Underflow" set
Failure: erf_downward (0x1p-10000): Exception "Underflow" set
Failure: erf_downward (0x3.8b7f12369ded54f38760a41abb84p-16384): Exception "Underflow" set
Failure: erf_downward (0x3.8b7f12369ded54f38760a41abb88p-16384): Exception "Underflow" set
Failure: erf_downward (0x3.8b7f12369ded54f4p-16384): Exception "Underflow" set
Failure: erf_downward (0x3.8b7f12369ded54f8p-16384): Exception "Underflow" set
Failure: erf_downward (0x3.8b7f12369ded5518p-16384): Exception "Underflow" set
Failure: erf_downward (0x3.8b7f12369ded551cp-16384): Exception "Underflow" set
Failure: erf_downward (0x3.8b7f12369ded552p-16384): Exception "Underflow" set
Failure: erf_downward (0x4p-16384): Exception "Underflow" set
Failure: erf_towardzero (-0x4p-16384): Exception "Underflow" set
Failure: erf_towardzero (0x1p-10000): Exception "Underflow" set
Failure: erf_towardzero (0x3.8b7f12369ded54f38760a41abb84p-16384): Exception "Underflow" set
Failure: erf_towardzero (0x3.8b7f12369ded54f38760a41abb88p-16384): Exception "Underflow" set
Failure: erf_towardzero (0x3.8b7f12369ded54f4p-16384): Exception "Underflow" set
Failure: erf_towardzero (0x3.8b7f12369ded54f8p-16384): Exception "Underflow" set
Failure: erf_towardzero (0x3.8b7f12369ded5518p-16384): Exception "Underflow" set
Failure: erf_towardzero (0x3.8b7f12369ded551cp-16384): Exception "Underflow" set
Failure: erf_towardzero (0x3.8b7f12369ded552p-16384): Exception "Underflow" set
Failure: erf_towardzero (0x4p-16384): Exception "Underflow" set
Failure: erf_upward (-0x4p-16384): Exception "Underflow" set
Failure: erf_upward (0x1p-10000): Exception "Underflow" set
Failure: erf_upward (0x3.8b7f12369ded54f38760a41abb84p-16384): Exception "Underflow" set
Failure: erf_upward (0x3.8b7f12369ded54f38760a41abb88p-16384): Exception "Underflow" set
Failure: erf_upward (0x3.8b7f12369ded54f4p-16384): Exception "Underflow" set
Failure: erf_upward (0x3.8b7f12369ded54f8p-16384): Exception "Underflow" set
Failure: erf_upward (0x3.8b7f12369ded5518p-16384): Exception "Underflow" set
Failure: erf_upward (0x3.8b7f12369ded551cp-16384): Exception "Underflow" set
Failure: erf_upward (0x3.8b7f12369ded552p-16384): Exception "Underflow" set
Failure: erf_upward (0x4p-16384): Exception "Underflow" set

Checked on x86_64-linux-gnu and aarch64-linux-gnu with gcc-15 and
clang-18.

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

diff --git a/sysdeps/ieee754/ldbl-128/s_erfl.c b/sysdeps/ieee754/ldbl-128/s_erfl.c
index 1cbedc249d54ab01569259daa6ba865938f8b630..e52d88bde3dadbffc3535a35f290a5a6eef432c3 100644 (file)
--- a/sysdeps/ieee754/ldbl-128/s_erfl.c
+++ b/sysdeps/ieee754/ldbl-128/s_erfl.c
@@ -778,7 +778,6 @@ __erfl (_Float128 x)
     }
   u.parts32.w0 = ix;
   a = u.value;
-  z = x * x;
   if (ix < 0x3ffec000)  /* a < 0.875 */
     {
       if (ix < 0x3fc60000) /* |x|<2**-57 */
@@ -792,6 +791,7 @@ __erfl (_Float128 x)
            }
          return x + efx * x;
        }
+      z = x * x;
       y = a + a * neval (z, TN1, NTN1) / deval (z, TD1, NTD1);
     }
   else