[PATCH 5/8] math: Fix potential underflow on ldbl-128 erfl

[Adhemerval Zanella]

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.
---
 sysdeps/ieee754/ldbl-128/s_erfl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sysdeps/ieee754/ldbl-128/s_erfl.c b/sysdeps/ieee754/ldbl-128/s_erfl.c
index 1cbedc249d..e52d88bde3 100644
--- 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
-- 
2.43.0