[PATCH] math: Add optimization barrier to ensure a1 + u.d is not reused [BZ #30664]

[John David Anglin]

This patch fixes the test fma fails seen on hppa with gcc-13 and
later.

There may be more instances of this problem still lurking in
sysdeps/ieee754/dbl-64/s_fma.c and other similar files.  Whether
they occur in practice is very hardware dependent.  I'm not able
to test changes to any other files on hppa.

Okay?

Dave
---

math: Add optimization barrier to ensure a1 + u.d is not reused [BZ #30664]

A number of fma tests started to fail on hppa when gcc was changed to
use Ranger rather than EVRP.  Eventually I found that the value of
a1 + u.d in this is block of code was being computed in FE_TOWARDZERO
mode and not the original rounding mode:

    if (TININESS_AFTER_ROUNDING)
      {
        w.d = a1 + u.d;
        if (w.ieee.exponent == 109)
          return w.d * 0x1p-108;
      }

This caused the exponent value to be wrong and the wrong return path
to be used.

Here we add an optimization barrier after the rounding mode is reset
to ensure that the previous value of a1 + u.d is not reused.

Signed-off-by: John David Anglin <dave.anglin@bell.net>

diff --git a/sysdeps/ieee754/dbl-64/s_fma.c b/sysdeps/ieee754/dbl-64/s_fma.c
index 20f617b996..42351c6b34 100644
--- a/sysdeps/ieee754/dbl-64/s_fma.c
+++ b/sysdeps/ieee754/dbl-64/s_fma.c
@@ -244,6 +244,9 @@ __fma (double x, double y, double z)
   /* Reset rounding mode and test for inexact simultaneously.  */
   int j = libc_feupdateenv_test (&env, FE_INEXACT) != 0;
 
+  /* Ensure value of a1 + u.d is not reused.  */
+  a1 = math_opt_barrier (a1);
+
   if (__glibc_likely (adjust == 0))
     {
       if ((u.ieee.mantissa1 & 1) == 0 && u.ieee.exponent != 0x7ff)
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