[PATCH, RFC] fma vs gcc 4.9

[Richard Henderson]

GCC 4.9 optimizes

  double a1 = z + m1;
  double t1 = a1 - z;
  double t2 = a1 - t1;
  t1 = m1 - t1;
  t2 = z - t2;
  double a2 = t1 + t2;
  feclearexcept (FE_INEXACT);

  if (a1 == 0 && m2 == 0)
    ... return ...

into

  double a1 = z + m1;
  feclearexcept (FE_INEXACT);

  if (a1 == 0 && m2 == 0)
    ... return ...

  double t1 = a1 - z;
  double t2 = a1 - t1;
  t1 = m1 - t1;
  t2 = z - t2;
  double a2 = t1 + t2;

because the later computation is partially redundant for the path leading to
the return.  I noticed this because, of course, the moved computation raises
INEXACT, leading to incorrect results.

At first I was simply going to add a math_force_eval to make sure everything is
complete before the feclearexcept, but on further reflection it seemed odd that
most of the computation is in fact redundant.

It seems to me that there's a typo on that exact zero test: a2 should be used,
not m2.  Correct, or have I mis-read the code?


r~
-------------- next part --------------
diff --git a/ChangeLog b/ChangeLog
index 5eb0d43..4a3e672 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2014-07-03  Richard Henderson  <rth@redhat.com>
 
+	* sysdeps/ieee754/dbl-64/s_fma.c (__fma): Use math_force_eval before
+	feclearexcept; use math_opt_barrier instead of open-coded asm; fix
+	typo in exact zero test.
+
 	* sysdeps/alpha/fpu/s_nearbyintf.c: Remove file.
 	* sysdeps/alpha/fpu/s_nearbyint.c (__nearbyint): Remove;
 	include sysdeps/ieee754/dbl-64/wordsize-64/s_nearbyint.c.
diff --git a/sysdeps/ieee754/dbl-64/s_fma.c b/sysdeps/ieee754/dbl-64/s_fma.c
index 389acd4..3bd7f71 100644
--- a/sysdeps/ieee754/dbl-64/s_fma.c
+++ b/sysdeps/ieee754/dbl-64/s_fma.c
@@ -198,16 +198,16 @@ __fma (double x, double y, double z)
   t1 = m1 - t1;
   t2 = z - t2;
   double a2 = t1 + t2;
+  /* Ensure the addition is not scheduled after feclearexcept call.  */
+  math_force_eval (a2);
   feclearexcept (FE_INEXACT);
 
-  /* If the result is an exact zero, ensure it has the correct
-     sign.  */
-  if (a1 == 0 && m2 == 0)
+  /* If the result is an exact zero, ensure it has the correct sign.  */
+  if (a1 == 0 && a2 == 0)
     {
       libc_feupdateenv (&env);
-      /* Ensure that round-to-nearest value of z + m1 is not
-	 reused.  */
-      asm volatile ("" : "=m" (z) : "m" (z));
+      /* Ensure that round-to-nearest value of z + m1 is not reused.  */
+      z = math_opt_barrier (z);
       return z + m1;
     }