fmal: workaround for OpenBSD's gcc

[Bruno Haible]

On OpenBSD 5.1/SPARC, I'm seeing this test failure:

  test-fma2.h:93: assertion failed
  FAIL: test-fmal2

It turns out that gcc has miscompiled the rpl_fmal function in lib/fmal.c.
More precisely, this code

               fsum += (DOUBLE)
                       ((sum[sum_len - k - 1] << (GMP_LIMB_BITS - shift))
                        | (sum[sum_len - k - 2] >> shift));

in a situation where the parenthesized expression would evaluate to
2^32 with 64-bit computation, or therefore to 0 when truncated to
'unsigned int' (the sum[] array element type is 'unsigned int'), increments
fsum by 262144.0L. It happens even without optimization.

(gdb) print shift
$38 = 1
(gdb) print fsum
$39 = 0
(gdb) next
876                                 fsum += (DOUBLE)
(gdb) print ( sum[sum_len - k - 1] << 31 ) | (sum[sum_len - k - 2] >> 1)
$40 = 4294967296
(gdb) print (unsigned int)(( sum[sum_len - k - 1] << 31 ) | (sum[sum_len - k - 
2] >> 1))
$41 = 0
(gdb) next
873                             while (--k >= 0)
(gdb) print fsum
$42 = 262144


2012-03-15  Bruno Haible  <br...@clisp.org>

        fmal: Avoid test failure on OpenBSD 5.1/SPARC64.
        * lib/fma.c (VOLATILE): New macro.
        (FUNC): Use it to work around a GCC compiler bug.

--- lib/fma.c.orig      Thu Mar 15 12:09:33 2012
+++ lib/fma.c   Thu Mar 15 12:06:55 2012
@@ -72,6 +72,13 @@
 # pragma fenv_access (off)
 #endif
 
+/* Work around GCC 4.2.1 bug on OpenBSD 5.1/SPARC64.  */
+#if defined __GNUC__ && defined __sparc__
+# define VOLATILE volatile
+#else
+# define VOLATILE
+#endif
+
 /* It is possible to write an implementation of fused multiply-add with
    floating-point operations alone.  See
      Sylvie Boldo, Guillaume Melquiond:
@@ -866,16 +873,22 @@
                     else
                       {
                         /* First loop round.  */
-                        fsum = (DOUBLE)
-                               ((sum[sum_len - k - 1] << (GMP_LIMB_BITS - 
shift))
-                                | (sum_len >= k + 2 ? sum[sum_len - k - 2] >> 
shift : 0));
+                        {
+                          VOLATILE mp_limb_t chunk =
+                            (sum[sum_len - k - 1] << (GMP_LIMB_BITS - shift))
+                            | (sum_len >= k + 2 ? sum[sum_len - k - 2] >> 
shift : 0);
+                          fsum = (DOUBLE) chunk;
+                        }
                         /* General loop.  */
                         while (--k >= 0)
                           {
                             fsum *= chunk_multiplier;
-                            fsum += (DOUBLE)
-                                    ((sum[sum_len - k - 1] << (GMP_LIMB_BITS - 
shift))
-                                     | (sum[sum_len - k - 2] >> shift));
+                            {
+                              VOLATILE mp_limb_t chunk =
+                                (sum[sum_len - k - 1] << (GMP_LIMB_BITS - 
shift))
+                                | (sum[sum_len - k - 2] >> shift);
+                              fsum += (DOUBLE) chunk;
+                            }
                           }
                       }
                   }