Fix powerpc software sqrt (bug 17964).

As Adhemerval noted in
<https://sourceware.org/ml/libc-alpha/2015-01/msg00451.html>, the
powerpc sqrt implementation for when _ARCH_PPCSQ is not defined is
inaccurate in some cases.

The problem is that this code relies on fused multiply-add, and relies
on the compiler contracting a * b + c to get a fused operation.  But
sysdeps/ieee754/dbl-64/Makefile disables contraction for e_sqrt.c,
because the implementation in that directory relies on *not* having
contracted operations.

While it would be possible to arrange makefiles so that an earlier
sysdeps directory can disable the setting in
sysdeps/ieee754/dbl-64/Makefile, it seems a lot cleaner to make the
dependence on fused operations explicit in the .c file.  GCC 4.6
introduced support for __builtin_fma on powerpc and other
architectures with such instructions, so we can rely on that; this
patch duly makes the code use __builtin_fma for all such fused
operations.

Tested for powerpc32 (hard float).

2015-02-12  Joseph Myers  <joseph@codesourcery.com>

	[BZ #17964]
	* sysdeps/powerpc/fpu/e_sqrt.c (__slow_ieee754_sqrt): Use
	__builtin_fma instead of relying on contraction of a * b + c.

diff --git a/ChangeLog b/ChangeLog
index ce62001..87a9f58 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2015-02-12  Joseph Myers  <joseph@codesourcery.com>
+
+       [BZ #17964]
+       * sysdeps/powerpc/fpu/e_sqrt.c (__slow_ieee754_sqrt): Use
+       __builtin_fma instead of relying on contraction of a * b + c.
+
 2015-02-12  Roland McGrath  <roland@hack.frob.com>
 
        * Makeconfig (ASFLAGS): Add -Werror=undef.

diff --git a/NEWS b/NEWS
index 37e17ad..11ca36b 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -9,7 +9,7 @@ Version 2.22
 
 * The following bugs are resolved with this release:
 
-  4719, 15467, 15790, 16560, 17912, 17932, 17944, 17949, 17965.
+  4719, 15467, 15790, 16560, 17912, 17932, 17944, 17949, 17964, 17965.
 \f
 Version 2.21
 

diff --git a/sysdeps/powerpc/fpu/e_sqrt.c b/sysdeps/powerpc/fpu/e_sqrt.c
index 0934faa..9b55ef8 100644 (file)
--- a/sysdeps/powerpc/fpu/e_sqrt.c
+++ b/sysdeps/powerpc/fpu/e_sqrt.c
@@ -99,38 +99,41 @@ __slow_ieee754_sqrt (double x)
          /* Here we have three Newton-Raphson iterations each of a
             division and a square root and the remainder of the
             argument reduction, all interleaved.   */
-         sd = -(sg * sg - sx);
+         sd = -__builtin_fma (sg, sg, -sx);
          fsgi = (xi0 + 0x40000000) >> 1 & 0x7ff00000;
          sy2 = sy + sy;
-         sg = sy * sd + sg;    /* 16-bit approximation to sqrt(sx). */
+         sg = __builtin_fma (sy, sd, sg);      /* 16-bit approximation to
+                                                  sqrt(sx). */
 
          /* schedule the INSERT_WORDS (fsg, fsgi, 0) to get separation
             between the store and the load.  */
          INSERT_WORDS (fsg, fsgi, 0);
          iw_u.parts.msw = fsgi;
          iw_u.parts.lsw = (0);
-         e = -(sy * sg - almost_half);
-         sd = -(sg * sg - sx);
+         e = -__builtin_fma (sy, sg, -almost_half);
+         sd = -__builtin_fma (sg, sg, -sx);
          if ((xi0 & 0x7ff00000) == 0)
            goto denorm;
-         sy = sy + e * sy2;
-         sg = sg + sy * sd;    /* 32-bit approximation to sqrt(sx).  */
+         sy = __builtin_fma (e, sy2, sy);
+         sg = __builtin_fma (sy, sd, sg);      /* 32-bit approximation to
+                                                  sqrt(sx).  */
          sy2 = sy + sy;
          /* complete the INSERT_WORDS (fsg, fsgi, 0) operation.  */
          fsg = iw_u.value;
-         e = -(sy * sg - almost_half);
-         sd = -(sg * sg - sx);
-         sy = sy + e * sy2;
+         e = -__builtin_fma (sy, sg, -almost_half);
+         sd = -__builtin_fma (sg, sg, -sx);
+         sy = __builtin_fma (e, sy2, sy);
          shx = sx * fsg;
-         sg = sg + sy * sd;    /* 64-bit approximation to sqrt(sx),
-                                  but perhaps rounded incorrectly.  */
+         sg = __builtin_fma (sy, sd, sg);      /* 64-bit approximation to
+                                                  sqrt(sx), but perhaps
+                                                  rounded incorrectly.  */
          sy2 = sy + sy;
          g = sg * fsg;
-         e = -(sy * sg - almost_half);
-         d = -(g * sg - shx);
-         sy = sy + e * sy2;
+         e = -__builtin_fma (sy, sg, -almost_half);
+         d = -__builtin_fma (g, sg, -shx);
+         sy = __builtin_fma (e, sy2, sy);
          fesetenv_register (fe);
-         return g + sy * d;
+         return __builtin_fma (sy, d, g);
        denorm:
          /* For denormalised numbers, we normalise, calculate the
             square root, and return an adjusted result.  */