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Fix powerpc software sqrtf (bug 17967) [committed]
- From: Joseph Myers <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Fri, 13 Feb 2015 16:21:28 +0000
- Subject: Fix powerpc software sqrtf (bug 17967) [committed]
- Authentication-results: sourceware.org; auth=none
Similarly to sqrt in
<https://sourceware.org/ml/libc-alpha/2015-02/msg00353.html>, the
powerpc sqrtf implementation for when _ARCH_PPCSQ is not defined also
relies on a * b + c being contracted into a fused multiply-add.
Although this contraction is not explicitly disabled for e_sqrtf.c, it
still seems appropriate to make the file explicit about its
requirements by using __builtin_fmaf; this patch does so.
Furthermore, it turns out that doing so fixes the observed inaccuracy
and missing exceptions (that is, that without explicit __builtin_fmaf
usage, it was not being compiled as intended).
Tested for powerpc32 (hard float). Committed.
2015-02-13 Joseph Myers <joseph@codesourcery.com>
[BZ #17967]
* sysdeps/powerpc/fpu/e_sqrtf.c (__slow_ieee754_sqrtf): Use
__builtin_fmaf instead of relying on contraction of a * b + c.
diff --git a/sysdeps/powerpc/fpu/e_sqrtf.c b/sysdeps/powerpc/fpu/e_sqrtf.c
index 034b6f5..a684cf9 100644
--- a/sysdeps/powerpc/fpu/e_sqrtf.c
+++ b/sysdeps/powerpc/fpu/e_sqrtf.c
@@ -87,26 +87,28 @@ __slow_ieee754_sqrtf (float 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_fmaf (sg, sg, -sx);
fsgi = (xi + 0x40000000) >> 1 & 0x7f800000;
sy2 = sy + sy;
- sg = sy * sd + sg; /* 16-bit approximation to sqrt(sx). */
- e = -(sy * sg - almost_half);
+ sg = __builtin_fmaf (sy, sd, sg); /* 16-bit approximation to
+ sqrt(sx). */
+ e = -__builtin_fmaf (sy, sg, -almost_half);
SET_FLOAT_WORD (fsg, fsgi);
- sd = -(sg * sg - sx);
- sy = sy + e * sy2;
+ sd = -__builtin_fmaf (sg, sg, -sx);
+ sy = __builtin_fmaf (e, sy2, sy);
if ((xi & 0x7f800000) == 0)
goto denorm;
shx = sx * fsg;
- sg = sg + sy * sd; /* 32-bit approximation to sqrt(sx),
- but perhaps rounded incorrectly. */
+ sg = __builtin_fmaf (sy, sd, sg); /* 32-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_fmaf (sy, sg, -almost_half);
+ d = -__builtin_fmaf (g, sg, -shx);
+ sy = __builtin_fmaf (e, sy2, sy);
fesetenv_register (fe);
- return g + sy * d;
+ return __builtin_fmaf (sy, d, g);
denorm:
/* For denormalised numbers, we normalise, calculate the
square root, and return an adjusted result. */
--
Joseph S. Myers
joseph@codesourcery.com