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Fix powerpc software sqrtf (bug 17967). 

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).

	[BZ #17967]
	* sysdeps/powerpc/fpu/e_sqrtf.c (__slow_ieee754_sqrtf): Use
	__builtin_fmaf instead of relying on contraction of a * b + c.
This commit is contained in:
Joseph Myers 2015-02-13 16:20:36 +00:00
parent 3f293d614c
commit 95c26233a1
3 changed files with 20 additions and 12 deletions
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6
ChangeLog
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@ -1,3 +1,9 @@
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.
2015-02-12 J William Piggott <elseifthen@gmx.com> 2015-02-12 J William Piggott <elseifthen@gmx.com>
[BZ #17969] [BZ #17969]

2
NEWS
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@ -10,7 +10,7 @@ Version 2.22
* The following bugs are resolved with this release: * The following bugs are resolved with this release:
4719, 15467, 15790, 16560, 17912, 17932, 17944, 17949, 17964, 17965, 4719, 15467, 15790, 16560, 17912, 17932, 17944, 17949, 17964, 17965,
17969. 17967, 17969.
Version 2.21 Version 2.21

24
sysdeps/powerpc/fpu/e_sqrtf.c
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@ -87,26 +87,28 @@ __slow_ieee754_sqrtf (float x)
/* Here we have three Newton-Raphson iterations each of a /* Here we have three Newton-Raphson iterations each of a
division and a square root and the remainder of the division and a square root and the remainder of the
argument reduction, all interleaved. */ argument reduction, all interleaved. */
sd = -(sg * sg - sx); sd = -__builtin_fmaf (sg, sg, -sx);
fsgi = (xi + 0x40000000) >> 1 & 0x7f800000; fsgi = (xi + 0x40000000) >> 1 & 0x7f800000;
sy2 = sy + sy; sy2 = sy + sy;
sg = sy * sd + sg; /* 16-bit approximation to sqrt(sx). */ sg = __builtin_fmaf (sy, sd, sg); /* 16-bit approximation to
e = -(sy * sg - almost_half); sqrt(sx). */
e = -__builtin_fmaf (sy, sg, -almost_half);
SET_FLOAT_WORD (fsg, fsgi); SET_FLOAT_WORD (fsg, fsgi);
sd = -(sg * sg - sx); sd = -__builtin_fmaf (sg, sg, -sx);
sy = sy + e * sy2; sy = __builtin_fmaf (e, sy2, sy);
if ((xi & 0x7f800000) == 0) if ((xi & 0x7f800000) == 0)
goto denorm; goto denorm;
shx = sx * fsg; shx = sx * fsg;
sg = sg + sy * sd; /* 32-bit approximation to sqrt(sx), sg = __builtin_fmaf (sy, sd, sg); /* 32-bit approximation to
but perhaps rounded incorrectly. */ sqrt(sx), but perhaps
rounded incorrectly. */
sy2 = sy + sy; sy2 = sy + sy;
g = sg * fsg; g = sg * fsg;
e = -(sy * sg - almost_half); e = -__builtin_fmaf (sy, sg, -almost_half);
d = -(g * sg - shx); d = -__builtin_fmaf (g, sg, -shx);
sy = sy + e * sy2; sy = __builtin_fmaf (e, sy2, sy);
fesetenv_register (fe); fesetenv_register (fe);
return g + sy * d; return __builtin_fmaf (sy, d, g);
denorm: denorm:
/* For denormalised numbers, we normalise, calculate the /* For denormalised numbers, we normalise, calculate the
square root, and return an adjusted result. */ square root, and return an adjusted result. */