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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.
This commit is contained in:
Joseph Myers 2015-02-12 23:05:37 +00:00
parent 96a157490c
commit e8bd5286c6
3 changed files with 25 additions and 16 deletions
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6
ChangeLog
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@ -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> 2015-02-12 Roland McGrath <roland@hack.frob.com>
* Makeconfig (ASFLAGS): Add -Werror=undef. * Makeconfig (ASFLAGS): Add -Werror=undef.

2
NEWS
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@ -9,7 +9,7 @@ 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, 17965. 4719, 15467, 15790, 16560, 17912, 17932, 17944, 17949, 17964, 17965.
Version 2.21 Version 2.21

33
sysdeps/powerpc/fpu/e_sqrt.c
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@ -99,38 +99,41 @@ __slow_ieee754_sqrt (double 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_fma (sg, sg, -sx);
fsgi = (xi0 + 0x40000000) >> 1 & 0x7ff00000; fsgi = (xi0 + 0x40000000) >> 1 & 0x7ff00000;
sy2 = sy + sy; 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 /* schedule the INSERT_WORDS (fsg, fsgi, 0) to get separation
between the store and the load. */ between the store and the load. */
INSERT_WORDS (fsg, fsgi, 0); INSERT_WORDS (fsg, fsgi, 0);
iw_u.parts.msw = fsgi; iw_u.parts.msw = fsgi;
iw_u.parts.lsw = (0); iw_u.parts.lsw = (0);
e = -(sy * sg - almost_half); e = -__builtin_fma (sy, sg, -almost_half);
sd = -(sg * sg - sx); sd = -__builtin_fma (sg, sg, -sx);
if ((xi0 & 0x7ff00000) == 0) if ((xi0 & 0x7ff00000) == 0)
goto denorm; goto denorm;
sy = sy + e * sy2; sy = __builtin_fma (e, sy2, sy);
sg = sg + sy * sd; /* 32-bit approximation to sqrt(sx). */ sg = __builtin_fma (sy, sd, sg); /* 32-bit approximation to
sqrt(sx). */
sy2 = sy + sy; sy2 = sy + sy;
/* complete the INSERT_WORDS (fsg, fsgi, 0) operation. */ /* complete the INSERT_WORDS (fsg, fsgi, 0) operation. */
fsg = iw_u.value; fsg = iw_u.value;
e = -(sy * sg - almost_half); e = -__builtin_fma (sy, sg, -almost_half);
sd = -(sg * sg - sx); sd = -__builtin_fma (sg, sg, -sx);
sy = sy + e * sy2; sy = __builtin_fma (e, sy2, sy);
shx = sx * fsg; shx = sx * fsg;
sg = sg + sy * sd; /* 64-bit approximation to sqrt(sx), sg = __builtin_fma (sy, sd, sg); /* 64-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_fma (sy, sg, -almost_half);
d = -(g * sg - shx); d = -__builtin_fma (g, sg, -shx);
sy = sy + e * sy2; sy = __builtin_fma (e, sy2, sy);
fesetenv_register (fe); fesetenv_register (fe);
return g + sy * d; return __builtin_fma (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. */