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.
e8bd5286c6