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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. |
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