From fa752c698146ca3e9f7747d33059fbef9bb02b0e Mon Sep 17 00:00:00 2001 From: Joseph Myers Date: Sat, 26 Sep 2015 00:27:06 +0000 Subject: Fix powf inaccuracy (bug 18956). The flt-32 version of powf can be inaccurate because of bugs in the extra-precision calculation of (x-1)/(x+1) or (x-1.5)/(x+1.5) as part of calculating log(x) with extra precision: a constant used (as part of adding 1 or 1.5 through integer arithmetic) is incorrect, and then the code fails to mask a computed high part before using it in arithmetic that relies on s_h*t_h being exactly representable. This patch fixes these bugs. Tested for x86_64 and x86. x86_64 ulps for powf removed and regenerated to reflect reduced ulps from the increased accuracy for existing tests. [BZ #18956] * sysdeps/ieee754/flt-32/e_powf.c (__ieee754_powf): Add 0x00400000 not 0x0040000 for high bit of mantissa. Mask with 0xfffff000 when extracting high part. * math/auto-libm-test-in: Add another test of pow. * math/auto-libm-test-out: Regenerated. * sysdeps/x86_64/fpu/libm-test-ulps: Update. --- sysdeps/ieee754/flt-32/e_powf.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'sysdeps/ieee754/flt-32') diff --git a/sysdeps/ieee754/flt-32/e_powf.c b/sysdeps/ieee754/flt-32/e_powf.c index 18042960c2..c72fe37d3b 100644 --- a/sysdeps/ieee754/flt-32/e_powf.c +++ b/sysdeps/ieee754/flt-32/e_powf.c @@ -166,7 +166,9 @@ __ieee754_powf(float x, float y) GET_FLOAT_WORD(is,s_h); SET_FLOAT_WORD(s_h,is&0xfffff000); /* t_h=ax+bp[k] High */ - SET_FLOAT_WORD(t_h,((ix>>1)|0x20000000)+0x0040000+(k<<21)); + SET_FLOAT_WORD (t_h, + ((((ix>>1)|0x20000000)+0x00400000+(k<<21)) + & 0xfffff000)); t_l = ax - (t_h-bp[k]); s_l = v*((u-s_h*t_h)-s_h*t_l); /* compute log(ax) */ -- cgit v1.2.3