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fix fmaf wrong result

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if double precision r=x*y+z is not a half way case between two single
precision floats or it is an exact result then fmaf returns (float)r.

however the exactness check was wrong when |x*y| < |z| and could cause
incorrectly rounded result in nearest rounding mode when r is a half
way case.

fmaf(-0x1.26524ep-54, -0x1.cb7868p+11, 0x1.d10f5ep-29)
was incorrectly rounded up to 0x1.d117ap-29 instead of 0x1.d1179ep-29.
(exact result is 0x1.d1179efffffffecp-29, r is 0x1.d1179fp-29)
This commit is contained in:
Szabolcs Nagy 2018-04-01 20:02:01 +00:00 committed by Rich Felker
parent 729fef0a93
commit 282b1cd266
1 changed files with 1 additions and 1 deletions
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src/math/fmaf.c
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@ -50,7 +50,7 @@ float fmaf(float x, float y, float z)
/* Common case: The double precision result is fine. */
if ((u.i & 0x1fffffff) != 0x10000000 || /* not a halfway case */
e == 0x7ff || /* NaN */
result - xy == z || /* exact */
(result - xy == z && result - z == xy) || /* exact */
fegetround() != FE_TONEAREST) /* not round-to-nearest */
{
/*