From 282b1cd26649d69de038111f5876853df6ddc345 Mon Sep 17 00:00:00 2001 From: Szabolcs Nagy Date: Sun, 1 Apr 2018 20:02:01 +0000 Subject: [PATCH] fix fmaf wrong result 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) --- src/math/fmaf.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/math/fmaf.c b/src/math/fmaf.c index aa57feb6..80f5cd8a 100644 --- a/src/math/fmaf.c +++ b/src/math/fmaf.c @@ -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 */ { /* -- 2.25.1