From 739babd775d4b69525e3494cad4310742a9b360a Mon Sep 17 00:00:00 2001 From: Joseph Myers Date: Fri, 14 Aug 2015 17:15:06 +0000 Subject: Fix fma spurious underflows (bug 18824). Various fma implementations have logic that, when computing fma (x, y, z) where z is large (so care needs taking to avoid internal overflow) but x * y is small, scale x * y up instead of down to avoid internal underflows resulting from scaling down. (In these cases, x * y is small enough that only its sign actually matters rather than the exact value.) The threshold for scaling up instead of down was correct for "if the unscaled values were multiplied, the low part of the multiplication could underflow", and the scaling was sufficient to ensure that the low part of the multiplication did not underflow (given that cases of very small x * y - less than half the least subnormal - were previously dealt with). However, the choice in the functions wasn't between scaling up or no scaling, but between scaling up and scaling down (scaling down actually being needed when x * y isn't so small compared to z and so the exact value does matter). Thus a larger threshold is needed to ensure that scaling down doesn't produce values the multiplication of whose low parts underflows. This patch increases the thresholds accordingly. Tested for x86_64, x86 and mips64 (with the MIPS version of s_fmal.c removed so that the ldbl-128 version gets tested instead of the soft-fp one). [BZ #18824] * sysdeps/ieee754/dbl-64/s_fma.c (__fma): Increase threshold for scaling x * y up instead of down. * sysdeps/ieee754/ldbl-128/s_fmal.c (__fmal): Likewise. * sysdeps/ieee754/ldbl-96/s_fmal.c (__fmal): Likewise. * math/auto-libm-test-in: Add more tests of fma. * math/auto-libm-test-out: Regenerated. --- math/auto-libm-test-in | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'math/auto-libm-test-in') diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in index d3a4834faa..44803714c9 100644 --- a/math/auto-libm-test-in +++ b/math/auto-libm-test-in @@ -1794,6 +1794,24 @@ fma 0x1.0000000000000000000000000001p-16382 -0x1.0000000000000000000000000001p-6 fma 0x1.0000000000000000000000000001p-16382 0x1.0000000000000000000000000001p-66 -0x1p16319 fma 0x1.0000000000000000000000000001p-16382 -0x1.0000000000000000000000000001p-66 -0x1p16319 +# Bug 6801: errno setting may be missing. +fma 0x1.fffffep-126 0x1.fffffep25 0x1.fffffep127 missing-errno +fma 0x1.fffffep-126 -0x1.fffffep25 0x1.fffffep127 +fma 0x1.fffffep-126 0x1.fffffep25 -0x1.fffffep127 +fma 0x1.fffffep-126 -0x1.fffffep25 -0x1.fffffep127 missing-errno +fma 0x1.fffffffffffffp-1022 0x1.fffffffffffffp54 0x1.fffffffffffffp1023 missing-errno +fma 0x1.fffffffffffffp-1022 -0x1.fffffffffffffp54 0x1.fffffffffffffp1023 +fma 0x1.fffffffffffffp-1022 0x1.fffffffffffffp54 -0x1.fffffffffffffp1023 +fma 0x1.fffffffffffffp-1022 -0x1.fffffffffffffp54 -0x1.fffffffffffffp1023 missing-errno +fma 0x1.fffffffffffffffep-16382 0x1.fffffffffffffffep65 0x1.fffffffffffffffep16383 missing-errno +fma 0x1.fffffffffffffffep-16382 -0x1.fffffffffffffffep65 0x1.fffffffffffffffep16383 +fma 0x1.fffffffffffffffep-16382 0x1.fffffffffffffffep65 -0x1.fffffffffffffffep16383 +fma 0x1.fffffffffffffffep-16382 -0x1.fffffffffffffffep65 -0x1.fffffffffffffffep16383 missing-errno +fma 0x1.ffffffffffffffffffffffffffffp-16382 0x1.ffffffffffffffffffffffffffffp114 0x1.ffffffffffffffffffffffffffffp16383 missing-errno +fma 0x1.ffffffffffffffffffffffffffffp-16382 -0x1.ffffffffffffffffffffffffffffp114 0x1.ffffffffffffffffffffffffffffp16383 +fma 0x1.ffffffffffffffffffffffffffffp-16382 0x1.ffffffffffffffffffffffffffffp114 -0x1.ffffffffffffffffffffffffffffp16383 +fma 0x1.ffffffffffffffffffffffffffffp-16382 -0x1.ffffffffffffffffffffffffffffp114 -0x1.ffffffffffffffffffffffffffffp16383 missing-errno + hypot 0 0 hypot 0 -0 hypot -0 0 -- cgit 1.4.1