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[APFloat][ADT] Fix sign handling logic for FMA results that truncate to zero.

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This patch adds a check for underflow when truncating results back to lower
precision at the end of an FMA. The additional sign handling logic in
APFloat::fusedMultiplyAdd should only be performed when the result of the
addition step of the FMA (in full precision) is exactly zero, not when the
result underflows to zero.

Unit tests for this case and related signed zero FMA results are included.

Fixes <rdar://problem/18925551>.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225123 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Lang Hames
2015-01-04 01:20:55 +00:00
parent b19a485253
commit a46dd583d6
2 changed files with 42 additions and 1 deletions
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2
lib/Support/APFloat.cpp
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@@ -1823,7 +1823,7 @@ APFloat::fusedMultiplyAdd(const APFloat &multiplicand,
/* If two numbers add (exactly) to zero, IEEE 754 decrees it is a
positive zero unless rounding to minus infinity, except that
adding two like-signed zeroes gives that zero. */
if (category == fcZero && sign != addend.sign)
if (category == fcZero && !(fs & opUnderflow) && sign != addend.sign)
sign = (rounding_mode == rmTowardNegative);
} else {
fs = multiplySpecials(multiplicand);