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

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>.

llvm-svn: 225123
This commit is contained in:
Lang Hames 2015-01-04 01:20:55 +00:00
parent 744cc5b5dd
commit 12b12e800b
2 changed files with 42 additions and 1 deletions
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2
llvm/lib/Support/APFloat.cpp
View File

@ -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);

41
llvm/unittests/ADT/APFloatTest.cpp
View File

@ -475,6 +475,47 @@ TEST(APFloatTest, FMA) {
EXPECT_EQ(12.0f, f1.convertToFloat());
}
// Test for correct zero sign when answer is exactly zero.
// fma(1.0, -1.0, 1.0) -> +ve 0.
{
APFloat f1(1.0);
APFloat f2(-1.0);
APFloat f3(1.0);
f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
EXPECT_TRUE(!f1.isNegative() && f1.isZero());
}
// Test for correct zero sign when answer is exactly zero and rounding towards
// negative.
// fma(1.0, -1.0, 1.0) -> +ve 0.
{
APFloat f1(1.0);
APFloat f2(-1.0);
APFloat f3(1.0);
f1.fusedMultiplyAdd(f2, f3, APFloat::rmTowardNegative);
EXPECT_TRUE(f1.isNegative() && f1.isZero());
}
// Test for correct (in this case -ve) sign when adding like signed zeros.
// Test fma(0.0, -0.0, -0.0) -> -ve 0.
{
APFloat f1(0.0);
APFloat f2(-0.0);
APFloat f3(-0.0);
f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
EXPECT_TRUE(f1.isNegative() && f1.isZero());
}
// Test -ve sign preservation when small negative results underflow.
{
APFloat f1(APFloat::IEEEdouble, "-0x1p-1074");
APFloat f2(APFloat::IEEEdouble, "+0x1p-1074");
APFloat f3(0.0);
f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
EXPECT_TRUE(f1.isNegative() && f1.isZero());
}
// Test x87 extended precision case from http://llvm.org/PR20728.
{
APFloat M1(APFloat::x87DoubleExtended, 1.0);
APFloat M2(APFloat::x87DoubleExtended, 1.0);