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[ADT] Fix PR20728 - Incorrect APFloat::fusedMultiplyAdd results for x86_fp80.

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As detailed at http://llvm.org/PR20728, due to an internal overflow in
APFloat::multiplySignificand the APFloat::fusedMultiplyAdd method can return
incorrect results for x87DoubleExtended (x86_fp80) values. This commonly
manifests as incorrect constant folding of libm fmal calls on x86. E.g.

fmal(1.0L, 1.0L, 3.0L) == 0.0L      (should be 4.0L)

This patch fixes PR20728 by adding an extra bit to the significand for
intermediate results of APFloat::multiplySignificand, avoiding the overflow.

llvm-svn: 222374
This commit is contained in:
Lang Hames 2014-11-19 19:15:41 +00:00
parent feb92b52fc
commit 2082c9d610
2 changed files with 36 additions and 12 deletions
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36
lib/Support/APFloat.cpp
View File

@ -926,7 +926,10 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
assert(semantics == rhs.semantics);
precision = semantics->precision;
newPartsCount = partCountForBits(precision * 2);
// Allocate space for twice as many bits as the original significand, plus one
// extra bit for the addition to overflow into.
newPartsCount = partCountForBits(precision * 2 + 1);
if (newPartsCount > 4)
fullSignificand = new integerPart[newPartsCount];
@ -948,11 +951,12 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
// *this = a23 . a22 ... a0 * 2^e1
// rhs = b23 . b22 ... b0 * 2^e2
// the result of multiplication is:
// *this = c47 c46 . c45 ... c0 * 2^(e1+e2)
// Note that there are two significant bits at the left-hand side of the
// radix point. Move the radix point toward left by one bit, and adjust
// exponent accordingly.
exponent += 1;
// *this = c48 c47 c46 . c45 ... c0 * 2^(e1+e2)
// Note that there are three significant bits at the left-hand side of the
// radix point: two for the multiplication, and an overflow bit for the
// addition (that will always be zero at this point). Move the radix point
// toward left by two bits, and adjust exponent accordingly.
exponent += 2;
if (addend && addend->isNonZero()) {
// The intermediate result of the multiplication has "2 * precision"
@ -964,13 +968,13 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
opStatus status;
unsigned int extendedPrecision;
/* Normalize our MSB. */
extendedPrecision = 2 * precision;
if (omsb != extendedPrecision) {
// Normalize our MSB to one below the top bit to allow for overflow.
extendedPrecision = 2 * precision + 1;
if (omsb != extendedPrecision - 1) {
assert(extendedPrecision > omsb);
APInt::tcShiftLeft(fullSignificand, newPartsCount,
extendedPrecision - omsb);
exponent -= extendedPrecision - omsb;
(extendedPrecision - 1) - omsb);
exponent -= (extendedPrecision - 1) - omsb;
}
/* Create new semantics. */
@ -987,6 +991,14 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
status = extendedAddend.convert(extendedSemantics, rmTowardZero, &ignored);
assert(status == opOK);
(void)status;
// Shift the significand of the addend right by one bit. This guarantees
// that the high bit of the significand is zero (same as fullSignificand),
// so the addition will overflow (if it does overflow at all) into the top bit.
lost_fraction = extendedAddend.shiftSignificandRight(1);
assert(lost_fraction == lfExactlyZero &&
"Lost precision while shifting addend for fused-multiply-add.");
lost_fraction = addOrSubtractSignificand(extendedAddend, false);
/* Restore our state. */
@ -1002,7 +1014,7 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
// having "precision" significant-bits. First, move the radix point from
// poision "2*precision - 1" to "precision - 1". The exponent need to be
// adjusted by "2*precision - 1" - "precision - 1" = "precision".
exponent -= precision;
exponent -= precision + 1;
// In case MSB resides at the left-hand side of radix point, shift the
// mantissa right by some amount to make sure the MSB reside right before

12
unittests/ADT/APFloatTest.cpp
View File

@ -474,6 +474,18 @@ TEST(APFloatTest, FMA) {
f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
EXPECT_EQ(12.0f, f1.convertToFloat());
}
{
APFloat M1(APFloat::x87DoubleExtended, 1.0);
APFloat M2(APFloat::x87DoubleExtended, 1.0);
APFloat A(APFloat::x87DoubleExtended, 3.0);
bool losesInfo = false;
M1.fusedMultiplyAdd(M1, A, APFloat::rmNearestTiesToEven);
M1.convert(APFloat::IEEEsingle, APFloat::rmNearestTiesToEven, &losesInfo);
EXPECT_FALSE(losesInfo);
EXPECT_EQ(4.0f, M1.convertToFloat());
}
}
TEST(APFloatTest, MinNum) {