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[APFloat] Fix fusedMultiplyAdd when `this` equals to `Addend` 

Up until now, the arguments to `fusedMultiplyAdd` are passed by
reference. We must save the `Addend` value on the beginning of the
function, before we modify `this`, as they may be the same reference.

To fix this, we now pass the `addend` parameter of `multiplySignificand`
by value (instead of by-ref), and have a default value of zero.

Fix PR44051.

Differential Revision: https://reviews.llvm.org/D70422
This commit is contained in:
Ehud Katz 2020-01-07 08:45:18 +02:00
parent 188f72ab20
commit 08de551f4f
3 changed files with 22 additions and 10 deletions
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3
llvm/include/llvm/ADT/APFloat.h
View File

@ -487,7 +487,8 @@ private:
integerPart addSignificand(const IEEEFloat &);
integerPart subtractSignificand(const IEEEFloat &, integerPart);
lostFraction addOrSubtractSignificand(const IEEEFloat &, bool subtract);
lostFraction multiplySignificand(const IEEEFloat &, const IEEEFloat *);
lostFraction multiplySignificand(const IEEEFloat &, IEEEFloat);
lostFraction multiplySignificand(const IEEEFloat&);
lostFraction divideSignificand(const IEEEFloat &);
void incrementSignificand();
void initialize(const fltSemantics *);

21
llvm/lib/Support/APFloat.cpp
View File

@ -992,7 +992,7 @@ IEEEFloat::integerPart IEEEFloat::subtractSignificand(const IEEEFloat &rhs,
on to the full-precision result of the multiplication. Returns the
lost fraction. */
lostFraction IEEEFloat::multiplySignificand(const IEEEFloat &rhs,
const IEEEFloat *addend) {
IEEEFloat addend) {
unsigned int omsb; // One, not zero, based MSB.
unsigned int partsCount, newPartsCount, precision;
integerPart *lhsSignificand;
@ -1036,7 +1036,7 @@ lostFraction IEEEFloat::multiplySignificand(const IEEEFloat &rhs,
// toward left by two bits, and adjust exponent accordingly.
exponent += 2;
if (addend && addend->isNonZero()) {
if (addend.isNonZero()) {
// The intermediate result of the multiplication has "2 * precision"
// signicant bit; adjust the addend to be consistent with mul result.
//
@ -1065,19 +1065,18 @@ lostFraction IEEEFloat::multiplySignificand(const IEEEFloat &rhs,
significand.parts = fullSignificand;
semantics = &extendedSemantics;
IEEEFloat extendedAddend(*addend);
status = extendedAddend.convert(extendedSemantics, rmTowardZero, &ignored);
status = addend.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);
lost_fraction = addend.shiftSignificandRight(1);
assert(lost_fraction == lfExactlyZero &&
"Lost precision while shifting addend for fused-multiply-add.");
lost_fraction = addOrSubtractSignificand(extendedAddend, false);
lost_fraction = addOrSubtractSignificand(addend, false);
/* Restore our state. */
if (newPartsCount == 1)
@ -1120,6 +1119,10 @@ lostFraction IEEEFloat::multiplySignificand(const IEEEFloat &rhs,
return lost_fraction;
}
lostFraction IEEEFloat::multiplySignificand(const IEEEFloat &rhs) {
return multiplySignificand(rhs, IEEEFloat(*semantics));
}
/* Multiply the significands of LHS and RHS to DST. */
lostFraction IEEEFloat::divideSignificand(const IEEEFloat &rhs) {
unsigned int bit, i, partsCount;
@ -1725,7 +1728,7 @@ IEEEFloat::opStatus IEEEFloat::multiply(const IEEEFloat &rhs,
fs = multiplySpecials(rhs);
if (isFiniteNonZero()) {
lostFraction lost_fraction = multiplySignificand(rhs, nullptr);
lostFraction lost_fraction = multiplySignificand(rhs);
fs = normalize(rounding_mode, lost_fraction);
if (lost_fraction != lfExactlyZero)
fs = (opStatus) (fs | opInexact);
@ -1826,7 +1829,7 @@ IEEEFloat::opStatus IEEEFloat::fusedMultiplyAdd(const IEEEFloat &multiplicand,
addend.isFinite()) {
lostFraction lost_fraction;
lost_fraction = multiplySignificand(multiplicand, &addend);
lost_fraction = multiplySignificand(multiplicand, addend);
fs = normalize(rounding_mode, lost_fraction);
if (lost_fraction != lfExactlyZero)
fs = (opStatus) (fs | opInexact);
@ -2449,7 +2452,7 @@ IEEEFloat::roundSignificandWithExponent(const integerPart *decSigParts,
if (exp >= 0) {
/* multiplySignificand leaves the precision-th bit set to 1. */
calcLostFraction = decSig.multiplySignificand(pow5, nullptr);
calcLostFraction = decSig.multiplySignificand(pow5);
powHUerr = powStatus != opOK;
} else {
calcLostFraction = decSig.divideSignificand(pow5);

8
llvm/unittests/ADT/APFloatTest.cpp
View File

@ -547,6 +547,14 @@ TEST(APFloatTest, FMA) {
f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
EXPECT_EQ(-8.85242279E-41f, f1.convertToFloat());
}
// Test using only a single instance of APFloat.
{
APFloat F(1.5);
F.fusedMultiplyAdd(F, F, APFloat::rmNearestTiesToEven);
EXPECT_EQ(3.75, F.convertToDouble());
}
}
TEST(APFloatTest, MinNum) {