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Fix a bug that APFloat::fusedMultiplyAdd() mistakenly evaluate "14.5f * -14.5f + 225.0f" to 225.0f. 

llvm-svn: 181715
This commit is contained in:
Shuxin Yang 2013-05-13 18:03:12 +00:00
parent 310e26a832
commit bbddbacd2e
2 changed files with 50 additions and 2 deletions
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29
llvm/lib/Support/APFloat.cpp
View File

@ -872,7 +872,21 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
omsb = APInt::tcMSB(fullSignificand, newPartsCount) + 1;
exponent += rhs.exponent;
// Assume the operands involved in the multiplication are single-precision
// FP, and the two multiplicants are:
// *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;
if (addend) {
// The intermediate result of the multiplication has "2 * precision"
// signicant bit; adjust the addend to be consistent with mul result.
//
Significand savedSignificand = significand;
const fltSemantics *savedSemantics = semantics;
fltSemantics extendedSemantics;
@ -880,8 +894,9 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
unsigned int extendedPrecision;
/* Normalize our MSB. */
extendedPrecision = precision + precision - 1;
extendedPrecision = 2 * precision;
if (omsb != extendedPrecision) {
assert(extendedPrecision > omsb);
APInt::tcShiftLeft(fullSignificand, newPartsCount,
extendedPrecision - omsb);
exponent -= extendedPrecision - omsb;
@ -912,8 +927,18 @@ APFloat::multiplySignificand(const APFloat &rhs, const APFloat *addend)
omsb = APInt::tcMSB(fullSignificand, newPartsCount) + 1;
}
exponent -= (precision - 1);
// Convert the result having "2 * precision" significant-bits back to the one
// 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;
// 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
// the radix point (i.e. "MSB . rest-significant-bits").
//
// Note that the result is not normalized when "omsb < precision". So, the
// caller needs to call APFloat::normalize() if normalized value is expected.
if (omsb > precision) {
unsigned int bits, significantParts;
lostFraction lf;

23
llvm/unittests/ADT/APFloatTest.cpp
View File

@ -33,6 +33,29 @@ static std::string convertToString(double d, unsigned Prec, unsigned Pad) {
namespace {
TEST(APFloatTest, FMA) {
APFloat::roundingMode rdmd = APFloat::rmNearestTiesToEven;
{
APFloat f1(14.5f);
APFloat f2(-14.5f);
APFloat f3(225.0f);
f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
EXPECT_EQ(14.75f, f1.convertToFloat());
}
{
APFloat Val2(2.0f);
APFloat f1((float)1.17549435e-38F);
APFloat f2((float)1.17549435e-38F);
f1.divide(Val2, rdmd);
f2.divide(Val2, rdmd);
APFloat f3(12.0f);
f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
EXPECT_EQ(12.0f, f1.convertToFloat());
}
}
TEST(APFloatTest, Denormal) {
APFloat::roundingMode rdmd = APFloat::rmNearestTiesToEven;