Fix a bug that APFloat::fusedMultiplyAdd() mistakenly evaluate "14.5f * -14.5f + 225.0f" to 225.0f.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181715 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp
index 6182e34..57e60da 100644
--- a/lib/Support/APFloat.cpp
+++ b/lib/Support/APFloat.cpp
@@ -872,7 +872,21 @@
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 @@
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 @@
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;