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//===-- lib/muldf3.c - Double-precision multiplication ------------*- C -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements double-precision soft-float multiplication
// with the IEEE-754 default rounding (to nearest, ties to even).
//
//===----------------------------------------------------------------------===//
#define DOUBLE_PRECISION
#include "fp_lib.h"
fp_t __muldf3(fp_t a, fp_t b) {
const unsigned int aExponent = toRep(a) >> significandBits & maxExponent;
const unsigned int bExponent = toRep(b) >> significandBits & maxExponent;
const rep_t productSign = (toRep(a) ^ toRep(b)) & signBit;
rep_t aSignificand = toRep(a) & significandMask;
rep_t bSignificand = toRep(b) & significandMask;
int scale = 0;
// Detect if a or b is zero, denormal, infinity, or NaN.
if (aExponent-1U >= maxExponent-1U || bExponent-1U >= maxExponent-1U) {
const rep_t aAbs = toRep(a) & absMask;
const rep_t bAbs = toRep(b) & absMask;
// NaN * anything = qNaN
if (aAbs > infRep) return fromRep(toRep(a) | quietBit);
// anything * NaN = qNaN
if (bAbs > infRep) return fromRep(toRep(b) | quietBit);
if (aAbs == infRep) {
// infinity * non-zero = +/- infinity
if (bAbs) return fromRep(aAbs | productSign);
// infinity * zero = NaN
else return fromRep(qnanRep);
}
if (bAbs == infRep) {
// non-zero * infinity = +/- infinity
if (aAbs) return fromRep(bAbs | productSign);
// zero * infinity = NaN
else return fromRep(qnanRep);
}
// zero * anything = +/- zero
if (!aAbs) return fromRep(productSign);
// anything * zero = +/- zero
if (!bAbs) return fromRep(productSign);
// one or both of a or b is denormal, the other (if applicable) is a
// normal number. Renormalize one or both of a and b, and set scale to
// include the necessary exponent adjustment.
if (aAbs < implicitBit) scale += normalize(&aSignificand);
if (bAbs < implicitBit) scale += normalize(&bSignificand);
}
// Or in the implicit significand bit. (If we fell through from the
// denormal path it was already set by normalize( ), but setting it twice
// won't hurt anything.)
aSignificand |= implicitBit;
bSignificand |= implicitBit;
// Get the significand of a*b. Before multiplying the significands, shift
// one of them left to left-align it in the field. Thus, the product will
// have (exponentBits + 2) integral digits, all but two of which must be
// zero. Normalizing this result is just a conditional left-shift by one
// and bumping the exponent accordingly.
rep_t productHi, productLo;
wideMultiply(aSignificand, bSignificand << exponentBits,
&productHi, &productLo);
int productExponent = aExponent + bExponent - exponentBias + scale;
// Normalize the significand, adjust exponent if needed.
if (productHi & implicitBit) productExponent++;
else wideLeftShift(&productHi, &productLo, 1);
// If we have overflowed the type, return +/- infinity.
if (productExponent >= maxExponent) return fromRep(infRep | productSign);
if (productExponent <= 0) {
// Result is denormal before rounding
//
// If the result is so small that it just underflows to zero, return
// a zero of the appropriate sign. Mathematically there is no need to
// handle this case separately, but we make it a special case to
// simplify the shift logic.
const int shift = 1 - productExponent;
if (shift >= typeWidth) return fromRep(productSign);
// Otherwise, shift the significand of the result so that the round
// bit is the high bit of productLo.
wideRightShiftWithSticky(&productHi, &productLo, shift);
}
else {
// Result is normal before rounding; insert the exponent.
productHi &= significandMask;
productHi |= (rep_t)productExponent << significandBits;
}
// Insert the sign of the result:
productHi |= productSign;
// Final rounding. The final result may overflow to infinity, or underflow
// to zero, but those are the correct results in those cases. We use the
// default IEEE-754 round-to-nearest, ties-to-even rounding mode.
if (productLo > signBit) productHi++;
if (productLo == signBit) productHi += productHi & 1;
return fromRep(productHi);
}
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