diff options
Diffstat (limited to 'lib/libcompiler_rt/divtf3.c')
| -rw-r--r-- | lib/libcompiler_rt/divtf3.c | 203 |
1 files changed, 203 insertions, 0 deletions
diff --git a/lib/libcompiler_rt/divtf3.c b/lib/libcompiler_rt/divtf3.c new file mode 100644 index 00000000000..e81dab826bd --- /dev/null +++ b/lib/libcompiler_rt/divtf3.c @@ -0,0 +1,203 @@ +//===-- lib/divtf3.c - Quad-precision division --------------------*- C -*-===// +// +// The LLVM Compiler Infrastructure +// +// This file is dual licensed under the MIT and the University of Illinois Open +// Source Licenses. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file implements quad-precision soft-float division +// with the IEEE-754 default rounding (to nearest, ties to even). +// +// For simplicity, this implementation currently flushes denormals to zero. +// It should be a fairly straightforward exercise to implement gradual +// underflow with correct rounding. +// +//===----------------------------------------------------------------------===// + +#define QUAD_PRECISION +#include "fp_lib.h" + +#if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT) +COMPILER_RT_ABI fp_t __divtf3(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 quotientSign = (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 / infinity = NaN + if (bAbs == infRep) return fromRep(qnanRep); + // infinity / anything else = +/- infinity + else return fromRep(aAbs | quotientSign); + } + + // anything else / infinity = +/- 0 + if (bAbs == infRep) return fromRep(quotientSign); + + if (!aAbs) { + // zero / zero = NaN + if (!bAbs) return fromRep(qnanRep); + // zero / anything else = +/- zero + else return fromRep(quotientSign); + } + // anything else / zero = +/- infinity + if (!bAbs) return fromRep(infRep | quotientSign); + + // 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; + int quotientExponent = aExponent - bExponent + scale; + + // Align the significand of b as a Q63 fixed-point number in the range + // [1, 2.0) and get a Q64 approximate reciprocal using a small minimax + // polynomial approximation: reciprocal = 3/4 + 1/sqrt(2) - b/2. This + // is accurate to about 3.5 binary digits. + const uint64_t q63b = bSignificand >> 49; + uint64_t recip64 = UINT64_C(0x7504f333F9DE6484) - q63b; + // 0x7504f333F9DE6484 / 2^64 + 1 = 3/4 + 1/sqrt(2) + + // Now refine the reciprocal estimate using a Newton-Raphson iteration: + // + // x1 = x0 * (2 - x0 * b) + // + // This doubles the number of correct binary digits in the approximation + // with each iteration. + uint64_t correction64; + correction64 = -((rep_t)recip64 * q63b >> 64); + recip64 = (rep_t)recip64 * correction64 >> 63; + correction64 = -((rep_t)recip64 * q63b >> 64); + recip64 = (rep_t)recip64 * correction64 >> 63; + correction64 = -((rep_t)recip64 * q63b >> 64); + recip64 = (rep_t)recip64 * correction64 >> 63; + correction64 = -((rep_t)recip64 * q63b >> 64); + recip64 = (rep_t)recip64 * correction64 >> 63; + correction64 = -((rep_t)recip64 * q63b >> 64); + recip64 = (rep_t)recip64 * correction64 >> 63; + + // recip64 might have overflowed to exactly zero in the preceeding + // computation if the high word of b is exactly 1.0. This would sabotage + // the full-width final stage of the computation that follows, so we adjust + // recip64 downward by one bit. + recip64--; + + // We need to perform one more iteration to get us to 112 binary digits; + // The last iteration needs to happen with extra precision. + const uint64_t q127blo = bSignificand << 15; + rep_t correction, reciprocal; + + // NOTE: This operation is equivalent to __multi3, which is not implemented + // in some architechure + rep_t r64q63, r64q127, r64cH, r64cL, dummy; + wideMultiply((rep_t)recip64, (rep_t)q63b, &dummy, &r64q63); + wideMultiply((rep_t)recip64, (rep_t)q127blo, &dummy, &r64q127); + + correction = -(r64q63 + (r64q127 >> 64)); + + uint64_t cHi = correction >> 64; + uint64_t cLo = correction; + + wideMultiply((rep_t)recip64, (rep_t)cHi, &dummy, &r64cH); + wideMultiply((rep_t)recip64, (rep_t)cLo, &dummy, &r64cL); + + reciprocal = r64cH + (r64cL >> 64); + + // We already adjusted the 64-bit estimate, now we need to adjust the final + // 128-bit reciprocal estimate downward to ensure that it is strictly smaller + // than the infinitely precise exact reciprocal. Because the computation + // of the Newton-Raphson step is truncating at every step, this adjustment + // is small; most of the work is already done. + reciprocal -= 2; + + // The numerical reciprocal is accurate to within 2^-112, lies in the + // interval [0.5, 1.0), and is strictly smaller than the true reciprocal + // of b. Multiplying a by this reciprocal thus gives a numerical q = a/b + // in Q127 with the following properties: + // + // 1. q < a/b + // 2. q is in the interval [0.5, 2.0) + // 3. the error in q is bounded away from 2^-113 (actually, we have a + // couple of bits to spare, but this is all we need). + + // We need a 128 x 128 multiply high to compute q, which isn't a basic + // operation in C, so we need to be a little bit fussy. + rep_t quotient, quotientLo; + wideMultiply(aSignificand << 2, reciprocal, "ient, "ientLo); + + // Two cases: quotient is in [0.5, 1.0) or quotient is in [1.0, 2.0). + // In either case, we are going to compute a residual of the form + // + // r = a - q*b + // + // We know from the construction of q that r satisfies: + // + // 0 <= r < ulp(q)*b + // + // if r is greater than 1/2 ulp(q)*b, then q rounds up. Otherwise, we + // already have the correct result. The exact halfway case cannot occur. + // We also take this time to right shift quotient if it falls in the [1,2) + // range and adjust the exponent accordingly. + rep_t residual; + rep_t qb; + + if (quotient < (implicitBit << 1)) { + wideMultiply(quotient, bSignificand, &dummy, &qb); + residual = (aSignificand << 113) - qb; + quotientExponent--; + } else { + quotient >>= 1; + wideMultiply(quotient, bSignificand, &dummy, &qb); + residual = (aSignificand << 112) - qb; + } + + const int writtenExponent = quotientExponent + exponentBias; + + if (writtenExponent >= maxExponent) { + // If we have overflowed the exponent, return infinity. + return fromRep(infRep | quotientSign); + } + else if (writtenExponent < 1) { + // Flush denormals to zero. In the future, it would be nice to add + // code to round them correctly. + return fromRep(quotientSign); + } + else { + const bool round = (residual << 1) >= bSignificand; + // Clear the implicit bit + rep_t absResult = quotient & significandMask; + // Insert the exponent + absResult |= (rep_t)writtenExponent << significandBits; + // Round + absResult += round; + // Insert the sign and return + const long double result = fromRep(absResult | quotientSign); + return result; + } +} + +#endif |