addsf3.c revision 296373
1//===-- lib/addsf3.c - Single-precision addition ------------------*- C -*-===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is dual licensed under the MIT and the University of Illinois Open 6// Source Licenses. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file implements single-precision soft-float addition with the IEEE-754 11// default rounding (to nearest, ties to even). 12// 13//===----------------------------------------------------------------------===// 14 15#define SINGLE_PRECISION 16#include "fp_lib.h" 17 18ARM_EABI_FNALIAS(fadd, addsf3) 19 20fp_t __addsf3(fp_t a, fp_t b) { 21 22 rep_t aRep = toRep(a); 23 rep_t bRep = toRep(b); 24 const rep_t aAbs = aRep & absMask; 25 const rep_t bAbs = bRep & absMask; 26 27 // Detect if a or b is zero, infinity, or NaN. 28 if (aAbs - 1U >= infRep - 1U || bAbs - 1U >= infRep - 1U) { 29 30 // NaN + anything = qNaN 31 if (aAbs > infRep) return fromRep(toRep(a) | quietBit); 32 // anything + NaN = qNaN 33 if (bAbs > infRep) return fromRep(toRep(b) | quietBit); 34 35 if (aAbs == infRep) { 36 // +/-infinity + -/+infinity = qNaN 37 if ((toRep(a) ^ toRep(b)) == signBit) return fromRep(qnanRep); 38 // +/-infinity + anything remaining = +/- infinity 39 else return a; 40 } 41 42 // anything remaining + +/-infinity = +/-infinity 43 if (bAbs == infRep) return b; 44 45 // zero + anything = anything 46 if (!aAbs) { 47 // but we need to get the sign right for zero + zero 48 if (!bAbs) return fromRep(toRep(a) & toRep(b)); 49 else return b; 50 } 51 52 // anything + zero = anything 53 if (!bAbs) return a; 54 } 55 56 // Swap a and b if necessary so that a has the larger absolute value. 57 if (bAbs > aAbs) { 58 const rep_t temp = aRep; 59 aRep = bRep; 60 bRep = temp; 61 } 62 63 // Extract the exponent and significand from the (possibly swapped) a and b. 64 int aExponent = aRep >> significandBits & maxExponent; 65 int bExponent = bRep >> significandBits & maxExponent; 66 rep_t aSignificand = aRep & significandMask; 67 rep_t bSignificand = bRep & significandMask; 68 69 // Normalize any denormals, and adjust the exponent accordingly. 70 if (aExponent == 0) aExponent = normalize(&aSignificand); 71 if (bExponent == 0) bExponent = normalize(&bSignificand); 72 73 // The sign of the result is the sign of the larger operand, a. If they 74 // have opposite signs, we are performing a subtraction; otherwise addition. 75 const rep_t resultSign = aRep & signBit; 76 const bool subtraction = (aRep ^ bRep) & signBit; 77 78 // Shift the significands to give us round, guard and sticky, and or in the 79 // implicit significand bit. (If we fell through from the denormal path it 80 // was already set by normalize( ), but setting it twice won't hurt 81 // anything.) 82 aSignificand = (aSignificand | implicitBit) << 3; 83 bSignificand = (bSignificand | implicitBit) << 3; 84 85 // Shift the significand of b by the difference in exponents, with a sticky 86 // bottom bit to get rounding correct. 87 const unsigned int align = aExponent - bExponent; 88 if (align) { 89 if (align < typeWidth) { 90 const bool sticky = bSignificand << (typeWidth - align); 91 bSignificand = bSignificand >> align | sticky; 92 } else { 93 bSignificand = 1; // sticky; b is known to be non-zero. 94 } 95 } 96 97 if (subtraction) { 98 aSignificand -= bSignificand; 99 100 // If a == -b, return +zero. 101 if (aSignificand == 0) return fromRep(0); 102 103 // If partial cancellation occured, we need to left-shift the result 104 // and adjust the exponent: 105 if (aSignificand < implicitBit << 3) { 106 const int shift = rep_clz(aSignificand) - rep_clz(implicitBit << 3); 107 aSignificand <<= shift; 108 aExponent -= shift; 109 } 110 } 111 112 else /* addition */ { 113 aSignificand += bSignificand; 114 115 // If the addition carried up, we need to right-shift the result and 116 // adjust the exponent: 117 if (aSignificand & implicitBit << 4) { 118 const bool sticky = aSignificand & 1; 119 aSignificand = aSignificand >> 1 | sticky; 120 aExponent += 1; 121 } 122 } 123 124 // If we have overflowed the type, return +/- infinity: 125 if (aExponent >= maxExponent) return fromRep(infRep | resultSign); 126 127 if (aExponent <= 0) { 128 // Result is denormal before rounding; the exponent is zero and we 129 // need to shift the significand. 130 const int shift = 1 - aExponent; 131 const bool sticky = aSignificand << (typeWidth - shift); 132 aSignificand = aSignificand >> shift | sticky; 133 aExponent = 0; 134 } 135 136 // Low three bits are round, guard, and sticky. 137 const int roundGuardSticky = aSignificand & 0x7; 138 139 // Shift the significand into place, and mask off the implicit bit. 140 rep_t result = aSignificand >> 3 & significandMask; 141 142 // Insert the exponent and sign. 143 result |= (rep_t)aExponent << significandBits; 144 result |= resultSign; 145 146 // Final rounding. The result may overflow to infinity, but that is the 147 // correct result in that case. 148 if (roundGuardSticky > 0x4) result++; 149 if (roundGuardSticky == 0x4) result += result & 1; 150 return fromRep(result); 151} 152