• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
1//===----- lib/fp_add_impl.inc - floaing point 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 soft-float addition with the IEEE-754 default rounding
11// (to nearest, ties to even).
12//
13//===----------------------------------------------------------------------===//
14
15#include "fp_lib.h"
16
17static __inline fp_t __addXf3__(fp_t a, fp_t b) {
18    rep_t aRep = toRep(a);
19    rep_t bRep = toRep(b);
20    const rep_t aAbs = aRep & absMask;
21    const rep_t bAbs = bRep & absMask;
22
23    // Detect if a or b is zero, infinity, or NaN.
24    if (aAbs - REP_C(1) >= infRep - REP_C(1) ||
25        bAbs - REP_C(1) >= infRep - REP_C(1)) {
26        // NaN + anything = qNaN
27        if (aAbs > infRep) return fromRep(toRep(a) | quietBit);
28        // anything + NaN = qNaN
29        if (bAbs > infRep) return fromRep(toRep(b) | quietBit);
30
31        if (aAbs == infRep) {
32            // +/-infinity + -/+infinity = qNaN
33            if ((toRep(a) ^ toRep(b)) == signBit) return fromRep(qnanRep);
34            // +/-infinity + anything remaining = +/- infinity
35            else return a;
36        }
37
38        // anything remaining + +/-infinity = +/-infinity
39        if (bAbs == infRep) return b;
40
41        // zero + anything = anything
42        if (!aAbs) {
43            // but we need to get the sign right for zero + zero
44            if (!bAbs) return fromRep(toRep(a) & toRep(b));
45            else return b;
46        }
47
48        // anything + zero = anything
49        if (!bAbs) return a;
50    }
51
52    // Swap a and b if necessary so that a has the larger absolute value.
53    if (bAbs > aAbs) {
54        const rep_t temp = aRep;
55        aRep = bRep;
56        bRep = temp;
57    }
58
59    // Extract the exponent and significand from the (possibly swapped) a and b.
60    int aExponent = aRep >> significandBits & maxExponent;
61    int bExponent = bRep >> significandBits & maxExponent;
62    rep_t aSignificand = aRep & significandMask;
63    rep_t bSignificand = bRep & significandMask;
64
65    // Normalize any denormals, and adjust the exponent accordingly.
66    if (aExponent == 0) aExponent = normalize(&aSignificand);
67    if (bExponent == 0) bExponent = normalize(&bSignificand);
68
69    // The sign of the result is the sign of the larger operand, a.  If they
70    // have opposite signs, we are performing a subtraction; otherwise addition.
71    const rep_t resultSign = aRep & signBit;
72    const bool subtraction = (aRep ^ bRep) & signBit;
73
74    // Shift the significands to give us round, guard and sticky, and or in the
75    // implicit significand bit.  (If we fell through from the denormal path it
76    // was already set by normalize( ), but setting it twice won't hurt
77    // anything.)
78    aSignificand = (aSignificand | implicitBit) << 3;
79    bSignificand = (bSignificand | implicitBit) << 3;
80
81    // Shift the significand of b by the difference in exponents, with a sticky
82    // bottom bit to get rounding correct.
83    const unsigned int align = aExponent - bExponent;
84    if (align) {
85        if (align < typeWidth) {
86            const bool sticky = bSignificand << (typeWidth - align);
87            bSignificand = bSignificand >> align | sticky;
88        } else {
89            bSignificand = 1; // sticky; b is known to be non-zero.
90        }
91    }
92    if (subtraction) {
93        aSignificand -= bSignificand;
94        // If a == -b, return +zero.
95        if (aSignificand == 0) return fromRep(0);
96
97        // If partial cancellation occured, we need to left-shift the result
98        // and adjust the exponent:
99        if (aSignificand < implicitBit << 3) {
100            const int shift = rep_clz(aSignificand) - rep_clz(implicitBit << 3);
101            aSignificand <<= shift;
102            aExponent -= shift;
103        }
104    }
105    else /* addition */ {
106        aSignificand += bSignificand;
107
108        // If the addition carried up, we need to right-shift the result and
109        // adjust the exponent:
110        if (aSignificand & implicitBit << 4) {
111            const bool sticky = aSignificand & 1;
112            aSignificand = aSignificand >> 1 | sticky;
113            aExponent += 1;
114        }
115    }
116
117    // If we have overflowed the type, return +/- infinity:
118    if (aExponent >= maxExponent) return fromRep(infRep | resultSign);
119
120    if (aExponent <= 0) {
121        // Result is denormal before rounding; the exponent is zero and we
122        // need to shift the significand.
123        const int shift = 1 - aExponent;
124        const bool sticky = aSignificand << (typeWidth - shift);
125        aSignificand = aSignificand >> shift | sticky;
126        aExponent = 0;
127    }
128
129    // Low three bits are round, guard, and sticky.
130    const int roundGuardSticky = aSignificand & 0x7;
131
132    // Shift the significand into place, and mask off the implicit bit.
133    rep_t result = aSignificand >> 3 & significandMask;
134
135    // Insert the exponent and sign.
136    result |= (rep_t)aExponent << significandBits;
137    result |= resultSign;
138
139    // Final rounding.  The result may overflow to infinity, but that is the
140    // correct result in that case.
141    if (roundGuardSticky > 0x4) result++;
142    if (roundGuardSticky == 0x4) result += result & 1;
143    return fromRep(result);
144}
145