addsf3.c revision 2d1fdb26e458c4ddc04155c1d421bced3ba90cd0 
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
20COMPILER_RT_ABI fp_t
21__addsf3(fp_t a, fp_t b) {
22
23    rep_t aRep = toRep(a);
24    rep_t bRep = toRep(b);
25    const rep_t aAbs = aRep & absMask;
26    const rep_t bAbs = bRep & absMask;
27
28    // Detect if a or b is zero, infinity, or NaN.
29    if (aAbs - 1U >= infRep - 1U || bAbs - 1U >= infRep - 1U) {
30
31        // NaN + anything = qNaN
32        if (aAbs > infRep) return fromRep(toRep(a) | quietBit);
33        // anything + NaN = qNaN
34        if (bAbs > infRep) return fromRep(toRep(b) | quietBit);
35
36        if (aAbs == infRep) {
37            // +/-infinity + -/+infinity = qNaN
38            if ((toRep(a) ^ toRep(b)) == signBit) return fromRep(qnanRep);
39            // +/-infinity + anything remaining = +/- infinity
40            else return a;
41        }
42
43        // anything remaining + +/-infinity = +/-infinity
44        if (bAbs == infRep) return b;
45
46        // zero + anything = anything
47        if (!aAbs) {
48            // but we need to get the sign right for zero + zero
49            if (!bAbs) return fromRep(toRep(a) & toRep(b));
50            else return b;
51        }
52
53        // anything + zero = anything
54        if (!bAbs) return a;
55    }
56
57    // Swap a and b if necessary so that a has the larger absolute value.
58    if (bAbs > aAbs) {
59        const rep_t temp = aRep;
60        aRep = bRep;
61        bRep = temp;
62    }
63
64    // Extract the exponent and significand from the (possibly swapped) a and b.
65    int aExponent = aRep >> significandBits & maxExponent;
66    int bExponent = bRep >> significandBits & maxExponent;
67    rep_t aSignificand = aRep & significandMask;
68    rep_t bSignificand = bRep & significandMask;
69
70    // Normalize any denormals, and adjust the exponent accordingly.
71    if (aExponent == 0) aExponent = normalize(&aSignificand);
72    if (bExponent == 0) bExponent = normalize(&bSignificand);
73
74    // The sign of the result is the sign of the larger operand, a.  If they
75    // have opposite signs, we are performing a subtraction; otherwise addition.
76    const rep_t resultSign = aRep & signBit;
77    const bool subtraction = (aRep ^ bRep) & signBit;
78
79    // Shift the significands to give us round, guard and sticky, and or in the
80    // implicit significand bit.  (If we fell through from the denormal path it
81    // was already set by normalize( ), but setting it twice won't hurt
82    // anything.)
83    aSignificand = (aSignificand | implicitBit) << 3;
84    bSignificand = (bSignificand | implicitBit) << 3;
85
86    // Shift the significand of b by the difference in exponents, with a sticky
87    // bottom bit to get rounding correct.
88    const unsigned int align = aExponent - bExponent;
89    if (align) {
90        if (align < typeWidth) {
91            const bool sticky = bSignificand << (typeWidth - align);
92            bSignificand = bSignificand >> align | sticky;
93        } else {
94            bSignificand = 1; // sticky; b is known to be non-zero.
95        }
96    }
97
98    if (subtraction) {
99        aSignificand -= bSignificand;
100
101        // If a == -b, return +zero.
102        if (aSignificand == 0) return fromRep(0);
103
104        // If partial cancellation occurred, we need to left-shift the result
105        // and adjust the exponent:
106        if (aSignificand < implicitBit << 3) {
107            const int shift = rep_clz(aSignificand) - rep_clz(implicitBit << 3);
108            aSignificand <<= shift;
109            aExponent -= shift;
110        }
111    }
112
113    else /* addition */ {
114        aSignificand += bSignificand;
115
116        // If the addition carried up, we need to right-shift the result and
117        // adjust the exponent:
118        if (aSignificand & implicitBit << 4) {
119            const bool sticky = aSignificand & 1;
120            aSignificand = aSignificand >> 1 | sticky;
121            aExponent += 1;
122        }
123    }
124
125    // If we have overflowed the type, return +/- infinity:
126    if (aExponent >= maxExponent) return fromRep(infRep | resultSign);
127
128    if (aExponent <= 0) {
129        // Result is denormal before rounding; the exponent is zero and we
130        // need to shift the significand.
131        const int shift = 1 - aExponent;
132        const bool sticky = aSignificand << (typeWidth - shift);
133        aSignificand = aSignificand >> shift | sticky;
134        aExponent = 0;
135    }
136
137    // Low three bits are round, guard, and sticky.
138    const int roundGuardSticky = aSignificand & 0x7;
139
140    // Shift the significand into place, and mask off the implicit bit.
141    rep_t result = aSignificand >> 3 & significandMask;
142
143    // Insert the exponent and sign.
144    result |= (rep_t)aExponent << significandBits;
145    result |= resultSign;
146
147    // Final rounding.  The result may overflow to infinity, but that is the
148    // correct result in that case.
149    if (roundGuardSticky > 0x4) result++;
150    if (roundGuardSticky == 0x4) result += result & 1;
151    return fromRep(result);
152}
153