src/base/condcodes.hh

   1 /*
   2  * Copyright (c) 2003-2005 The Regents of The University of Michigan
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions are
   7  * met: redistributions of source code must retain the above copyright
   8  * notice, this list of conditions and the following disclaimer;
   9  * redistributions in binary form must reproduce the above copyright
  10  * notice, this list of conditions and the following disclaimer in the
  11  * documentation and/or other materials provided with the distribution;
  12  * neither the name of the copyright holders nor the names of its
  13  * contributors may be used to endorse or promote products derived from
  14  * this software without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  17  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  18  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  19  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  20  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  21  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  22  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  23  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  24  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  25  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  26  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  27  */
  28
  29 #ifndef __BASE_CONDCODE_HH__
  30 #define __BASE_CONDCODE_HH__
  31
  32 #include "base/bitfield.hh"
  33
  34 /**
  35  * Calculate the carry flag from an addition. This should work even when
  36  * a carry value is also added in.
  37  *
  38  * Parameters:
  39  *   dest: The result value of the addition.
  40  *   src1: One of the addends that was added.
  41  *   src2: The other addend that was added in.
  42  *
  43  * Rationale:
  44  *   This code analyzes the most sig. bits of the source addends and result,
  45  *   and deduces the carry out flag from them without needing the carry in bit.
  46  *
  47  *   Observe that we have four cases after an addition regarding the carry
  48  *   in and carry out bits:
  49  *
  50  *   If we have no carry in but a carry out:
  51  *     src1 and src2 must both be 1, with the result bit being 0. Hence,
  52  *     ~0 + 1 + 1 => 11, which has a high second bit. We return true.
  53  *
  54  *   If we have a carry in and a carry out:
  55  *     src1 and src2 can either be 1 and 0, or vice versa. In this case,
  56  *     the addition with the carry in gives a result bit of 0 but a carry out.
  57  *     Hence,
  58  *     ~0 + 1 + 0 => 10, or ~0 + 0 + 1 => 10. We return true.
  59  *
  60  *     Or, src1 and src2 can both be one. Along with the carry, this gives
  61  *     a result of 1 and a carry out of 1. Hence,
  62  *     ~1 + 1 + 1 => 10. We return true.
  63  *
  64  *   If we have no carry in and no carry out:
  65  *     src1 and src2 can either be 1 and 0, 0 and 1, or 0 and 0.
  66  *     In the first two cases the result bit is 1, which when negated does not
  67  *     contribute to the sum algorithm at all. In the last case the result bit
  68  *     is zero, but neither src1 nor src2 contribute to the sum either. Hence,
  69  *     ~1 + 1 + 0 => 1,
  70  *     ~1 + 0 + 1 => 1,
  71  *     ~0 + 0 + 0 => 1.
  72  *     So we return false for all of these cases.
  73  *
  74  *   If we have a carry in, but no carry out:
  75  *     src1 and src2 can neither be 1. So the overall result bit is 1. Hence:
  76  *     ~1 + 0 + 0 => 0. We return false.
  77  */
  78 static inline bool
  79 findCarry(int width, uint64_t dest, uint64_t src1, uint64_t src2)
  80 {
  81     int shift = width - 1;
  82     return ((~(dest >> shift) & 1) +
  83             ((src1 >> shift) & 1) +
  84             ((src2 >> shift) & 1)) & 0x2;
  85 }
  86
  87 /**
  88  * Calculate the overflow flag from an addition.
  89  */
  90 static inline bool
  91 findOverflow(int width, uint64_t dest, uint64_t src1, uint64_t src2)
  92 {
  93     int shift = width - 1;
  94     return ((src1 ^ ~src2) & (src1 ^ dest)) & (1ULL << shift);
  95 }
  96
  97 /**
  98  * Calculate the parity of a value. 1 is for odd parity and 0 is for even.
  99  *
 100  * Parameters:
 101  *   dest: a value to be tested.
 102  *
 103  * Rationale:
 104  *   findParity simply performs bitwise XOR operations on each "pair" of bits
 105  *   in the dest parameter; the procedure being that a pair of ones will be
 106  *   XOR'ed out of the intermediate value.
 107  *
 108  *   This process is repeated until one last pair of bits are XOR'ed together.
 109  *   If the intermediate is still one, then there is exactly one high bit
 110  *   which does not have a corresponding high bit. Therefore, the value must
 111  *   have odd parity, and we return 1 accordingly. Otherwise we return 0.
 112  */
 113 static inline bool
 114 findParity(int width, uint64_t dest)
 115 {
 116     dest &= mask(width);
 117     dest ^= (dest >> 32);
 118     dest ^= (dest >> 16);
 119     dest ^= (dest >> 8);
 120     dest ^= (dest >> 4);
 121     dest ^= (dest >> 2);
 122     dest ^= (dest >> 1);
 123     return dest & 1;
 124 }
 125
 126 /**
 127  * Calculate the negative flag.
 128  */
 129 static inline bool
 130 findNegative(int width, uint64_t dest)
 131 {
 132     return bits(dest, width - 1);
 133 }
 134
 135 /**
 136  * Calculate the zero flag.
 137  */
 138 static inline bool
 139 findZero(int width, uint64_t dest)
 140 {
 141     return !(dest & mask(width));
 142 }
 143
 144 #endif // __BASE_CONDCODE_HH__