1 2 /*--------------------------------------------------------------------*/ 3 /*--- Branch predictor simulation cg_branchpred.c ---*/ 4 /*--------------------------------------------------------------------*/ 5 6 /* 7 This file is part of Cachegrind, a Valgrind tool for cache 8 profiling programs. 9 10 Copyright (C) 2002-2010 Nicholas Nethercote 11 njn (at) valgrind.org 12 13 This program is free software; you can redistribute it and/or 14 modify it under the terms of the GNU General Public License as 15 published by the Free Software Foundation; either version 2 of the 16 License, or (at your option) any later version. 17 18 This program is distributed in the hope that it will be useful, but 19 WITHOUT ANY WARRANTY; without even the implied warranty of 20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 21 General Public License for more details. 22 23 You should have received a copy of the GNU General Public License 24 along with this program; if not, write to the Free Software 25 Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 26 02111-1307, USA. 27 28 The GNU General Public License is contained in the file COPYING. 29 */ 30 31 32 /* This file contains the actual branch predictor simulator and its 33 associated state. As with cg_sim.c it is #included directly into 34 cg_main.c. It provides: 35 36 - a taken/not-taken predictor for conditional branches 37 - a branch target address predictor for indirect branches 38 39 Function return-address prediction is not modelled, on the basis 40 that return stack predictors almost always predict correctly, and 41 also that it is difficult for Valgrind to robustly identify 42 function calls and returns. 43 */ 44 45 /* How many bits at the bottom of an instruction address are 46 guaranteed to be zero? */ 47 #if defined(VGA_ppc32) || defined(VGA_ppc64) || defined(VGA_arm) 48 # define N_IADDR_LO_ZERO_BITS 2 49 #elif defined(VGA_x86) || defined(VGA_amd64) 50 # define N_IADDR_LO_ZERO_BITS 0 51 #else 52 # error "Unsupported architecture" 53 #endif 54 55 56 /* Get a taken/not-taken prediction for the instruction (presumably a 57 conditional branch) at instr_addr. Once that's done, update the 58 predictor state based on whether or not it was actually taken, as 59 indicated by 'taken'. Finally, return 1 for a mispredict and 0 for 60 a successful predict. 61 62 The predictor is an array of 16k (== 2^14) 2-bit saturating 63 counters. Given the address of the branch instruction, the array 64 index to use is computed both from the low order bits of the branch 65 instruction's address, and the global history - that is, from the 66 taken/not-taken behaviour of the most recent few branches. This 67 makes the predictor able to correlate this branch's behaviour with 68 that of other branches. 69 70 TODO: use predictor written by someone who understands this stuff. 71 Perhaps it would be better to move to a standard GShare predictor 72 and/or tournament predictor. 73 */ 74 /* The index is composed of N_HIST bits at the top and N_IADD bits at 75 the bottom. These numbers chosen somewhat arbitrarily, but note 76 that making N_IADD_BITS too small (eg 4) can cause large amounts of 77 aliasing, and hence misprediction, particularly if the history bits 78 are mostly unchanging. */ 79 #define N_HIST_BITS 7 80 #define N_IADD_BITS 7 81 82 #define N_BITS (N_HIST_BITS + N_IADD_BITS) 83 #define N_COUNTERS (1 << N_BITS) 84 85 static UWord shift_register = 0; /* Contains global history */ 86 static UChar counters[N_COUNTERS]; /* Counter array; presumably auto-zeroed */ 87 88 89 static ULong do_cond_branch_predict ( Addr instr_addr, Word takenW ) 90 { 91 UWord indx; 92 Bool predicted_taken, actually_taken, mispredict; 93 94 const UWord hist_mask = (1 << N_HIST_BITS) - 1; 95 const UWord iadd_mask = (1 << N_IADD_BITS) - 1; 96 UWord hist_bits = shift_register & hist_mask; 97 UWord iadd_bits = (instr_addr >> N_IADDR_LO_ZERO_BITS) 98 & iadd_mask; 99 100 tl_assert(hist_bits <= hist_mask); 101 tl_assert(iadd_bits <= iadd_mask); 102 indx = (hist_bits << N_IADD_BITS) | iadd_bits; 103 tl_assert(indx < N_COUNTERS); 104 if (0) VG_(printf)("index = %d\n", (Int)indx); 105 106 tl_assert(takenW <= 1); 107 predicted_taken = counters[ indx ] >= 2; 108 actually_taken = takenW > 0; 109 110 mispredict = (actually_taken && (!predicted_taken)) 111 || ((!actually_taken) && predicted_taken); 112 113 shift_register <<= 1; 114 shift_register |= (actually_taken ? 1 : 0); 115 116 if (actually_taken) { 117 if (counters[indx] < 3) 118 counters[indx]++; 119 } else { 120 if (counters[indx] > 0) 121 counters[indx]--; 122 } 123 124 tl_assert(counters[indx] <= 3); 125 126 return mispredict ? 1 : 0; 127 } 128 129 130 /* A very simple indirect branch predictor. Use the branch's address 131 to index a table which records the previous target address for this 132 branch (or whatever aliased with it) and use that as the 133 prediction. */ 134 #define N_BTAC_BITS 9 135 #define N_BTAC (1 << N_BTAC_BITS) 136 static Addr btac[N_BTAC]; /* BTAC; presumably auto-zeroed */ 137 138 static ULong do_ind_branch_predict ( Addr instr_addr, Addr actual ) 139 { 140 Bool mispredict; 141 const UWord mask = (1 << N_BTAC_BITS) - 1; 142 UWord indx = (instr_addr >> N_IADDR_LO_ZERO_BITS) 143 & mask; 144 tl_assert(indx < N_BTAC); 145 mispredict = btac[indx] != actual; 146 btac[indx] = actual; 147 return mispredict ? 1 : 0; 148 } 149 150 151 /*--------------------------------------------------------------------*/ 152 /*--- end cg_branchpred.c ---*/ 153 /*--------------------------------------------------------------------*/ 154 155
