144 lines
4.0 KiB
C
144 lines
4.0 KiB
C
/* Branch prediction routines for the GNU compiler.
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Copyright (C) 2000 Free Software Foundation, Inc.
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This file is part of GNU CC.
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GNU CC is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU CC is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU CC; see the file COPYING. If not, write to
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the Free Software Foundation, 59 Temple Place - Suite 330,
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Boston, MA 02111-1307, USA. */
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/* References:
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[1] "Branch Prediction for Free"
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Ball and Larus; PLDI '93.
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[2] "Static Branch Frequency and Program Profile Analysis"
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Wu and Larus; MICRO-27.
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[3] "Corpus-based Static Branch Prediction"
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Calder, Grunwald, Lindsay, Martin, Mozer, and Zorn; PLDI '95.
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*/
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#include "config.h"
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#include "system.h"
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#include "tree.h"
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#include "rtl.h"
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#include "tm_p.h"
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#include "basic-block.h"
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#include "insn-config.h"
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#include "regs.h"
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#include "hard-reg-set.h"
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#include "flags.h"
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#include "output.h"
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#include "function.h"
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#include "except.h"
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#include "toplev.h"
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#include "recog.h"
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#include "insn-flags.h"
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#include "expr.h"
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/* Statically estimate the probability that a branch will be taken.
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??? In the next revision there will be a number of other predictors added
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from the above references. Further, each heuristic will be factored out
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into its own function for clarity (and to facilitate the combination of
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predictions). */
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void
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estimate_probability (loops_info)
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struct loops *loops_info;
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{
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int i;
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/* Try to predict out blocks in a loop that are not part of a natural loop */
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for (i = 0; i < loops_info->num; i++)
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{
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int j;
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for (j = loops_info->array[i].header->index;
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j <= loops_info->array[i].latch->index;
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++j)
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{
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edge e;
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if (! TEST_BIT (loops_info->array[i].nodes, j))
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for (e = BASIC_BLOCK(j)->pred; e; e = e->pred_next)
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if (TEST_BIT (loops_info->array[i].nodes, e->src->index))
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{
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rtx last_insn = BLOCK_END (e->src->index);
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rtx cond, earliest;
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if (GET_CODE (last_insn) != JUMP_INSN
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|| ! condjump_p (last_insn) || simplejump_p (last_insn))
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continue;
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cond = get_condition (last_insn, &earliest);
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if (!cond)
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continue;
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if (! find_reg_note (last_insn, REG_BR_PROB, 0))
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REG_NOTES (last_insn)
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= gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (REG_BR_PROB_BASE),
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REG_NOTES (last_insn));
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}
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}
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}
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/* Try to predict condjumps using same algorithm as mostly_true_jump */
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for (i = 0; i < n_basic_blocks - 1; i++)
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{
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rtx last_insn = BLOCK_END (i);
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rtx cond, earliest;
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int prob = 0;
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if (GET_CODE (last_insn) != JUMP_INSN
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|| ! condjump_p (last_insn) || simplejump_p (last_insn))
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continue;
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cond = get_condition (last_insn, &earliest);
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if (! cond)
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continue;
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/* EQ tests are usually false and NE tests are usually true. Also,
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most quantities are positive, so we can make the appropriate guesses
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about signed comparisons against zero. */
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switch (GET_CODE (cond))
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{
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case CONST_INT:
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/* Unconditional branch. */
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prob = REG_BR_PROB_BASE / 2;
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case EQ:
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prob = REG_BR_PROB_BASE / 10;
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case NE:
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prob = REG_BR_PROB_BASE / 2;
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case LE:
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case LT:
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if (XEXP (cond, 1) == const0_rtx)
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prob = REG_BR_PROB_BASE / 10;
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break;
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case GE:
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case GT:
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if (XEXP (cond, 1) == const0_rtx
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|| (GET_CODE (XEXP (cond, 1)) == CONST_INT
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&& INTVAL (XEXP (cond, 1)) == -1))
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prob = REG_BR_PROB_BASE / 2;
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break;
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default:
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prob = 0;
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}
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if (! find_reg_note (last_insn, REG_BR_PROB, 0))
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REG_NOTES (last_insn)
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= gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob),
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REG_NOTES (last_insn));
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}
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}
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