/* Branch prediction routines for the GNU compiler. Copyright (C) 2000 Free Software Foundation, Inc. This file is part of GNU CC. GNU CC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU CC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU CC; see the file COPYING. If not, write to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ /* References: [1] "Branch Prediction for Free" Ball and Larus; PLDI '93. [2] "Static Branch Frequency and Program Profile Analysis" Wu and Larus; MICRO-27. [3] "Corpus-based Static Branch Prediction" Calder, Grunwald, Lindsay, Martin, Mozer, and Zorn; PLDI '95. */ #include "config.h" #include "system.h" #include "tree.h" #include "rtl.h" #include "tm_p.h" #include "basic-block.h" #include "insn-config.h" #include "regs.h" #include "hard-reg-set.h" #include "flags.h" #include "output.h" #include "function.h" #include "except.h" #include "toplev.h" #include "recog.h" #include "insn-flags.h" #include "expr.h" /* Statically estimate the probability that a branch will be taken. ??? In the next revision there will be a number of other predictors added from the above references. Further, each heuristic will be factored out into its own function for clarity (and to facilitate the combination of predictions). */ void estimate_probability (loops_info) struct loops *loops_info; { int i; /* Try to predict out blocks in a loop that are not part of a natural loop. */ for (i = 0; i < loops_info->num; i++) { int j; for (j = loops_info->array[i].first->index; j <= loops_info->array[i].last->index; ++j) { edge e; if (! TEST_BIT (loops_info->array[i].nodes, j)) for (e = BASIC_BLOCK(j)->pred; e; e = e->pred_next) if (TEST_BIT (loops_info->array[i].nodes, e->src->index)) { rtx last_insn = BLOCK_END (e->src->index); rtx cond, earliest; if (GET_CODE (last_insn) != JUMP_INSN || ! condjump_p (last_insn) || simplejump_p (last_insn)) continue; cond = get_condition (last_insn, &earliest); if (! cond) continue; if (! find_reg_note (last_insn, REG_BR_PROB, 0)) REG_NOTES (last_insn) = gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (REG_BR_PROB_BASE), REG_NOTES (last_insn)); } } } /* Attempt to predict conditional jumps using a number of heuristics. For each conditional jump, we try each heuristic in a fixed order. If more than one heuristic applies to a particular branch, the first is used as the prediction for the branch. */ for (i = 0; i < n_basic_blocks - 1; i++) { rtx last_insn = BLOCK_END (i); rtx cond, earliest; int prob = 0; if (GET_CODE (last_insn) != JUMP_INSN || ! condjump_p (last_insn) || simplejump_p (last_insn)) continue; cond = get_condition (last_insn, &earliest); if (! cond) continue; /* Try "pointer heuristic." A comparison ptr == 0 is predicted as false. Similarly, a comparison ptr1 == ptr2 is predicted as false. */ prob = 0; switch (GET_CODE (cond)) { case EQ: if (GET_CODE (XEXP (cond, 0)) == REG && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 0))) && (XEXP (cond, 1) == const0_rtx || (GET_CODE (XEXP (cond, 1)) == REG && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 1)))))) prob = REG_BR_PROB_BASE / 10; break; case NE: if (GET_CODE (XEXP (cond, 0)) == REG && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 0))) && (XEXP (cond, 1) == const0_rtx || (GET_CODE (XEXP (cond, 1)) == REG && REGNO_POINTER_FLAG (REGNO (XEXP (cond, 1)))))) prob = REG_BR_PROB_BASE / 2; break; default: prob = 0; } if (prob && ! find_reg_note (last_insn, REG_BR_PROB, 0)) REG_NOTES (last_insn) = gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob), REG_NOTES (last_insn)); /* Try "opcode heuristic." EQ tests are usually false and NE tests are usually true. Also, most quantities are positive, so we can make the appropriate guesses about signed comparisons against zero. */ switch (GET_CODE (cond)) { case CONST_INT: /* Unconditional branch. */ prob = REG_BR_PROB_BASE / 2; break; case EQ: prob = REG_BR_PROB_BASE / 10; break; case NE: prob = REG_BR_PROB_BASE / 2; break; case LE: case LT: if (XEXP (cond, 1) == const0_rtx) prob = REG_BR_PROB_BASE / 10; break; case GE: case GT: if (XEXP (cond, 1) == const0_rtx || (GET_CODE (XEXP (cond, 1)) == CONST_INT && INTVAL (XEXP (cond, 1)) == -1)) prob = REG_BR_PROB_BASE / 2; break; default: prob = 0; } if (! find_reg_note (last_insn, REG_BR_PROB, 0)) REG_NOTES (last_insn) = gen_rtx_EXPR_LIST (REG_BR_PROB, GEN_INT (prob), REG_NOTES (last_insn)); } } /* __builtin_expect dropped tokens into the insn stream describing expected values of registers. Generate branch probabilities based off these values. */ static rtx find_expected_value PARAMS ((rtx, rtx)); void expected_value_to_br_prob () { rtx insn, cond, ev = NULL_RTX, ev_reg; for (insn = get_insns (); insn ; insn = NEXT_INSN (insn)) { switch (GET_CODE (insn)) { case NOTE: /* Look for expected value notes. */ if (NOTE_LINE_NUMBER (insn) == NOTE_INSN_EXPECTED_VALUE) { ev = NOTE_EXPECTED_VALUE (insn); ev_reg = XEXP (ev, 0); } continue; case CODE_LABEL: /* Never propagate across labels. */ ev = NULL_RTX; continue; default: /* Look for insns that clobber the EV register. */ if (ev && reg_set_p (ev_reg, insn)) ev = NULL_RTX; continue; case JUMP_INSN: /* Look for simple conditional branches. If we havn't got an expected value yet, no point going further. */ if (GET_CODE (insn) != JUMP_INSN || ev == NULL_RTX) continue; if (! condjump_p (insn) || simplejump_p (insn)) continue; break; } /* Collect the branch condition, hopefully relative to EV_REG. */ cond = XEXP (SET_SRC (PATTERN (insn)), 0); cond = canonicalize_condition (insn, cond, 0, NULL, ev_reg); if (! cond || XEXP (cond, 0) != ev_reg) continue; /* Substitute and simplify. Given that the expression we're building involves two constants, we should wind up with either true or false. */ cond = gen_rtx_fmt_ee (GET_CODE (cond), VOIDmode, XEXP (ev, 1), XEXP (cond, 1)); cond = simplify_rtx (cond); /* Turn the condition into a scaled branch probability. */ if (cond == const1_rtx) cond = GEN_INT (REG_BR_PROB_BASE); else if (cond != const0_rtx) abort (); REG_NOTES (insn) = alloc_EXPR_LIST (REG_BR_PROB, cond, REG_NOTES (insn)); } }