gcc/predict.def

/* Definitions for the branch prediction routines in the GNU compiler.
   Copyright (C) 2001-2024 Free Software Foundation, Inc.

This file is part of GCC.

GCC 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 3, or (at your option) any later
version.

GCC 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 GCC; see the file COPYING3.  If not see
<http://www.gnu.org/licenses/>.  */

/* Before including this file, you should define a macro:

     DEF_PREDICTOR (ENUM, NAME, HITRATE)

   This macro will be called once for each predictor.  The ENUM will
   be of type `enum br_predictor', and will enumerate all supported
   predictors.  The order of DEF_PREDICTOR calls is important, as
   in the first match combining heuristics, the predictor appearing
   first in this file will win.

   NAME is used in the debugging output to determine predictor type.

   HITRATE is the probability that edge predicted by predictor as taken
   will be really taken (so it should be always above
   REG_BR_PROB_BASE / 2).  */


/* A value used as final outcome of all heuristics.  */
DEF_PREDICTOR (PRED_COMBINED, "combined", PROB_ALWAYS, 0)

/* An outcome estimated by Dempster-Shaffer theory.  */
DEF_PREDICTOR (PRED_DS_THEORY, "DS theory", PROB_ALWAYS, 0)

/* A combined heuristics using probability determined by first
   matching heuristics from this list.  */
DEF_PREDICTOR (PRED_FIRST_MATCH, "first match", PROB_ALWAYS, 0)

/* Heuristic applying when no heuristic below applies.  */
DEF_PREDICTOR (PRED_NO_PREDICTION, "no prediction", PROB_ALWAYS, 0)

/* Mark unconditional jump as taken.  */
DEF_PREDICTOR (PRED_UNCONDITIONAL, "unconditional jump", PROB_ALWAYS,
	       PRED_FLAG_FIRST_MATCH)

/* Use number of loop iterations determined by # of iterations
   analysis to set probability.  We don't want to use Dempster-Shaffer
   theory here, as the predictions is exact.  */
DEF_PREDICTOR (PRED_LOOP_ITERATIONS, "loop iterations", PROB_UNINITIALIZED,
	       PRED_FLAG_FIRST_MATCH)

/* Hints provided by user via __builtin_expect_with_probability.  */
DEF_PREDICTOR (PRED_BUILTIN_EXPECT_WITH_PROBABILITY,
	       "__builtin_expect_with_probability", PROB_UNINITIALIZED,
	       PRED_FLAG_FIRST_MATCH)

/* Assume that any given atomic operation has low contention,
   and thus the compare-and-swap operation succeeds. */
DEF_PREDICTOR (PRED_COMPARE_AND_SWAP, "compare and swap", PROB_VERY_LIKELY,
	       PRED_FLAG_FIRST_MATCH)

/* Hints dropped by user via __builtin_expect feature.  Note: the
   probability of PROB_VERY_LIKELY is now overwritten by param
   builtin_expect_probability with a default value of HITRATE(90).
   Refer to param.def for details.  */
DEF_PREDICTOR (PRED_BUILTIN_EXPECT, "__builtin_expect", PROB_VERY_LIKELY,
	       PRED_FLAG_FIRST_MATCH)

/* Branches to hot labels are likely.  */
DEF_PREDICTOR (PRED_HOT_LABEL, "hot label", HITRATE (90),
	       PRED_FLAG_FIRST_MATCH)

/* Branches to cold labels are extremely unlikely.  */
DEF_PREDICTOR (PRED_COLD_LABEL, "cold label", HITRATE (90),
	       PRED_FLAG_FIRST_MATCH)

/* Return value of malloc function is almost always non-null.  */
DEF_PREDICTOR (PRED_MALLOC_NONNULL, "malloc returned non-NULL", \
	       PROB_VERY_LIKELY, PRED_FLAG_FIRST_MATCH)

/* Use number of loop iterations guessed by the contents of the loop.  */
DEF_PREDICTOR (PRED_LOOP_ITERATIONS_GUESSED, "guessed loop iterations",
	       PROB_UNINITIALIZED, PRED_FLAG_FIRST_MATCH)

/* Use number of loop iterations guessed by the contents of the loop.  */
DEF_PREDICTOR (PRED_LOOP_ITERATIONS_MAX, "guessed loop iterations",
	       PROB_UNINITIALIZED, PRED_FLAG_FIRST_MATCH)

/* Prediction which is an outcome of combining multiple value predictions.  */
DEF_PREDICTOR (PRED_COMBINED_VALUE_PREDICTIONS,
	       "combined value predictions", PROB_UNINITIALIZED, 0)

/* Prediction which is an outcome of combining multiple value predictions
   on PHI statement (this is less accurate since we do not know reverse
   edge probabilities at that time).  */
DEF_PREDICTOR (PRED_COMBINED_VALUE_PREDICTIONS_PHI,
	       "combined value predictions", PROB_UNINITIALIZED, 0)

/* Branch containing goto is probably not taken.  */
DEF_PREDICTOR (PRED_CONTINUE, "continue", HITRATE (67), 0)

/* Branch to basic block containing call marked by noreturn attribute.  */
DEF_PREDICTOR (PRED_NORETURN, "noreturn call", PROB_VERY_LIKELY,
	       PRED_FLAG_FIRST_MATCH)

/* Branch to basic block containing call marked by cold function attribute.  */
DEF_PREDICTOR (PRED_COLD_FUNCTION, "cold function call", PROB_VERY_LIKELY,
	       PRED_FLAG_FIRST_MATCH)

/* Edge causing loop to terminate is probably not taken.  */
DEF_PREDICTOR (PRED_LOOP_EXIT, "loop exit", HITRATE (89),
	       PRED_FLAG_FIRST_MATCH)

/* Same as LOOP_EXIT but for loops containing recursive call.  */
DEF_PREDICTOR (PRED_LOOP_EXIT_WITH_RECURSION, "loop exit with recursion",
	       HITRATE (78), PRED_FLAG_FIRST_MATCH)

/* Edge causing loop to terminate by computing value used by later
   conditional.  */
DEF_PREDICTOR (PRED_LOOP_EXTRA_EXIT, "extra loop exit", HITRATE (67),
	       PRED_FLAG_FIRST_MATCH)

/* Pointers are usually not NULL.  */
DEF_PREDICTOR (PRED_POINTER, "pointer", HITRATE (70), 0)
DEF_PREDICTOR (PRED_TREE_POINTER, "pointer (on trees)", HITRATE (70), 0)

/* NE is probable, EQ not etc...  */
DEF_PREDICTOR (PRED_OPCODE_POSITIVE, "opcode values positive", HITRATE (59), 0)
DEF_PREDICTOR (PRED_OPCODE_NONEQUAL, "opcode values nonequal", HITRATE (66), 0)
DEF_PREDICTOR (PRED_FPOPCODE, "fp_opcode", HITRATE (90), 0)
DEF_PREDICTOR (PRED_TREE_OPCODE_POSITIVE, "opcode values positive (on trees)",
	       HITRATE (59), 0)
DEF_PREDICTOR (PRED_TREE_OPCODE_NONEQUAL, "opcode values nonequal (on trees)",
	       HITRATE (66), 0)
DEF_PREDICTOR (PRED_TREE_FPOPCODE, "fp_opcode (on trees)", HITRATE (90), 0)

/* Branch guarding call is probably taken.  */
DEF_PREDICTOR (PRED_CALL, "call", HITRATE (67), 0)

/* Call predictors are for now ignored, lets leave the predictor
   to measure its benefit.  */
DEF_PREDICTOR (PRED_INDIR_CALL, "indirect call", PROB_EVEN, 0)
DEF_PREDICTOR (PRED_POLYMORPHIC_CALL, "polymorphic call", PROB_EVEN, 0)
DEF_PREDICTOR (PRED_RECURSIVE_CALL, "recursive call", PROB_EVEN, 0)

/* Branch causing function to terminate is probably not taken.  */
DEF_PREDICTOR (PRED_TREE_EARLY_RETURN, "early return (on trees)", HITRATE (66),
	       0)

/* Branch containing goto is probably not taken.  */
DEF_PREDICTOR (PRED_GOTO, "goto", HITRATE (66), 0)

/* Branch ending with return constant is probably not taken.  */
DEF_PREDICTOR (PRED_CONST_RETURN, "const return", HITRATE (65), 0)

/* Branch ending with return negative constant is probably not taken.  */
DEF_PREDICTOR (PRED_NEGATIVE_RETURN, "negative return", HITRATE (98), 0)

/* Branch ending with return; is probably not taken */
DEF_PREDICTOR (PRED_NULL_RETURN, "null return", HITRATE (71), 0)

/* Branches to compare induction variable to a loop bound is
   extremely likely.  */
DEF_PREDICTOR (PRED_LOOP_IV_COMPARE_GUESS, "guess loop iv compare",
	       HITRATE (64), 0)

/* Use number of loop iterations determined by # of iterations analysis
   to set probability of branches that compares IV to loop bound variable.  */
DEF_PREDICTOR (PRED_LOOP_IV_COMPARE, "loop iv compare", PROB_UNINITIALIZED,
	       PRED_FLAG_FIRST_MATCH)

/* In the following code
   for (loop1)
     if (cond)
       for (loop2)
	 body;
   guess that cond is unlikely.  */
DEF_PREDICTOR (PRED_LOOP_GUARD, "loop guard", HITRATE (73), 0)

/* Same but for loops containing recursion.  */
DEF_PREDICTOR (PRED_LOOP_GUARD_WITH_RECURSION, "loop guard with recursion",
	       HITRATE (85), 0)

/* The following predictors are used in Fortran. */

/* Branch leading to an integer overflow are extremely unlikely.  */
DEF_PREDICTOR (PRED_FORTRAN_OVERFLOW, "Fortran overflow", PROB_ALWAYS,
	       PRED_FLAG_FIRST_MATCH)

/* Branch leading to a failure status are unlikely.  This can occur for out
   of memory.  This predictor only occurs when the user explicitly asked
   for a return status.  By default, the code aborts,
   which is handled via PRED_NORETURN.  */
DEF_PREDICTOR (PRED_FORTRAN_FAIL_ALLOC, "Fortran fail alloc",
	       PROB_VERY_LIKELY, 0)

/* Predictor is used for an allocation of an already allocated memory or
   deallocating an already deallocated allocatable.  */
DEF_PREDICTOR (PRED_FORTRAN_REALLOC, "Fortran repeated allocation/deallocation",
	       PROB_LIKELY, 0)

/* Branch leading to an I/O failure status are unlikely.  This predictor is
   used for I/O failures such as for invalid unit numbers.  This predictor
   only occurs when the user explicitly asked for a return status.  By default,
   the code aborts, which is handled via PRED_NORETURN.  */
DEF_PREDICTOR (PRED_FORTRAN_FAIL_IO, "Fortran fail IO", HITRATE (85), 0)

/* Branch leading to a run-time warning message which is printed only once
   are unlikely.  The print-warning branch itself can be likely or unlikely.  */
DEF_PREDICTOR (PRED_FORTRAN_WARN_ONCE, "Fortran warn once", HITRATE (75), 0)

/* Branch belonging to a zero-sized array.  */
DEF_PREDICTOR (PRED_FORTRAN_SIZE_ZERO, "Fortran zero-sized array", \
	       HITRATE (99), 0)

/* Branch belonging to an invalid bound index, in a context where it is
   standard conform and well defined but rather pointless and, hence, rather
   unlikely to occur.  */
DEF_PREDICTOR (PRED_FORTRAN_INVALID_BOUND, "Fortran invalid bound", \
	       HITRATE (90), 0)

/* Branch belonging to the handling of absent optional arguments.  This
   predictor is used when an optional dummy argument, associated with an
   absent argument, is passed on as actual argument to another procedure,
   which in turn has an optional argument.  */
DEF_PREDICTOR (PRED_FORTRAN_ABSENT_DUMMY, "Fortran absent dummy", \
	       HITRATE (60), 0)

/* Fortran DO statement generates a pre-header guard:
   empty = (step > 0 ? to < from : to > from), which can be predicted
   to be very likely.  */
DEF_PREDICTOR (PRED_FORTRAN_LOOP_PREHEADER, "Fortran loop preheader", \
	       HITRATE (99), 0)

/* Fortran assumed size arrays can be non-contiguous, so they need
   to be repacked.  */

DEF_PREDICTOR (PRED_FORTRAN_CONTIGUOUS, "Fortran contiguous", \
	       HITRATE (75), 0)