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323 lines
8.0 KiB
C
323 lines
8.0 KiB
C
/* Basic block reordering routines for the GNU compiler.
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Copyright (C) 2000, 2002 Free Software Foundation, Inc.
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This file is part of GCC.
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GCC is free software; you can redistribute it and/or modify it
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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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GCC is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
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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 GCC; see the file COPYING. If not, write to the Free
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Software Foundation, 59 Temple Place - Suite 330, Boston, MA
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02111-1307, USA. */
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/* References:
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"Profile Guided Code Positioning"
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Pettis and Hanson; PLDI '90.
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TODO:
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(1) Consider:
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if (p) goto A; // predict taken
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foo ();
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A:
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if (q) goto B; // predict taken
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bar ();
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B:
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baz ();
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return;
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We'll currently reorder this as
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if (!p) goto C;
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A:
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if (!q) goto D;
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B:
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baz ();
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return;
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D:
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bar ();
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goto B;
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C:
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foo ();
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goto A;
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A better ordering is
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if (!p) goto C;
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if (!q) goto D;
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B:
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baz ();
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return;
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C:
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foo ();
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if (q) goto B;
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D:
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bar ();
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goto B;
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This requires that we be able to duplicate the jump at A, and
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adjust the graph traversal such that greedy placement doesn't
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fix D before C is considered.
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(2) Coordinate with shorten_branches to minimize the number of
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long branches.
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(3) Invent a method by which sufficiently non-predicted code can
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be moved to either the end of the section or another section
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entirely. Some sort of NOTE_INSN note would work fine.
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This completely scroggs all debugging formats, so the user
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would have to explicitly ask for it.
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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 "hard-reg-set.h"
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#include "basic-block.h"
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#include "flags.h"
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#include "output.h"
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#include "cfglayout.h"
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#include "function.h"
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#include "target.h"
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/* Local function prototypes. */
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static void make_reorder_chain PARAMS ((void));
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static basic_block make_reorder_chain_1 PARAMS ((basic_block, basic_block));
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static basic_block maybe_duplicate_computed_goto_succ PARAMS ((basic_block));
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/* Compute an ordering for a subgraph beginning with block BB. Record the
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ordering in RBI()->index and chained through RBI()->next. */
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static void
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make_reorder_chain ()
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{
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basic_block prev = NULL;
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basic_block next, bb;
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/* Loop until we've placed every block. */
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do
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{
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next = NULL;
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/* Find the next unplaced block. */
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/* ??? Get rid of this loop, and track which blocks are not yet
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placed more directly, so as to avoid the O(N^2) worst case.
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Perhaps keep a doubly-linked list of all to-be-placed blocks;
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remove from the list as we place. The head of that list is
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what we're looking for here. */
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FOR_EACH_BB (bb)
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if (! RBI (bb)->visited)
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{
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next = bb;
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break;
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}
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if (next)
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prev = make_reorder_chain_1 (next, prev);
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}
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while (next);
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RBI (prev)->next = NULL;
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}
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/* If the successor is our artificial computed_jump block, duplicate it. */
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static inline basic_block
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maybe_duplicate_computed_goto_succ (bb)
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basic_block bb;
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{
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edge e;
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basic_block next;
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/* Note that we can't rely on computed_goto_common_label still being in
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the instruction stream -- cfgloop.c likes to munge things about. But
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we can still use it's non-null-ness to avoid a fruitless search. */
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if (!cfun->computed_goto_common_label)
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return NULL;
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/* Only want to duplicate when coming from a simple branch. */
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e = bb->succ;
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if (!e || e->succ_next)
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return NULL;
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/* Only duplicate if we've already layed out this block once. */
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next = e->dest;
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if (!RBI (next)->visited)
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return NULL;
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/* See if the block contains only a computed branch. */
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if ((next->head == next->end
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|| next_active_insn (next->head) == next->end)
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&& computed_jump_p (next->end))
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{
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if (rtl_dump_file)
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fprintf (rtl_dump_file, "Duplicating block %d after %d\n",
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next->index, bb->index);
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return cfg_layout_duplicate_bb (next, e);
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}
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return NULL;
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}
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/* A helper function for make_reorder_chain.
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We do not follow EH edges, or non-fallthru edges to noreturn blocks.
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These are assumed to be the error condition and we wish to cluster
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all of them at the very end of the function for the benefit of cache
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locality for the rest of the function.
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??? We could do slightly better by noticing earlier that some subgraph
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has all paths leading to noreturn functions, but for there to be more
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than one block in such a subgraph is rare. */
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static basic_block
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make_reorder_chain_1 (bb, prev)
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basic_block bb;
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basic_block prev;
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{
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edge e;
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basic_block next;
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rtx note;
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/* Mark this block visited. */
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if (prev)
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{
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restart:
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RBI (prev)->next = bb;
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if (rtl_dump_file && prev->next_bb != bb)
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fprintf (rtl_dump_file, "Reordering block %d after %d\n",
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bb->index, prev->index);
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}
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else
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{
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if (bb->prev_bb != ENTRY_BLOCK_PTR)
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abort ();
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}
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RBI (bb)->visited = 1;
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prev = bb;
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if (bb->succ == NULL)
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return prev;
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/* Find the most probable block. */
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next = NULL;
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if (any_condjump_p (bb->end)
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&& (note = find_reg_note (bb->end, REG_BR_PROB, 0)) != NULL)
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{
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int taken, probability;
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edge e_taken, e_fall;
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probability = INTVAL (XEXP (note, 0));
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taken = probability > REG_BR_PROB_BASE / 2;
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/* Find the normal taken edge and the normal fallthru edge.
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Note, conditional jumps with other side effects may not
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be fully optimized. In this case it is possible for
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the conditional jump to branch to the same location as
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the fallthru path.
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We should probably work to improve optimization of that
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case; however, it seems silly not to also deal with such
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problems here if they happen to occur. */
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e_taken = e_fall = NULL;
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for (e = bb->succ; e ; e = e->succ_next)
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{
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if (e->flags & EDGE_FALLTHRU)
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e_fall = e;
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else if (! (e->flags & EDGE_EH))
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e_taken = e;
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}
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next = ((taken && e_taken) ? e_taken : e_fall)->dest;
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}
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/* If the successor is our artificial computed_jump block, duplicate it. */
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else
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next = maybe_duplicate_computed_goto_succ (bb);
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/* In the absence of a prediction, disturb things as little as possible
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by selecting the old "next" block from the list of successors. If
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there had been a fallthru edge, that will be the one. */
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/* Note that the fallthru block may not be next any time we eliminate
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forwarder blocks. */
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if (! next)
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{
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for (e = bb->succ; e ; e = e->succ_next)
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if (e->flags & EDGE_FALLTHRU)
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{
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next = e->dest;
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break;
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}
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else if (e->dest == bb->next_bb)
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{
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if (! (e->flags & (EDGE_ABNORMAL_CALL | EDGE_EH)))
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next = e->dest;
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}
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}
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/* Make sure we didn't select a silly next block. */
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if (! next || next == EXIT_BLOCK_PTR || RBI (next)->visited)
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next = NULL;
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/* Recurse on the successors. Unroll the last call, as the normal
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case is exactly one or two edges, and we can tail recurse. */
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for (e = bb->succ; e; e = e->succ_next)
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if (e->dest != EXIT_BLOCK_PTR
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&& ! RBI (e->dest)->visited
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&& e->dest->succ
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&& ! (e->flags & (EDGE_ABNORMAL_CALL | EDGE_EH)))
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{
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if (next)
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{
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prev = make_reorder_chain_1 (next, prev);
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next = RBI (e->dest)->visited ? NULL : e->dest;
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}
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else
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next = e->dest;
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}
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if (next)
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{
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bb = next;
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goto restart;
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}
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return prev;
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}
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/* Reorder basic blocks. The main entry point to this file. */
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void
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reorder_basic_blocks ()
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{
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if (n_basic_blocks <= 1)
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return;
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if ((* targetm.cannot_modify_jumps_p) ())
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return;
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cfg_layout_initialize ();
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make_reorder_chain ();
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if (rtl_dump_file)
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dump_flow_info (rtl_dump_file);
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cfg_layout_finalize ();
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}
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