1 /* Basic block reordering routines for the GNU compiler.
2 Copyright (C) 2000 Free Software Foundation, Inc.
4 This file is part of GCC.
6 GCC is free software; you can redistribute it and/or modify it
7 under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2, or (at your option)
11 GCC is distributed in the hope that it will be useful, but WITHOUT
12 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
13 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
14 License for more details.
16 You should have received a copy of the GNU General Public License
17 along with GCC; see the file COPYING. If not, write to the Free
18 Software Foundation, 59 Temple Place - Suite 330, Boston, MA
23 "Profile Guided Code Positioning"
24 Pettis and Hanson; PLDI '90.
30 if (p) goto A; // predict taken
33 if (q) goto B; // predict taken
39 We'll currently reorder this as
68 This requires that we be able to duplicate the jump at A, and
69 adjust the graph traversal such that greedy placement doesn't
70 fix D before C is considered.
72 (2) Coordinate with shorten_branches to minimize the number of
75 (3) Invent a method by which sufficiently non-predicted code can
76 be moved to either the end of the section or another section
77 entirely. Some sort of NOTE_INSN note would work fine.
79 This completely scroggs all debugging formats, so the user
80 would have to explicitly ask for it.
87 #include "hard-reg-set.h"
88 #include "basic-block.h"
91 #include "cfglayout.h"
94 #define HAVE_epilogue 0
97 /* Local function prototypes. */
98 static void make_reorder_chain PARAMS ((void));
99 static basic_block make_reorder_chain_1 PARAMS ((basic_block, basic_block));
101 /* Compute an ordering for a subgraph beginning with block BB. Record the
102 ordering in RBI()->index and chained through RBI()->next. */
105 make_reorder_chain ()
107 basic_block last_block = NULL;
108 basic_block prev = NULL;
109 int nbb_m1 = n_basic_blocks - 1;
112 /* If we've not got epilogue in RTL, we must fallthru to the exit.
113 Force the last block to be at the end. */
114 /* ??? Some ABIs (e.g. MIPS) require the return insn to be at the
115 end of the function for stack unwinding purposes. */
118 last_block = BASIC_BLOCK (nbb_m1);
119 RBI (last_block)->visited = 1;
123 /* Loop until we've placed every block. */
130 /* Find the next unplaced block. */
131 /* ??? Get rid of this loop, and track which blocks are not yet
132 placed more directly, so as to avoid the O(N^2) worst case.
133 Perhaps keep a doubly-linked list of all to-be-placed blocks;
134 remove from the list as we place. The head of that list is
135 what we're looking for here. */
137 for (i = 0; i <= nbb_m1 && !next; ++i)
139 basic_block bb = BASIC_BLOCK (i);
140 if (! RBI (bb)->visited)
144 prev = make_reorder_chain_1 (next, prev);
148 /* Terminate the chain. */
151 RBI (prev)->next = last_block;
154 RBI (prev)->next = NULL;
157 /* A helper function for make_reorder_chain.
159 We do not follow EH edges, or non-fallthru edges to noreturn blocks.
160 These are assumed to be the error condition and we wish to cluster
161 all of them at the very end of the function for the benefit of cache
162 locality for the rest of the function.
164 ??? We could do slightly better by noticing earlier that some subgraph
165 has all paths leading to noreturn functions, but for there to be more
166 than one block in such a subgraph is rare. */
169 make_reorder_chain_1 (bb, prev)
177 /* Mark this block visited. */
181 RBI (prev)->next = bb;
183 if (rtl_dump_file && prev->index + 1 != bb->index)
184 fprintf (rtl_dump_file, "Reordering block %d after %d\n",
185 bb->index, prev->index);
192 RBI (bb)->visited = 1;
195 if (bb->succ == NULL)
198 /* Find the most probable block. */
201 if (any_condjump_p (bb->end)
202 && (note = find_reg_note (bb->end, REG_BR_PROB, 0)) != NULL)
204 int taken, probability;
205 edge e_taken, e_fall;
207 probability = INTVAL (XEXP (note, 0));
208 taken = probability > REG_BR_PROB_BASE / 2;
210 /* Find the normal taken edge and the normal fallthru edge.
212 Note, conditional jumps with other side effects may not
213 be fully optimized. In this case it is possible for
214 the conditional jump to branch to the same location as
217 We should probably work to improve optimization of that
218 case; however, it seems silly not to also deal with such
219 problems here if they happen to occur. */
221 e_taken = e_fall = NULL;
222 for (e = bb->succ; e ; e = e->succ_next)
224 if (e->flags & EDGE_FALLTHRU)
226 else if (! (e->flags & EDGE_EH))
230 next = (taken ? e_taken : e_fall)->dest;
233 /* In the absence of a prediction, disturb things as little as possible
234 by selecting the old "next" block from the list of successors. If
235 there had been a fallthru edge, that will be the one. */
238 for (e = bb->succ; e ; e = e->succ_next)
239 if (e->dest->index == bb->index + 1)
241 if ((e->flags & EDGE_FALLTHRU)
243 && ! (e->flags & (EDGE_ABNORMAL_CALL | EDGE_EH))))
249 /* Make sure we didn't select a silly next block. */
250 if (! next || next == EXIT_BLOCK_PTR || RBI (next)->visited)
253 /* Recurse on the successors. Unroll the last call, as the normal
254 case is exactly one or two edges, and we can tail recurse. */
255 for (e = bb->succ; e; e = e->succ_next)
256 if (e->dest != EXIT_BLOCK_PTR
257 && ! RBI (e->dest)->visited
259 && ! (e->flags & (EDGE_ABNORMAL_CALL | EDGE_EH)))
263 prev = make_reorder_chain_1 (next, prev);
264 next = RBI (e->dest)->visited ? NULL : e->dest;
278 /* Reorder basic blocks. The main entry point to this file. */
281 reorder_basic_blocks ()
283 if (n_basic_blocks <= 1)
286 cfg_layout_initialize ();
288 make_reorder_chain ();
291 dump_flow_info (rtl_dump_file);
293 cfg_layout_finalize ();