1 /* Basic block reordering routines for the GNU compiler.
2 Copyright (C) 2000, 2002 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 /* Local function prototypes. */
95 static void make_reorder_chain
PARAMS ((void));
96 static basic_block make_reorder_chain_1
PARAMS ((basic_block
, basic_block
));
98 /* Compute an ordering for a subgraph beginning with block BB. Record the
99 ordering in RBI()->index and chained through RBI()->next. */
102 make_reorder_chain ()
104 basic_block prev
= NULL
;
105 basic_block next
, bb
;
107 /* Loop until we've placed every block. */
112 /* Find the next unplaced block. */
113 /* ??? Get rid of this loop, and track which blocks are not yet
114 placed more directly, so as to avoid the O(N^2) worst case.
115 Perhaps keep a doubly-linked list of all to-be-placed blocks;
116 remove from the list as we place. The head of that list is
117 what we're looking for here. */
120 if (! RBI (bb
)->visited
)
127 prev
= make_reorder_chain_1 (next
, prev
);
130 RBI (prev
)->next
= NULL
;
133 /* A helper function for make_reorder_chain.
135 We do not follow EH edges, or non-fallthru edges to noreturn blocks.
136 These are assumed to be the error condition and we wish to cluster
137 all of them at the very end of the function for the benefit of cache
138 locality for the rest of the function.
140 ??? We could do slightly better by noticing earlier that some subgraph
141 has all paths leading to noreturn functions, but for there to be more
142 than one block in such a subgraph is rare. */
145 make_reorder_chain_1 (bb
, prev
)
153 /* Mark this block visited. */
157 RBI (prev
)->next
= bb
;
159 if (rtl_dump_file
&& prev
->next_bb
!= bb
)
160 fprintf (rtl_dump_file
, "Reordering block %d after %d\n",
161 bb
->sindex
, prev
->sindex
);
165 if (bb
->prev_bb
!= ENTRY_BLOCK_PTR
)
168 RBI (bb
)->visited
= 1;
171 if (bb
->succ
== NULL
)
174 /* Find the most probable block. */
177 if (any_condjump_p (bb
->end
)
178 && (note
= find_reg_note (bb
->end
, REG_BR_PROB
, 0)) != NULL
)
180 int taken
, probability
;
181 edge e_taken
, e_fall
;
183 probability
= INTVAL (XEXP (note
, 0));
184 taken
= probability
> REG_BR_PROB_BASE
/ 2;
186 /* Find the normal taken edge and the normal fallthru edge.
188 Note, conditional jumps with other side effects may not
189 be fully optimized. In this case it is possible for
190 the conditional jump to branch to the same location as
193 We should probably work to improve optimization of that
194 case; however, it seems silly not to also deal with such
195 problems here if they happen to occur. */
197 e_taken
= e_fall
= NULL
;
198 for (e
= bb
->succ
; e
; e
= e
->succ_next
)
200 if (e
->flags
& EDGE_FALLTHRU
)
202 else if (! (e
->flags
& EDGE_EH
))
206 next
= ((taken
&& e_taken
) ? e_taken
: e_fall
)->dest
;
209 /* In the absence of a prediction, disturb things as little as possible
210 by selecting the old "next" block from the list of successors. If
211 there had been a fallthru edge, that will be the one. */
214 for (e
= bb
->succ
; e
; e
= e
->succ_next
)
215 if (e
->dest
== bb
->next_bb
)
217 if ((e
->flags
& EDGE_FALLTHRU
)
219 && ! (e
->flags
& (EDGE_ABNORMAL_CALL
| EDGE_EH
))))
225 /* Make sure we didn't select a silly next block. */
226 if (! next
|| next
== EXIT_BLOCK_PTR
|| RBI (next
)->visited
)
229 /* Recurse on the successors. Unroll the last call, as the normal
230 case is exactly one or two edges, and we can tail recurse. */
231 for (e
= bb
->succ
; e
; e
= e
->succ_next
)
232 if (e
->dest
!= EXIT_BLOCK_PTR
233 && ! RBI (e
->dest
)->visited
235 && ! (e
->flags
& (EDGE_ABNORMAL_CALL
| EDGE_EH
)))
239 prev
= make_reorder_chain_1 (next
, prev
);
240 next
= RBI (e
->dest
)->visited
? NULL
: e
->dest
;
254 /* Reorder basic blocks. The main entry point to this file. */
257 reorder_basic_blocks ()
259 if (num_basic_blocks
<= 1)
262 if ((* targetm
.cannot_modify_jumps_p
) ())
265 cfg_layout_initialize ();
267 make_reorder_chain ();
270 dump_flow_info (rtl_dump_file
);
272 cfg_layout_finalize ();