freebsd-dev/contrib/gcc/bb-reorder.c

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/* Basic block reordering routines for the GNU compiler.
Copyright (C) 2000, 2002 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 2, 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 COPYING. If not, write to the Free
Software Foundation, 59 Temple Place - Suite 330, Boston, MA
02111-1307, USA. */
/* References:
"Profile Guided Code Positioning"
Pettis and Hanson; PLDI '90.
TODO:
(1) Consider:
if (p) goto A; // predict taken
foo ();
A:
if (q) goto B; // predict taken
bar ();
B:
baz ();
return;
We'll currently reorder this as
if (!p) goto C;
A:
if (!q) goto D;
B:
baz ();
return;
D:
bar ();
goto B;
C:
foo ();
goto A;
A better ordering is
if (!p) goto C;
if (!q) goto D;
B:
baz ();
return;
C:
foo ();
if (q) goto B;
D:
bar ();
goto B;
This requires that we be able to duplicate the jump at A, and
adjust the graph traversal such that greedy placement doesn't
fix D before C is considered.
(2) Coordinate with shorten_branches to minimize the number of
long branches.
(3) Invent a method by which sufficiently non-predicted code can
be moved to either the end of the section or another section
entirely. Some sort of NOTE_INSN note would work fine.
This completely scroggs all debugging formats, so the user
would have to explicitly ask for it.
*/
#include "config.h"
#include "system.h"
#include "tree.h"
#include "rtl.h"
#include "hard-reg-set.h"
#include "basic-block.h"
#include "flags.h"
#include "output.h"
#include "cfglayout.h"
#include "target.h"
/* Local function prototypes. */
static void make_reorder_chain PARAMS ((void));
static basic_block make_reorder_chain_1 PARAMS ((basic_block, basic_block));
/* Compute an ordering for a subgraph beginning with block BB. Record the
ordering in RBI()->index and chained through RBI()->next. */
static void
make_reorder_chain ()
{
basic_block prev = NULL;
int nbb_m1 = n_basic_blocks - 1;
basic_block next;
/* Loop until we've placed every block. */
do
{
int i;
next = NULL;
/* Find the next unplaced block. */
/* ??? Get rid of this loop, and track which blocks are not yet
placed more directly, so as to avoid the O(N^2) worst case.
Perhaps keep a doubly-linked list of all to-be-placed blocks;
remove from the list as we place. The head of that list is
what we're looking for here. */
for (i = 0; i <= nbb_m1 && !next; ++i)
{
basic_block bb = BASIC_BLOCK (i);
if (! RBI (bb)->visited)
next = bb;
}
if (next)
prev = make_reorder_chain_1 (next, prev);
}
while (next);
RBI (prev)->next = NULL;
}
/* A helper function for make_reorder_chain.
We do not follow EH edges, or non-fallthru edges to noreturn blocks.
These are assumed to be the error condition and we wish to cluster
all of them at the very end of the function for the benefit of cache
locality for the rest of the function.
??? We could do slightly better by noticing earlier that some subgraph
has all paths leading to noreturn functions, but for there to be more
than one block in such a subgraph is rare. */
static basic_block
make_reorder_chain_1 (bb, prev)
basic_block bb;
basic_block prev;
{
edge e;
basic_block next;
rtx note;
/* Mark this block visited. */
if (prev)
{
restart:
RBI (prev)->next = bb;
if (rtl_dump_file && prev->index + 1 != bb->index)
fprintf (rtl_dump_file, "Reordering block %d after %d\n",
bb->index, prev->index);
}
else
{
if (bb->index != 0)
abort ();
}
RBI (bb)->visited = 1;
prev = bb;
if (bb->succ == NULL)
return prev;
/* Find the most probable block. */
next = NULL;
if (any_condjump_p (bb->end)
&& (note = find_reg_note (bb->end, REG_BR_PROB, 0)) != NULL)
{
int taken, probability;
edge e_taken, e_fall;
probability = INTVAL (XEXP (note, 0));
taken = probability > REG_BR_PROB_BASE / 2;
/* Find the normal taken edge and the normal fallthru edge.
Note, conditional jumps with other side effects may not
be fully optimized. In this case it is possible for
the conditional jump to branch to the same location as
the fallthru path.
We should probably work to improve optimization of that
case; however, it seems silly not to also deal with such
problems here if they happen to occur. */
e_taken = e_fall = NULL;
for (e = bb->succ; e ; e = e->succ_next)
{
if (e->flags & EDGE_FALLTHRU)
e_fall = e;
else if (! (e->flags & EDGE_EH))
e_taken = e;
}
next = (taken ? e_taken : e_fall)->dest;
}
/* In the absence of a prediction, disturb things as little as possible
by selecting the old "next" block from the list of successors. If
there had been a fallthru edge, that will be the one. */
/* Note that the fallthru block may not be next any time we eliminate
forwarder blocks. */
if (! next)
{
for (e = bb->succ; e ; e = e->succ_next)
if (e->flags & EDGE_FALLTHRU)
{
next = e->dest;
break;
}
else if (e->dest->index == bb->index + 1)
{
if (! (e->flags & (EDGE_ABNORMAL_CALL | EDGE_EH)))
next = e->dest;
}
}
/* Make sure we didn't select a silly next block. */
if (! next || next == EXIT_BLOCK_PTR || RBI (next)->visited)
next = NULL;
/* Recurse on the successors. Unroll the last call, as the normal
case is exactly one or two edges, and we can tail recurse. */
for (e = bb->succ; e; e = e->succ_next)
if (e->dest != EXIT_BLOCK_PTR
&& ! RBI (e->dest)->visited
&& e->dest->succ
&& ! (e->flags & (EDGE_ABNORMAL_CALL | EDGE_EH)))
{
if (next)
{
prev = make_reorder_chain_1 (next, prev);
next = RBI (e->dest)->visited ? NULL : e->dest;
}
else
next = e->dest;
}
if (next)
{
bb = next;
goto restart;
}
return prev;
}
/* Reorder basic blocks. The main entry point to this file. */
void
reorder_basic_blocks ()
{
if (n_basic_blocks <= 1)
return;
if ((* targetm.cannot_modify_jumps_p) ())
return;
cfg_layout_initialize ();
make_reorder_chain ();
if (rtl_dump_file)
dump_flow_info (rtl_dump_file);
cfg_layout_finalize ();
}