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/* Thread edges through blocks and update the control flow and SSA graphs.
Copyright (C) 2004, 2005 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
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. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "flags.h"
#include "rtl.h"
#include "tm_p.h"
#include "ggc.h"
#include "basic-block.h"
#include "output.h"
#include "errors.h"
#include "expr.h"
#include "function.h"
#include "diagnostic.h"
#include "tree-flow.h"
#include "tree-dump.h"
#include "tree-pass.h"
/* APPLE LOCAL 4538899 mainline */
#include "cfgloop.h"
/* Given a block B, update the CFG and SSA graph to reflect redirecting
one or more in-edges to B to instead reach the destination of an
out-edge from B while preserving any side effects in B.
i.e., given A->B and B->C, change A->B to be A->C yet still preserve the
side effects of executing B.
1. Make a copy of B (including its outgoing edges and statements). Call
the copy B'. Note B' has no incoming edges or PHIs at this time.
2. Remove the control statement at the end of B' and all outgoing edges
except B'->C.
3. Add a new argument to each PHI in C with the same value as the existing
argument associated with edge B->C. Associate the new PHI arguments
with the edge B'->C.
4. For each PHI in B, find or create a PHI in B' with an identical
PHI_RESULT. Add an argument to the PHI in B' which has the same
value as the PHI in B associated with the edge A->B. Associate
the new argument in the PHI in B' with the edge A->B.
5. Change the edge A->B to A->B'.
5a. This automatically deletes any PHI arguments associated with the
edge A->B in B.
5b. This automatically associates each new argument added in step 4
with the edge A->B'.
6. Repeat for other incoming edges into B.
7. Put the duplicated resources in B and all the B' blocks into SSA form.
Note that block duplication can be minimized by first collecting the
the set of unique destination blocks that the incoming edges should
be threaded to. Block duplication can be further minimized by using
B instead of creating B' for one destination if all edges into B are
going to be threaded to a successor of B.
We further reduce the number of edges and statements we create by
not copying all the outgoing edges and the control statement in
step #1. We instead create a template block without the outgoing
edges and duplicate the template. */
/* Steps #5 and #6 of the above algorithm are best implemented by walking
all the incoming edges which thread to the same destination edge at
the same time. That avoids lots of table lookups to get information
for the destination edge.
To realize that implementation we create a list of incoming edges
which thread to the same outgoing edge. Thus to implement steps
#5 and #6 we traverse our hash table of outgoing edge information.
For each entry we walk the list of incoming edges which thread to
the current outgoing edge. */
struct el
edge e;
struct el *next;
/* Main data structure recording information regarding B's duplicate
blocks. */
/* We need to efficiently record the unique thread destinations of this
block and specific information associated with those destinations. We
may have many incoming edges threaded to the same outgoing edge. This
can be naturally implemented with a hash table. */
struct redirection_data
/* A duplicate of B with the trailing control statement removed and which
targets a single successor of B. */
basic_block dup_block;
/* An outgoing edge from B. DUP_BLOCK will have OUTGOING_EDGE->dest as
its single successor. */
edge outgoing_edge;
/* A list of incoming edges which we want to thread to
struct el *incoming_edges;
/* Flag indicating whether or not we should create a duplicate block
for this thread destination. This is only true if we are threading
all incoming edges and thus are using BB itself as a duplicate block. */
bool do_not_duplicate;
/* Main data structure to hold information for duplicates of BB. */
static htab_t redirection_data;
/* APPLE LOCAL begin 4538899 mainline */
bool rediscover_loops_after_threading;
/* APPLE LOCAL end 4538899 mainline */
/* Data structure of information to pass to hash table traversal routines. */
struct local_info
/* The current block we are working on. */
basic_block bb;
/* A template copy of BB with no outgoing edges or control statement that
we use for creating copies. */
basic_block template_block;
/* APPLE LOCAL begin 4538899 mainline */
/* TRUE if we thread one or more jumps, FALSE otherwise. */
bool jumps_threaded;
/* APPLE LOCAL end 4538899 mainline */
/* Remove the last statement in block BB if it is a control statement
Also remove all outgoing edges except the edge which reaches DEST_BB.
If DEST_BB is NULL, then remove all outgoing edges. */
static void
remove_ctrl_stmt_and_useless_edges (basic_block bb, basic_block dest_bb)
block_stmt_iterator bsi;
edge e;
edge_iterator ei;
bsi = bsi_last (bb);
/* If the duplicate ends with a control statement, then remove it.
Note that if we are duplicating the template block rather than the
original basic block, then the duplicate might not have any real
statements in it. */
if (!bsi_end_p (bsi)
&& bsi_stmt (bsi)
&& (TREE_CODE (bsi_stmt (bsi)) == COND_EXPR
|| TREE_CODE (bsi_stmt (bsi)) == SWITCH_EXPR))
bsi_remove (&bsi);
for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); )
if (e->dest != dest_bb)
remove_edge (e);
ei_next (&ei);
/* Create a duplicate of BB which only reaches the destination of the edge
stored in RD. Record the duplicate block in RD. */
static void
create_block_for_threading (basic_block bb, struct redirection_data *rd)
/* We can use the generic block duplication code and simply remove
the stuff we do not need. */
rd->dup_block = duplicate_block (bb, NULL);
/* Zero out the profile, since the block is unreachable for now. */
rd->dup_block->frequency = 0;
rd->dup_block->count = 0;
/* The call to duplicate_block will copy everything, including the
useless COND_EXPR or SWITCH_EXPR at the end of BB. We just remove
the useless COND_EXPR or SWITCH_EXPR here rather than having a
specialized block copier. We also remove all outgoing edges
from the duplicate block. The appropriate edge will be created
later. */
remove_ctrl_stmt_and_useless_edges (rd->dup_block, NULL);
/* Hashing and equality routines for our hash table. */
static hashval_t
redirection_data_hash (const void *p)
edge e = ((struct redirection_data *)p)->outgoing_edge;
return e->dest->index;
static int
redirection_data_eq (const void *p1, const void *p2)
edge e1 = ((struct redirection_data *)p1)->outgoing_edge;
edge e2 = ((struct redirection_data *)p2)->outgoing_edge;
return e1 == e2;
/* Given an outgoing edge E lookup and return its entry in our hash table.
If INSERT is true, then we insert the entry into the hash table if
it is not already present. INCOMING_EDGE is added to the list of incoming
edges associated with E in the hash table. */
static struct redirection_data *
lookup_redirection_data (edge e, edge incoming_edge, bool insert)
void **slot;
struct redirection_data *elt;
/* Build a hash table element so we can see if E is already
in the table. */
elt = xmalloc (sizeof (struct redirection_data));
elt->outgoing_edge = e;
elt->dup_block = NULL;
elt->do_not_duplicate = false;
elt->incoming_edges = NULL;
slot = htab_find_slot (redirection_data, elt, insert);
/* This will only happen if INSERT is false and the entry is not
in the hash table. */
if (slot == NULL)
free (elt);
return NULL;
/* This will only happen if E was not in the hash table and
INSERT is true. */
if (*slot == NULL)
*slot = (void *)elt;
elt->incoming_edges = xmalloc (sizeof (struct el));
elt->incoming_edges->e = incoming_edge;
elt->incoming_edges->next = NULL;
return elt;
/* E was in the hash table. */
/* Free ELT as we do not need it anymore, we will extract the
relevant entry from the hash table itself. */
free (elt);
/* Get the entry stored in the hash table. */
elt = (struct redirection_data *) *slot;
/* If insertion was requested, then we need to add INCOMING_EDGE
to the list of incoming edges associated with E. */
if (insert)
struct el *el = xmalloc (sizeof (struct el));
el->next = elt->incoming_edges;
el->e = incoming_edge;
elt->incoming_edges = el;
return elt;
/* Given a duplicate block and its single destination (both stored
in RD). Create an edge between the duplicate and its single
Add an additional argument to any PHI nodes at the single
destination. */
static void
create_edge_and_update_destination_phis (struct redirection_data *rd)
edge e = make_edge (rd->dup_block, rd->outgoing_edge->dest, EDGE_FALLTHRU);
tree phi;
/* If there are any PHI nodes at the destination of the outgoing edge
from the duplicate block, then we will need to add a new argument
to them. The argument should have the same value as the argument
associated with the outgoing edge stored in RD. */
for (phi = phi_nodes (e->dest); phi; phi = PHI_CHAIN (phi))
int indx = rd->outgoing_edge->dest_idx;
add_phi_arg (phi, PHI_ARG_DEF_TREE (phi, indx), e);
/* Hash table traversal callback routine to create duplicate blocks. */
static int
create_duplicates (void **slot, void *data)
struct redirection_data *rd = (struct redirection_data *) *slot;
struct local_info *local_info = (struct local_info *)data;
/* If this entry should not have a duplicate created, then there's
nothing to do. */
if (rd->do_not_duplicate)
return 1;
/* Create a template block if we have not done so already. Otherwise
use the template to create a new block. */
if (local_info->template_block == NULL)
create_block_for_threading (local_info->bb, rd);
local_info->template_block = rd->dup_block;
/* We do not create any outgoing edges for the template. We will
take care of that in a later traversal. That way we do not
create edges that are going to just be deleted. */
create_block_for_threading (local_info->template_block, rd);
/* Go ahead and wire up outgoing edges and update PHIs for the duplicate
block. */
create_edge_and_update_destination_phis (rd);
/* Keep walking the hash table. */
return 1;
/* We did not create any outgoing edges for the template block during
block creation. This hash table traversal callback creates the
outgoing edge for the template block. */
static int
fixup_template_block (void **slot, void *data)
struct redirection_data *rd = (struct redirection_data *) *slot;
struct local_info *local_info = (struct local_info *)data;
/* If this is the template block, then create its outgoing edges
and halt the hash table traversal. */
if (rd->dup_block && rd->dup_block == local_info->template_block)
create_edge_and_update_destination_phis (rd);
return 0;
return 1;
/* APPLE LOCAL begin 4538899 mainline */
/* Not all jump threading requests are useful. In particular some
jump threading requests can create irreducible regions which are
This routine will examine the BB's incoming edges for jump threading
requests which, if acted upon, would create irreducible regions. Any
such jump threading requests found will be pruned away. */
static void
prune_undesirable_thread_requests (basic_block bb)
edge e;
edge_iterator ei;
bool may_create_irreducible_region = false;
unsigned int num_outgoing_edges_into_loop = 0;
/* For the heuristics below, we need to know if BB has more than
one outgoing edge into a loop. */
FOR_EACH_EDGE (e, ei, bb->succs)
num_outgoing_edges_into_loop += ((e->flags & EDGE_LOOP_EXIT) == 0);
if (num_outgoing_edges_into_loop > 1)
edge backedge = NULL;
/* Consider the effect of threading the edge (0, 1) to 2 on the left
CFG to produce the right CFG:
0 0
| |
1<--+ 2<--------+
/ \ | | |
2 3 | 4<----+ |
\ / | / \ | |
4---+ E 1-- | --+
| | |
E 3---+
Threading the (0, 1) edge to 2 effectively creates two loops
(2, 4, 1) and (4, 1, 3) which are neither disjoint nor nested.
This is not good.
However, we do need to be able to thread (0, 1) to 2 or 3
in the left CFG below (which creates the middle and right
CFGs with nested loops).
0 0 0
| | |
1<--+ 2<----+ 3<-+<-+
/| | | | | | |
2 | | 3<-+ | 1--+ |
\| | | | | | |
3---+ 1--+--+ 2-----+
A safe heuristic appears to be to only allow threading if BB
has a single incoming backedge from one of its direct successors. */
FOR_EACH_EDGE (e, ei, bb->preds)
if (e->flags & EDGE_DFS_BACK)
if (backedge)
backedge = NULL;
backedge = e;
if (backedge && find_edge (bb, backedge->src))
may_create_irreducible_region = true;
edge dest = NULL;
/* If we thread across the loop entry block (BB) into the
loop and BB is still reached from outside the loop, then
we would create an irreducible CFG. Consider the effect
of threading the edge (1, 4) to 5 on the left CFG to produce
the right CFG
0 0
/ \ / \
1 2 1 2
\ / | |
4<----+ 5<->4
/ \ | |
E 5---+ E
Threading the (1, 4) edge to 5 creates two entry points
into the loop (4, 5) (one from block 1, the other from
block 2). A classic irreducible region.
So look at all of BB's incoming edges which are not
backedges and which are not threaded to the loop exit.
If that subset of incoming edges do not all thread
to the same block, then threading any of them will create
an irreducible region. */
FOR_EACH_EDGE (e, ei, bb->preds)
edge e2;
/* We ignore back edges for now. This may need refinement
as threading a backedge creates an inner loop which
we would need to verify has a single entry point.
If all backedges thread to new locations, then this
block will no longer have incoming backedges and we
need not worry about creating irreducible regions
by threading through BB. I don't think this happens
enough in practice to worry about it. */
if (e->flags & EDGE_DFS_BACK)
/* If the incoming edge threads to the loop exit, then it
is clearly safe. */
e2 = e->aux;
if (e2 && (e2->flags & EDGE_LOOP_EXIT))
/* E enters the loop header and is not threaded. We can
not allow any other incoming edges to thread into
the loop as that would create an irreducible region. */
if (!e2)
may_create_irreducible_region = true;
/* We know that this incoming edge threads to a block inside
the loop. This edge must thread to the same target in
the loop as any previously seen threaded edges. Otherwise
we will create an irreducible region. */
if (!dest)
dest = e2;
else if (e2 != dest)
may_create_irreducible_region = true;
/* If we might create an irreducible region, then cancel any of
the jump threading requests for incoming edges which are
not backedges and which do not thread to the exit block. */
if (may_create_irreducible_region)
FOR_EACH_EDGE (e, ei, bb->preds)
edge e2;
/* Ignore back edges. */
if (e->flags & EDGE_DFS_BACK)
e2 = e->aux;
/* If this incoming edge was not threaded, then there is
nothing to do. */
if (!e2)
/* If this incoming edge threaded to the loop exit,
then it can be ignored as it is safe. */
if (e2->flags & EDGE_LOOP_EXIT)
if (e2)
/* This edge threaded into the loop and the jump thread
request must be cancelled. */
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, " Not threading jump %d --> %d to %d\n",
e->src->index, e->dest->index, e2->dest->index);
e->aux = NULL;
/* APPLE LOCAL end 4538899 mainline */
/* Hash table traversal callback to redirect each incoming edge
associated with this hash table element to its new destination. */
static int
redirect_edges (void **slot, void *data)
struct redirection_data *rd = (struct redirection_data *) *slot;
struct local_info *local_info = (struct local_info *)data;
struct el *next, *el;
/* Walk over all the incoming edges associated associated with this
hash table entry. */
for (el = rd->incoming_edges; el; el = next)
edge e = el->e;
/* Go ahead and free this element from the list. Doing this now
avoids the need for another list walk when we destroy the hash
table. */
next = el->next;
free (el);
/* Go ahead and clear E->aux. It's not needed anymore and failure
to clear it will cause all kinds of unpleasant problems later. */
e->aux = NULL;
if (rd->dup_block)
edge e2;
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, " Threaded jump %d --> %d to %d\n",
e->src->index, e->dest->index, rd->dup_block->index);
/* Redirect the incoming edge to the appropriate duplicate
block. */
e2 = redirect_edge_and_branch (e, rd->dup_block);
flush_pending_stmts (e2);
if ((dump_file && (dump_flags & TDF_DETAILS))
&& e->src != e2->src)
fprintf (dump_file, " basic block %d created\n", e2->src->index);
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, " Threaded jump %d --> %d to %d\n",
e->src->index, e->dest->index, local_info->bb->index);
/* We are using BB as the duplicate. Remove the unnecessary
outgoing edges and statements from BB. */
remove_ctrl_stmt_and_useless_edges (local_info->bb,
/* And fixup the flags on the single remaining edge. */
EDGE_SUCC (local_info->bb, 0)->flags
/* APPLE LOCAL 4538899 mainline */
EDGE_SUCC (local_info->bb, 0)->flags |= EDGE_FALLTHRU;
/* APPLE LOCAL begin 4538899 mainline */
/* Indicate that we actually threaded one or more jumps. */
if (rd->incoming_edges)
local_info->jumps_threaded = true;
/* APPLE LOCAL end 4538899 mainline */
return 1;
/* BB is a block which ends with a COND_EXPR or SWITCH_EXPR and when BB
is reached via one or more specific incoming edges, we know which
outgoing edge from BB will be traversed.
We want to redirect those incoming edges to the target of the
appropriate outgoing edge. Doing so avoids a conditional branch
and may expose new optimization opportunities. Note that we have
to update dominator tree and SSA graph after such changes.
The key to keeping the SSA graph update manageable is to duplicate
the side effects occurring in BB so that those side effects still
occur on the paths which bypass BB after redirecting edges.
We accomplish this by creating duplicates of BB and arranging for
the duplicates to unconditionally pass control to one specific
successor of BB. We then revector the incoming edges into BB to
the appropriate duplicate of BB.
BB and its duplicates will have assignments to the same set of
SSA_NAMEs. Right now, we just call into rewrite_ssa_into_ssa
to update the SSA graph for those names.
We are also going to experiment with a true incremental update
scheme for the duplicated resources. One of the interesting
properties we can exploit here is that all the resources set
in BB will have the same IDFS, so we have one IDFS computation
per block with incoming threaded edges, which can lower the
cost of the true incremental update algorithm. */
/* APPLE LOCAL 4538899 mainline */
static bool
thread_block (basic_block bb)
/* E is an incoming edge into BB that we may or may not want to
redirect to a duplicate of BB. */
edge e;
edge_iterator ei;
struct local_info local_info;
/* APPLE LOCAL begin 4538899 mainline */
/* FOUND_BACKEDGE indicates that we found an incoming backedge
into BB, in which case we may ignore certain jump threads
to avoid creating irreducible regions. */
bool found_backedge = false;
/* APPLE LOCAL end 4538899 mainline */
/* ALL indicates whether or not all incoming edges into BB should
be threaded to a duplicate of BB. */
bool all = true;
/* To avoid scanning a linear array for the element we need we instead
use a hash table. For normal code there should be no noticeable
difference. However, if we have a block with a large number of
incoming and outgoing edges such linear searches can get expensive. */
redirection_data = htab_create (EDGE_COUNT (bb->succs),
/* APPLE LOCAL begin 4538899 mainline */
FOR_EACH_EDGE (e, ei, bb->preds)
found_backedge |= ((e->flags & EDGE_DFS_BACK) != 0);
/* If BB has incoming backedges, then threading across BB might
introduce an irreducible region, which would be undesirable
as that inhibits various optimizations later. Prune away
any jump threading requests which we know will result in
an irreducible region. */
if (found_backedge)
prune_undesirable_thread_requests (bb);
/* APPLE LOCAL end 4538899 mainline */
/* Record each unique threaded destination into a hash table for
efficient lookups. */
FOR_EACH_EDGE (e, ei, bb->preds)
if (!e->aux)
all = false;
edge e2 = e->aux;
/* APPLE LOCAL begin 4538899 mainline */
/* If we thread to a loop exit edge, then we will need to
rediscover the loop exit edges. While it may seem that
the new edge is a loop exit edge, that is not the case.
Consider threading the edge (5,6) to E in the CFG on the
left which creates the CFG on the right:
0<--+ 0<---+
/ \ | / \ |
1 2 | 1 2 |
/ \ | | / \ | |
3 4 | | 3 4 6--+
\ / | | \ /
5 | | 5
\ / | |
6---+ E
After threading, the edge (0, 1) is the loop exit edge and
the nodes 0, 2, 6 are the only nodes in the loop. */
if (e2->flags & EDGE_LOOP_EXIT)
rediscover_loops_after_threading = true;
/* APPLE LOCAL end 4538899 mainline */
/* Insert the outgoing edge into the hash table if it is not
already in the hash table. */
lookup_redirection_data (e2, e, true);
/* If we are going to thread all incoming edges to an outgoing edge, then
BB will become unreachable. Rather than just throwing it away, use
it for one of the duplicates. Mark the first incoming edge with the
DO_NOT_DUPLICATE attribute. */
if (all)
edge e = EDGE_PRED (bb, 0)->aux;
lookup_redirection_data (e, NULL, false)->do_not_duplicate = true;
/* Now create duplicates of BB.
Note that for a block with a high outgoing degree we can waste
a lot of time and memory creating and destroying useless edges.
So we first duplicate BB and remove the control structure at the
tail of the duplicate as well as all outgoing edges from the
duplicate. We then use that duplicate block as a template for
the rest of the duplicates. */
local_info.template_block = NULL; = bb;
/* APPLE LOCAL 4538899 mainline */
local_info.jumps_threaded = false;
htab_traverse (redirection_data, create_duplicates, &local_info);
/* The template does not have an outgoing edge. Create that outgoing
edge and update PHI nodes as the edge's target as necessary.
We do this after creating all the duplicates to avoid creating
unnecessary edges. */
htab_traverse (redirection_data, fixup_template_block, &local_info);
/* The hash table traversals above created the duplicate blocks (and the
statements within the duplicate blocks). This loop creates PHI nodes for
the duplicated blocks and redirects the incoming edges into BB to reach
the duplicates of BB. */
htab_traverse (redirection_data, redirect_edges, &local_info);
/* Done with this block. Clear REDIRECTION_DATA. */
htab_delete (redirection_data);
redirection_data = NULL;
/* APPLE LOCAL begin 4538899 mainline */
/* Indicate to our caller whether or not any jumps were threaded. */
return local_info.jumps_threaded;
/* APPLE LOCAL end 4538899 mainline */
/* Walk through all blocks and thread incoming edges to the block's
destinations as requested. This is the only entry point into this
Blocks which have one or more incoming edges have INCOMING_EDGE_THREADED
set in the block's annotation.
Each edge that should be threaded has the new destination edge stored in
the original edge's AUX field.
This routine (or one of its callees) will clear INCOMING_EDGE_THREADED
in the block annotations and the AUX field in the edges.
It is the caller's responsibility to fix the dominance information
and rewrite duplicated SSA_NAMEs back into SSA form.
Returns true if one or more edges were threaded, false otherwise. */
thread_through_all_blocks (void)
basic_block bb;
bool retval = false;
/* APPLE LOCAL begin 4538899 mainline */
rediscover_loops_after_threading = false;
/* APPLE LOCAL end 4538899 mainline */
if (bb_ann (bb)->incoming_edge_threaded
&& EDGE_COUNT (bb->preds) > 0)
/* APPLE LOCAL 4538899 mainline */
retval |= thread_block (bb);
bb_ann (bb)->incoming_edge_threaded = false;
return retval;