/* Thread edges through blocks and update the control flow and SSA graphs.
- Copyright (C) 2004, 2005 Free Software Foundation, Inc.
+ Copyright (C) 2004, 2005, 2006 Free Software Foundation, Inc.
This file is part of GCC.
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. */
+the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+Boston, MA 02110-1301, USA. */
#include "config.h"
#include "system.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"
+#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
/* A template copy of BB with no outgoing edges or control statement that
we use for creating copies. */
basic_block template_block;
+
+ /* TRUE if we thread one or more jumps, FALSE otherwise. */
+ bool jumps_threaded;
};
+/* Passes which use the jump threading code register jump threading
+ opportunities as they are discovered. We keep the registered
+ jump threading opportunities in this vector as edge pairs
+ (original_edge, target_edge). */
+DEF_VEC_ALLOC_P(edge,heap);
+static VEC(edge,heap) *threaded_edges;
+
+
+/* Jump threading statistics. */
+
+struct thread_stats_d
+{
+ unsigned long num_threaded_edges;
+};
+
+struct thread_stats_d thread_stats;
+
+
/* 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. */
if (!bsi_end_p (bsi)
&& bsi_stmt (bsi)
&& (TREE_CODE (bsi_stmt (bsi)) == COND_EXPR
+ || TREE_CODE (bsi_stmt (bsi)) == GOTO_EXPR
|| TREE_CODE (bsi_stmt (bsi)) == SWITCH_EXPR))
- bsi_remove (&bsi);
+ bsi_remove (&bsi, true);
for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); )
{
{
/* We can use the generic block duplication code and simply remove
the stuff we do not need. */
- rd->dup_block = duplicate_block (bb, NULL);
+ rd->dup_block = duplicate_block (bb, NULL, NULL);
/* Zero out the profile, since the block is unreachable for now. */
rd->dup_block->frequency = 0;
edges associated with E in the hash table. */
static struct redirection_data *
-lookup_redirection_data (edge e, edge incoming_edge, bool insert)
+lookup_redirection_data (edge e, edge incoming_edge, enum insert_option 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 = XNEW (struct redirection_data);
elt->outgoing_edge = e;
elt->dup_block = NULL;
elt->do_not_duplicate = false;
if (*slot == NULL)
{
*slot = (void *)elt;
- elt->incoming_edges = xmalloc (sizeof (struct el));
+ elt->incoming_edges = XNEW (struct el);
elt->incoming_edges->e = incoming_edge;
elt->incoming_edges->next = NULL;
return elt;
to the list of incoming edges associated with E. */
if (insert)
{
- struct el *el = xmalloc (sizeof (struct el));
+ struct el *el = XNEW (struct el);
el->next = elt->incoming_edges;
el->e = incoming_edge;
elt->incoming_edges = el;
edge e = make_edge (rd->dup_block, rd->outgoing_edge->dest, EDGE_FALLTHRU);
tree phi;
+ e->probability = REG_BR_PROB_BASE;
+ e->count = rd->dup_block->count;
+
/* 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
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);
+ add_phi_arg (phi, PHI_ARG_DEF (phi, indx), e);
}
}
return 1;
}
+/* Not all jump threading requests are useful. In particular some
+ jump threading requests can create irreducible regions which are
+ undesirable.
+
+ 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;
+ break;
+ }
+ else
+ {
+ backedge = e;
+ }
+ }
+ }
+
+ if (backedge && find_edge (bb, backedge->src))
+ ;
+ else
+ may_create_irreducible_region = true;
+ }
+ else
+ {
+ 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)
+ continue;
+
+ /* If the incoming edge threads to the loop exit, then it
+ is clearly safe. */
+ e2 = e->aux;
+ if (e2 && (e2->flags & EDGE_LOOP_EXIT))
+ continue;
+
+ /* 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;
+ break;
+ }
+
+ /* 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;
+ break;
+ }
+ }
+ }
+
+ /* 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)
+ continue;
+
+ e2 = e->aux;
+
+ /* If this incoming edge was not threaded, then there is
+ nothing to do. */
+ if (!e2)
+ continue;
+
+ /* If this incoming edge threaded to the loop exit,
+ then it can be ignored as it is safe. */
+ if (e2->flags & EDGE_LOOP_EXIT)
+ continue;
+
+ 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;
+ }
+ }
+ }
+}
+
/* Hash table traversal callback to redirect each incoming edge
associated with this hash table element to its new destination. */
to clear it will cause all kinds of unpleasant problems later. */
e->aux = NULL;
+ thread_stats.num_threaded_edges++;
+
if (rd->dup_block)
{
edge e2;
fprintf (dump_file, " Threaded jump %d --> %d to %d\n",
e->src->index, e->dest->index, rd->dup_block->index);
+ rd->dup_block->count += e->count;
+ rd->dup_block->frequency += EDGE_FREQUENCY (e);
+ EDGE_SUCC (rd->dup_block, 0)->count += e->count;
/* Redirect the incoming edge to the appropriate duplicate
block. */
e2 = redirect_edge_and_branch (e, rd->dup_block);
rd->outgoing_edge->dest);
/* And fixup the flags on the single remaining edge. */
- EDGE_SUCC (local_info->bb, 0)->flags
- &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
- EDGE_SUCC (local_info->bb, 0)->flags |= EDGE_FALLTHRU;
+ single_succ_edge (local_info->bb)->flags
+ &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE | EDGE_ABNORMAL);
+ single_succ_edge (local_info->bb)->flags |= EDGE_FALLTHRU;
}
}
+
+ /* Indicate that we actually threaded one or more jumps. */
+ if (rd->incoming_edges)
+ local_info->jumps_threaded = true;
+
return 1;
}
+/* Return true if this block has no executable statements other than
+ a simple ctrl flow instruction. When the number of outgoing edges
+ is one, this is equivalent to a "forwarder" block. */
+
+static bool
+redirection_block_p (basic_block bb)
+{
+ block_stmt_iterator bsi;
+
+ /* Advance to the first executable statement. */
+ bsi = bsi_start (bb);
+ while (!bsi_end_p (bsi)
+ && (TREE_CODE (bsi_stmt (bsi)) == LABEL_EXPR
+ || IS_EMPTY_STMT (bsi_stmt (bsi))))
+ bsi_next (&bsi);
+
+ /* Check if this is an empty block. */
+ if (bsi_end_p (bsi))
+ return true;
+
+ /* Test that we've reached the terminating control statement. */
+ return bsi_stmt (bsi)
+ && (TREE_CODE (bsi_stmt (bsi)) == COND_EXPR
+ || TREE_CODE (bsi_stmt (bsi)) == GOTO_EXPR
+ || TREE_CODE (bsi_stmt (bsi)) == SWITCH_EXPR);
+}
+
/* 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.
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.
+ SSA_NAMEs. Right now, we just call into update_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
per block with incoming threaded edges, which can lower the
cost of the true incremental update algorithm. */
-static void
+static bool
thread_block (basic_block bb)
{
/* E is an incoming edge into BB that we may or may not want to
edge_iterator ei;
struct local_info local_info;
+ /* 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;
+
/* ALL indicates whether or not all incoming edges into BB should
be threaded to a duplicate of BB. */
bool all = true;
+ /* If optimizing for size, only thread this block if we don't have
+ to duplicate it or it's an otherwise empty redirection block. */
+ if (optimize_size
+ && EDGE_COUNT (bb->preds) > 1
+ && !redirection_block_p (bb))
+ {
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ e->aux = NULL;
+ return false;
+ }
+
/* 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
redirection_data_eq,
free);
+ 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);
+
/* Record each unique threaded destination into a hash table for
efficient lookups. */
FOR_EACH_EDGE (e, ei, bb->preds)
else
{
edge e2 = e->aux;
+ update_bb_profile_for_threading (e->dest, EDGE_FREQUENCY (e),
+ e->count, e->aux);
/* Insert the outgoing edge into the hash table if it is not
already in the hash table. */
- lookup_redirection_data (e2, e, true);
+ lookup_redirection_data (e2, e, INSERT);
}
}
if (all)
{
edge e = EDGE_PRED (bb, 0)->aux;
- lookup_redirection_data (e, NULL, false)->do_not_duplicate = true;
+ lookup_redirection_data (e, NULL, NO_INSERT)->do_not_duplicate = true;
}
/* Now create duplicates of BB.
the rest of the duplicates. */
local_info.template_block = NULL;
local_info.bb = bb;
+ local_info.jumps_threaded = false;
htab_traverse (redirection_data, create_duplicates, &local_info);
/* The template does not have an outgoing edge. Create that outgoing
/* Done with this block. Clear REDIRECTION_DATA. */
htab_delete (redirection_data);
redirection_data = NULL;
+
+ /* Indicate to our caller whether or not any jumps were threaded. */
+ return local_info.jumps_threaded;
}
-/* Walk through all blocks and thread incoming edges to the block's
- destinations as requested. This is the only entry point into this
- file.
+/* Walk through the registered jump threads and convert them into a
+ form convenient for this pass.
- Blocks which have one or more incoming edges have INCOMING_EDGE_THREADED
- set in the block's annotation.
+ Any block which has incoming edges threaded to outgoing edges
+ will have its entry in THREADED_BLOCK set.
- Each edge that should be threaded has the new destination edge stored in
- the original edge's AUX field.
+ Any threaded edge will have its new outgoing 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.
+ This form avoids the need to walk all the edges in the CFG to
+ discover blocks which need processing and avoids unnecessary
+ hash table lookups to map from threaded edge to new target. */
+
+static void
+mark_threaded_blocks (bitmap threaded_blocks)
+{
+ unsigned int i;
+
+ for (i = 0; i < VEC_length (edge, threaded_edges); i += 2)
+ {
+ edge e = VEC_index (edge, threaded_edges, i);
+ edge e2 = VEC_index (edge, threaded_edges, i + 1);
+
+ e->aux = e2;
+ bitmap_set_bit (threaded_blocks, e->dest->index);
+ }
+}
+
+
+/* Walk through all blocks and thread incoming edges to the appropriate
+ outgoing edge for each edge pair recorded in THREADED_EDGES.
It is the caller's responsibility to fix the dominance information
and rewrite duplicated SSA_NAMEs back into SSA form.
bool
thread_through_all_blocks (void)
{
- basic_block bb;
bool retval = false;
+ unsigned int i;
+ bitmap_iterator bi;
+ bitmap threaded_blocks;
+
+ if (threaded_edges == NULL)
+ return false;
+
+ threaded_blocks = BITMAP_ALLOC (NULL);
+ memset (&thread_stats, 0, sizeof (thread_stats));
- FOR_EACH_BB (bb)
+ mark_threaded_blocks (threaded_blocks);
+
+ EXECUTE_IF_SET_IN_BITMAP (threaded_blocks, 0, i, bi)
{
- if (bb_ann (bb)->incoming_edge_threaded)
- {
- thread_block (bb);
- retval = true;
- bb_ann (bb)->incoming_edge_threaded = false;
- }
+ basic_block bb = BASIC_BLOCK (i);
+
+ if (EDGE_COUNT (bb->preds) > 0)
+ retval |= thread_block (bb);
}
+
+ if (dump_file && (dump_flags & TDF_STATS))
+ fprintf (dump_file, "\nJumps threaded: %lu\n",
+ thread_stats.num_threaded_edges);
+
+ BITMAP_FREE (threaded_blocks);
+ threaded_blocks = NULL;
+ VEC_free (edge, heap, threaded_edges);
+ threaded_edges = NULL;
return retval;
}
+
+/* Register a jump threading opportunity. We queue up all the jump
+ threading opportunities discovered by a pass and update the CFG
+ and SSA form all at once.
+
+ E is the edge we can thread, E2 is the new target edge. ie, we
+ are effectively recording that E->dest can be changed to E2->dest
+ after fixing the SSA graph. */
+
+void
+register_jump_thread (edge e, edge e2)
+{
+ if (threaded_edges == NULL)
+ threaded_edges = VEC_alloc (edge, heap, 10);
+
+ VEC_safe_push (edge, heap, threaded_edges, e);
+ VEC_safe_push (edge, heap, threaded_edges, e2);
+}
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