#include "tm_p.h"
#include "ggc.h"
#include "basic-block.h"
+#include "cfgloop.h"
#include "output.h"
#include "errors.h"
#include "expr.h"
{
struct dom_walk_data walk_data;
unsigned int i;
+ struct loops loops_info;
memset (&opt_stats, 0, sizeof (opt_stats));
calculate_dominance_info (CDI_DOMINATORS);
+ /* We need to know which edges exit loops so that we can
+ aggressively thread through loop headers to an exit
+ edge. */
+ flow_loops_find (&loops_info);
+ mark_loop_exit_edges (&loops_info);
+ flow_loops_free (&loops_info);
+
+ /* Clean up the CFG so that any forwarder blocks created by loop
+ canonicalization are removed. */
+ cleanup_tree_cfg ();
+
/* If we prove certain blocks are unreachable, then we want to
repeat the dominator optimization process as PHI nodes may
have turned into copies which allows better propagation of
/* Optimize the dominator tree. */
cfg_altered = false;
+ /* We need accurate information regarding back edges in the CFG
+ for jump threading. */
+ mark_dfs_back_edges ();
+
/* Recursively walk the dominator tree optimizing statements. */
walk_dominator_tree (&walk_data, ENTRY_BLOCK_PTR);
}
if (cfg_altered)
- free_dominance_info (CDI_DOMINATORS);
+ free_dominance_info (CDI_DOMINATORS);
+
cfg_altered |= cleanup_tree_cfg ();
+
+ if (rediscover_loops_after_threading)
+ {
+ /* Rerun basic loop analysis to discover any newly
+ created loops and update the set of exit edges. */
+ rediscover_loops_after_threading = false;
+ flow_loops_find (&loops_info);
+ mark_loop_exit_edges (&loops_info);
+ flow_loops_free (&loops_info);
+
+ /* Remove any forwarder blocks inserted by loop
+ header canonicalization. */
+ cleanup_tree_cfg ();
+ }
+
calculate_dominance_info (CDI_DOMINATORS);
rewrite_ssa_into_ssa ();
#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
/* Main data structure to hold information for duplicates of BB. */
static htab_t redirection_data;
+bool rediscover_loops_after_threading;
+
/* Data structure of information to pass to hash table traversal routines. */
struct local_info
{
/* 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;
};
/* Remove the last statement in block BB if it is a control statement
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. */
/* And fixup the flags on the single remaining edge. */
EDGE_SUCC (local_info->bb, 0)->flags
- &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
+ &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE | EDGE_ABNORMAL);
EDGE_SUCC (local_info->bb, 0)->flags |= EDGE_FALLTHRU;
}
}
+
+ /* Indicate that we actually threaded one or more jumps. */
+ if (rd->incoming_edges)
+ local_info->jumps_threaded = true;
+
return 1;
}
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;
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)
{
edge e2 = e->aux;
+ /* 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
+ |
+ 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;
+
/* Insert the outgoing edge into the hash table if it is not
already in the hash table. */
lookup_redirection_data (e2, e, true);
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
basic_block bb;
bool retval = false;
+ rediscover_loops_after_threading = false;
+
FOR_EACH_BB (bb)
{
if (bb_ann (bb)->incoming_edge_threaded)
{
- thread_block (bb);
- retval = true;
+ retval |= thread_block (bb);
bb_ann (bb)->incoming_edge_threaded = false;
+
}
}
+
return retval;
}
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