/* Thread edges through blocks and update the control flow and SSA graphs. Copyright (C) 2004, 2005, 2006 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, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, 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 "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 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 OUTGOING_EDGE->dest. */ 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; /* 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; /* 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. */ 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)) == GOTO_EXPR || TREE_CODE (bsi_stmt (bsi)) == SWITCH_EXPR)) bsi_remove (&bsi, true); for (ei = ei_start (bb->succs); (e = ei_safe_edge (ei)); ) { if (e->dest != dest_bb) remove_edge (e); else 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, 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, 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 = XNEW (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 = XNEW (struct el); elt->incoming_edges->e = incoming_edge; elt->incoming_edges->next = NULL; return elt; } /* E was in the hash table. */ else { /* 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 = XNEW (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 destination. 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; 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 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 (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. */ } else { 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; } /* 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. */ 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; thread_stats.num_threaded_edges++; 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); 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); 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); } else { 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, rd->outgoing_edge->dest); /* And fixup the flags on the single remaining edge. */ 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. 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 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 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. */ 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; /* 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 incoming and outgoing edges such linear searches can get expensive. */ redirection_data = htab_create (EDGE_COUNT (bb->succs), redirection_data_hash, 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) { if (!e->aux) { all = false; } 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, INSERT); } } /* 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, NO_INSERT)->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; 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 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; /* Indicate to our caller whether or not any jumps were threaded. */ return local_info.jumps_threaded; } /* Walk through the registered jump threads and convert them into a form convenient for this pass. Any block which has incoming edges threaded to outgoing edges will have its entry in THREADED_BLOCK set. Any threaded edge will have its new outgoing edge stored in the original edge's AUX field. 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. Returns true if one or more edges were threaded, false otherwise. */ bool thread_through_all_blocks (void) { 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)); mark_threaded_blocks (threaded_blocks); EXECUTE_IF_SET_IN_BITMAP (threaded_blocks, 0, i, bi) { 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); }