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[/] [openrisc/] [trunk/] [gnu-dev/] [or1k-gcc/] [gcc/] [tree-ssa-threadupdate.c] - Rev 751
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/* Thread edges through blocks and update the control flow and SSA graphs. Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010, 201 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 3, 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 COPYING3. If not see <http://www.gnu.org/licenses/>. */ #include "config.h" #include "system.h" #include "coretypes.h" #include "tm.h" #include "tree.h" #include "flags.h" #include "tm_p.h" #include "basic-block.h" #include "output.h" #include "function.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 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 had code to do this at one time, but I'm not convinced it is correct with the changes to avoid mucking up the loop structure (which may cancel threading requests, thus a block which we thought was going to become unreachable may still be reachable). This code was also going to get ugly with the introduction of the ability for a single jump thread request to bypass multiple blocks. 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; edge intermediate_edge; /* A list of incoming edges which we want to thread to OUTGOING_EDGE->dest. */ struct el *incoming_edges; }; /* 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). */ static VEC(edge,heap) *threaded_edges; /* When we start updating the CFG for threading, data necessary for jump threading is attached to the AUX field for the incoming edge. Use these macros to access the underlying structure attached to the AUX field. */ #define THREAD_TARGET(E) ((edge *)(E)->aux)[0] #define THREAD_TARGET2(E) ((edge *)(E)->aux)[1] /* 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) { gimple_stmt_iterator gsi; edge e; edge_iterator ei; gsi = gsi_last_bb (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 (!gsi_end_p (gsi) && gsi_stmt (gsi) && (gimple_code (gsi_stmt (gsi)) == GIMPLE_COND || gimple_code (gsi_stmt (gsi)) == GIMPLE_GOTO || gimple_code (gsi_stmt (gsi)) == GIMPLE_SWITCH)) gsi_remove (&gsi, 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. Record the duplicate block in RD. */ static void create_block_for_threading (basic_block bb, struct redirection_data *rd) { edge_iterator ei; edge e; /* 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); FOR_EACH_EDGE (e, ei, rd->dup_block->succs) e->aux = NULL; /* Zero out the profile, since the block is unreachable for now. */ rd->dup_block->frequency = 0; rd->dup_block->count = 0; } /* Hashing and equality routines for our hash table. */ static hashval_t redirection_data_hash (const void *p) { edge e = ((const struct redirection_data *)p)->outgoing_edge; return e->dest->index; } static int redirection_data_eq (const void *p1, const void *p2) { edge e1 = ((const struct redirection_data *)p1)->outgoing_edge; edge e2 = ((const struct redirection_data *)p2)->outgoing_edge; edge e3 = ((const struct redirection_data *)p1)->intermediate_edge; edge e4 = ((const struct redirection_data *)p2)->intermediate_edge; return e1 == e2 && e3 == e4; } /* 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, 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->intermediate_edge = THREAD_TARGET2 (e) ? THREAD_TARGET (e) : NULL; elt->outgoing_edge = THREAD_TARGET2 (e) ? THREAD_TARGET2 (e) : THREAD_TARGET (e); elt->dup_block = NULL; 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 = e; 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 = e; elt->incoming_edges = el; } return elt; } } /* For each PHI in BB, copy the argument associated with SRC_E to TGT_E. */ static void copy_phi_args (basic_block bb, edge src_e, edge tgt_e) { gimple_stmt_iterator gsi; int src_indx = src_e->dest_idx; for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi)) { gimple phi = gsi_stmt (gsi); source_location locus = gimple_phi_arg_location (phi, src_indx); add_phi_arg (phi, gimple_phi_arg_def (phi, src_indx), tgt_e, locus); } } /* We have recently made a copy of ORIG_BB, including its outgoing edges. The copy is NEW_BB. Every PHI node in every direct successor of ORIG_BB has a new argument associated with edge from NEW_BB to the successor. Initialize the PHI argument so that it is equal to the PHI argument associated with the edge from ORIG_BB to the successor. */ static void update_destination_phis (basic_block orig_bb, basic_block new_bb) { edge_iterator ei; edge e; FOR_EACH_EDGE (e, ei, orig_bb->succs) { edge e2 = find_edge (new_bb, e->dest); copy_phi_args (e->dest, e, e2); } } /* 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, basic_block bb) { edge e = make_edge (bb, rd->outgoing_edge->dest, EDGE_FALLTHRU); rescan_loop_exit (e, true, false); e->probability = REG_BR_PROB_BASE; e->count = bb->count; if (rd->outgoing_edge->aux) { e->aux = (edge *) XNEWVEC (edge, 2); THREAD_TARGET(e) = THREAD_TARGET (rd->outgoing_edge); THREAD_TARGET2(e) = THREAD_TARGET2 (rd->outgoing_edge); } else { e->aux = NULL; } /* 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. */ copy_phi_args (e->dest, rd->outgoing_edge, e); } /* Wire up the outgoing edges from the duplicate block and update any PHIs as needed. */ static void fix_duplicate_block_edges (struct redirection_data *rd, struct local_info *local_info) { /* If we were threading through an joiner block, then we want to keep its control statement and redirect an outgoing edge. Else we want to remove the control statement & edges, then create a new outgoing edge. In both cases we may need to update PHIs. */ if (THREAD_TARGET2 (rd->incoming_edges->e)) { edge victim; edge e2; edge e = rd->incoming_edges->e; /* This updates the PHIs at the destination of the duplicate block. */ update_destination_phis (local_info->bb, rd->dup_block); /* Find the edge from the duplicate block to the block we're threading through. That's the edge we want to redirect. */ victim = find_edge (rd->dup_block, THREAD_TARGET (e)->dest); e2 = redirect_edge_and_branch (victim, THREAD_TARGET2 (e)->dest); /* If we redirected the edge, then we need to copy PHI arguments at the target. If the edge already existed (e2 != victim case), then the PHIs in the target already have the correct arguments. */ if (e2 == victim) copy_phi_args (e2->dest, THREAD_TARGET2 (e), e2); } else { remove_ctrl_stmt_and_useless_edges (rd->dup_block, NULL); create_edge_and_update_destination_phis (rd, rd->dup_block); } } /* 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; /* 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. */ fix_duplicate_block_edges (rd, local_info); } /* 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 halt the traversal after updating it appropriately. If we were threading through an joiner block, then we want to keep its control statement and redirect an outgoing edge. Else we want to remove the control statement & edges, then create a new outgoing edge. In both cases we may need to update PHIs. */ if (rd->dup_block && rd->dup_block == local_info->template_block) { fix_duplicate_block_edges (rd, local_info); return 0; } return 1; } /* 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); thread_stats.num_threaded_edges++; /* If we are threading through a joiner block, then we have to find the edge we want to redirect and update some PHI nodes. */ if (THREAD_TARGET2 (e)) { edge e2; /* We want to redirect the incoming edge to the joiner block (E) to instead reach the duplicate of the joiner block. */ e2 = redirect_edge_and_branch (e, rd->dup_block); flush_pending_stmts (e2); } else 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; /* Excessive jump threading may make frequencies large enough so the computation overflows. */ if (rd->dup_block->frequency < BB_FREQ_MAX * 2) 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); gcc_assert (e == e2); flush_pending_stmts (e2); } /* Go ahead and clear E->aux. It's not needed anymore and failure to clear it will cause all kinds of unpleasant problems later. */ free (e->aux); e->aux = NULL; } /* 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) { gimple_stmt_iterator gsi; /* Advance to the first executable statement. */ gsi = gsi_start_bb (bb); while (!gsi_end_p (gsi) && (gimple_code (gsi_stmt (gsi)) == GIMPLE_LABEL || is_gimple_debug (gsi_stmt (gsi)) || gimple_nop_p (gsi_stmt (gsi)))) gsi_next (&gsi); /* Check if this is an empty block. */ if (gsi_end_p (gsi)) return true; /* Test that we've reached the terminating control statement. */ return gsi_stmt (gsi) && (gimple_code (gsi_stmt (gsi)) == GIMPLE_COND || gimple_code (gsi_stmt (gsi)) == GIMPLE_GOTO || gimple_code (gsi_stmt (gsi)) == GIMPLE_SWITCH); } /* 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. If NOLOOP_ONLY is true, we only perform the threading as long as it does not affect the structure of the loops in a nontrivial way. */ static bool thread_block (basic_block bb, bool noloop_only) { /* E is an incoming edge into BB that we may or may not want to redirect to a duplicate of BB. */ edge e, e2; edge_iterator ei; struct local_info local_info; struct loop *loop = bb->loop_father; /* 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); /* If we thread the latch of the loop to its exit, the loop ceases to exist. Make sure we do not restrict ourselves in order to preserve this loop. */ if (loop->header == bb) { e = loop_latch_edge (loop); if (e->aux) e2 = THREAD_TARGET (e); else e2 = NULL; if (e2 && loop_exit_edge_p (loop, e2)) { loop->header = NULL; loop->latch = NULL; } } /* Record each unique threaded destination into a hash table for efficient lookups. */ FOR_EACH_EDGE (e, ei, bb->preds) { if (e->aux == NULL) continue; if (THREAD_TARGET2 (e)) e2 = THREAD_TARGET2 (e); else e2 = THREAD_TARGET (e); if (!e2 /* If NOLOOP_ONLY is true, we only allow threading through the header of a loop to exit edges. */ || (noloop_only && bb == bb->loop_father->header && (!loop_exit_edge_p (bb->loop_father, e2) || THREAD_TARGET2 (e)))) continue; if (e->dest == e2->src) update_bb_profile_for_threading (e->dest, EDGE_FREQUENCY (e), e->count, THREAD_TARGET (e)); /* Insert the outgoing edge into the hash table if it is not already in the hash table. */ lookup_redirection_data (e, INSERT); } /* We do not update dominance info. */ free_dominance_info (CDI_DOMINATORS); /* 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; } /* Threads edge E through E->dest to the edge THREAD_TARGET (E). Returns the copy of E->dest created during threading, or E->dest if it was not necessary to copy it (E is its single predecessor). */ static basic_block thread_single_edge (edge e) { basic_block bb = e->dest; edge eto = THREAD_TARGET (e); struct redirection_data rd; free (e->aux); e->aux = NULL; thread_stats.num_threaded_edges++; if (single_pred_p (bb)) { /* If BB has just a single predecessor, we should only remove the control statements at its end, and successors except for ETO. */ remove_ctrl_stmt_and_useless_edges (bb, eto->dest); /* And fixup the flags on the single remaining edge. */ eto->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE | EDGE_ABNORMAL); eto->flags |= EDGE_FALLTHRU; return bb; } /* Otherwise, we need to create a copy. */ if (e->dest == eto->src) update_bb_profile_for_threading (bb, EDGE_FREQUENCY (e), e->count, eto); rd.outgoing_edge = eto; create_block_for_threading (bb, &rd); remove_ctrl_stmt_and_useless_edges (rd.dup_block, NULL); create_edge_and_update_destination_phis (&rd, rd.dup_block); 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); single_succ_edge (rd.dup_block)->count = e->count; redirect_edge_and_branch (e, rd.dup_block); flush_pending_stmts (e); return rd.dup_block; } /* Callback for dfs_enumerate_from. Returns true if BB is different from STOP and DBDS_CE_STOP. */ static basic_block dbds_ce_stop; static bool dbds_continue_enumeration_p (const_basic_block bb, const void *stop) { return (bb != (const_basic_block) stop && bb != dbds_ce_stop); } /* Evaluates the dominance relationship of latch of the LOOP and BB, and returns the state. */ enum bb_dom_status { /* BB does not dominate latch of the LOOP. */ DOMST_NONDOMINATING, /* The LOOP is broken (there is no path from the header to its latch. */ DOMST_LOOP_BROKEN, /* BB dominates the latch of the LOOP. */ DOMST_DOMINATING }; static enum bb_dom_status determine_bb_domination_status (struct loop *loop, basic_block bb) { basic_block *bblocks; unsigned nblocks, i; bool bb_reachable = false; edge_iterator ei; edge e; /* This function assumes BB is a successor of LOOP->header. If that is not the case return DOMST_NONDOMINATING which is always safe. */ { bool ok = false; FOR_EACH_EDGE (e, ei, bb->preds) { if (e->src == loop->header) { ok = true; break; } } if (!ok) return DOMST_NONDOMINATING; } if (bb == loop->latch) return DOMST_DOMINATING; /* Check that BB dominates LOOP->latch, and that it is back-reachable from it. */ bblocks = XCNEWVEC (basic_block, loop->num_nodes); dbds_ce_stop = loop->header; nblocks = dfs_enumerate_from (loop->latch, 1, dbds_continue_enumeration_p, bblocks, loop->num_nodes, bb); for (i = 0; i < nblocks; i++) FOR_EACH_EDGE (e, ei, bblocks[i]->preds) { if (e->src == loop->header) { free (bblocks); return DOMST_NONDOMINATING; } if (e->src == bb) bb_reachable = true; } free (bblocks); return (bb_reachable ? DOMST_DOMINATING : DOMST_LOOP_BROKEN); } /* Thread jumps through the header of LOOP. Returns true if cfg changes. If MAY_PEEL_LOOP_HEADERS is false, we avoid threading from entry edges to the inside of the loop. */ static bool thread_through_loop_header (struct loop *loop, bool may_peel_loop_headers) { basic_block header = loop->header; edge e, tgt_edge, latch = loop_latch_edge (loop); edge_iterator ei; basic_block tgt_bb, atgt_bb; enum bb_dom_status domst; /* We have already threaded through headers to exits, so all the threading requests now are to the inside of the loop. We need to avoid creating irreducible regions (i.e., loops with more than one entry block), and also loop with several latch edges, or new subloops of the loop (although there are cases where it might be appropriate, it is difficult to decide, and doing it wrongly may confuse other optimizers). We could handle more general cases here. However, the intention is to preserve some information about the loop, which is impossible if its structure changes significantly, in a way that is not well understood. Thus we only handle few important special cases, in which also updating of the loop-carried information should be feasible: 1) Propagation of latch edge to a block that dominates the latch block of a loop. This aims to handle the following idiom: first = 1; while (1) { if (first) initialize; first = 0; body; } After threading the latch edge, this becomes first = 1; if (first) initialize; while (1) { first = 0; body; } The original header of the loop is moved out of it, and we may thread the remaining edges through it without further constraints. 2) All entry edges are propagated to a single basic block that dominates the latch block of the loop. This aims to handle the following idiom (normally created for "for" loops): i = 0; while (1) { if (i >= 100) break; body; i++; } This becomes i = 0; while (1) { body; i++; if (i >= 100) break; } */ /* Threading through the header won't improve the code if the header has just one successor. */ if (single_succ_p (header)) goto fail; if (latch->aux) { if (THREAD_TARGET2 (latch)) goto fail; tgt_edge = THREAD_TARGET (latch); tgt_bb = tgt_edge->dest; } else if (!may_peel_loop_headers && !redirection_block_p (loop->header)) goto fail; else { tgt_bb = NULL; tgt_edge = NULL; FOR_EACH_EDGE (e, ei, header->preds) { if (!e->aux) { if (e == latch) continue; /* If latch is not threaded, and there is a header edge that is not threaded, we would create loop with multiple entries. */ goto fail; } if (THREAD_TARGET2 (e)) goto fail; tgt_edge = THREAD_TARGET (e); atgt_bb = tgt_edge->dest; if (!tgt_bb) tgt_bb = atgt_bb; /* Two targets of threading would make us create loop with multiple entries. */ else if (tgt_bb != atgt_bb) goto fail; } if (!tgt_bb) { /* There are no threading requests. */ return false; } /* Redirecting to empty loop latch is useless. */ if (tgt_bb == loop->latch && empty_block_p (loop->latch)) goto fail; } /* The target block must dominate the loop latch, otherwise we would be creating a subloop. */ domst = determine_bb_domination_status (loop, tgt_bb); if (domst == DOMST_NONDOMINATING) goto fail; if (domst == DOMST_LOOP_BROKEN) { /* If the loop ceased to exist, mark it as such, and thread through its original header. */ loop->header = NULL; loop->latch = NULL; return thread_block (header, false); } if (tgt_bb->loop_father->header == tgt_bb) { /* If the target of the threading is a header of a subloop, we need to create a preheader for it, so that the headers of the two loops do not merge. */ if (EDGE_COUNT (tgt_bb->preds) > 2) { tgt_bb = create_preheader (tgt_bb->loop_father, 0); gcc_assert (tgt_bb != NULL); } else tgt_bb = split_edge (tgt_edge); } if (latch->aux) { /* First handle the case latch edge is redirected. */ loop->latch = thread_single_edge (latch); gcc_assert (single_succ (loop->latch) == tgt_bb); loop->header = tgt_bb; /* Thread the remaining edges through the former header. */ thread_block (header, false); } else { basic_block new_preheader; /* Now consider the case entry edges are redirected to the new entry block. Remember one entry edge, so that we can find the new preheader (its destination after threading). */ FOR_EACH_EDGE (e, ei, header->preds) { if (e->aux) break; } /* The duplicate of the header is the new preheader of the loop. Ensure that it is placed correctly in the loop hierarchy. */ set_loop_copy (loop, loop_outer (loop)); thread_block (header, false); set_loop_copy (loop, NULL); new_preheader = e->dest; /* Create the new latch block. This is always necessary, as the latch must have only a single successor, but the original header had at least two successors. */ loop->latch = NULL; mfb_kj_edge = single_succ_edge (new_preheader); loop->header = mfb_kj_edge->dest; latch = make_forwarder_block (tgt_bb, mfb_keep_just, NULL); loop->header = latch->dest; loop->latch = latch->src; } return true; fail: /* We failed to thread anything. Cancel the requests. */ FOR_EACH_EDGE (e, ei, header->preds) { free (e->aux); e->aux = NULL; } return false; } /* 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; bitmap_iterator bi; bitmap tmp = BITMAP_ALLOC (NULL); basic_block bb; edge e; edge_iterator ei; for (i = 0; i < VEC_length (edge, threaded_edges); i += 3) { edge e = VEC_index (edge, threaded_edges, i); edge *x = (edge *) XNEWVEC (edge, 2); e->aux = x; THREAD_TARGET (e) = VEC_index (edge, threaded_edges, i + 1); THREAD_TARGET2 (e) = VEC_index (edge, threaded_edges, i + 2); bitmap_set_bit (tmp, e->dest->index); } /* If optimizing for size, only thread through block if we don't have to duplicate it or it's an otherwise empty redirection block. */ if (optimize_function_for_size_p (cfun)) { EXECUTE_IF_SET_IN_BITMAP (tmp, 0, i, bi) { bb = BASIC_BLOCK (i); if (EDGE_COUNT (bb->preds) > 1 && !redirection_block_p (bb)) { FOR_EACH_EDGE (e, ei, bb->preds) { free (e->aux); e->aux = NULL; } } else bitmap_set_bit (threaded_blocks, i); } } else bitmap_copy (threaded_blocks, tmp); BITMAP_FREE(tmp); } /* 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. If MAY_PEEL_LOOP_HEADERS is false, we avoid threading edges through loop headers if it does not simplify the loop. Returns true if one or more edges were threaded, false otherwise. */ bool thread_through_all_blocks (bool may_peel_loop_headers) { bool retval = false; unsigned int i; bitmap_iterator bi; bitmap threaded_blocks; struct loop *loop; loop_iterator li; /* We must know about loops in order to preserve them. */ gcc_assert (current_loops != NULL); if (threaded_edges == NULL) return false; threaded_blocks = BITMAP_ALLOC (NULL); memset (&thread_stats, 0, sizeof (thread_stats)); mark_threaded_blocks (threaded_blocks); initialize_original_copy_tables (); /* First perform the threading requests that do not affect loop structure. */ 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, true); } /* Then perform the threading through loop headers. We start with the innermost loop, so that the changes in cfg we perform won't affect further threading. */ FOR_EACH_LOOP (li, loop, LI_FROM_INNERMOST) { if (!loop->header || !bitmap_bit_p (threaded_blocks, loop->header->index)) continue; retval |= thread_through_loop_header (loop, may_peel_loop_headers); } statistics_counter_event (cfun, "Jumps threaded", thread_stats.num_threaded_edges); free_original_copy_tables (); BITMAP_FREE (threaded_blocks); threaded_blocks = NULL; VEC_free (edge, heap, threaded_edges); threaded_edges = NULL; if (retval) loops_state_set (LOOPS_NEED_FIXUP); 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, i.e., 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, edge e3) { /* This can occur if we're jumping to a constant address or or something similar. Just get out now. */ if (e2 == NULL) return; if (threaded_edges == NULL) threaded_edges = VEC_alloc (edge, heap, 15); if (dump_file && (dump_flags & TDF_DETAILS) && e->dest != e2->src) fprintf (dump_file, " Registering jump thread around one or more intermediate blocks\n"); VEC_safe_push (edge, heap, threaded_edges, e); VEC_safe_push (edge, heap, threaded_edges, e2); VEC_safe_push (edge, heap, threaded_edges, e3); }
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