/* Generic SSA value propagation engine. Copyright (C) 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. Contributed by Diego Novillo <dnovillo@redhat.com> 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, 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 "timevar.h" #include "tree-dump.h" #include "tree-flow.h" #include "tree-pass.h" #include "tree-ssa-propagate.h" #include "langhooks.h" #include "varray.h" #include "vec.h" /* This file implements a generic value propagation engine based on the same propagation used by the SSA-CCP algorithm [1]. Propagation is performed by simulating the execution of every statement that produces the value being propagated. Simulation proceeds as follows: 1- Initially, all edges of the CFG are marked not executable and the CFG worklist is seeded with all the statements in the entry basic block (block 0). 2- Every statement S is simulated with a call to the call-back function SSA_PROP_VISIT_STMT. This evaluation may produce 3 results: SSA_PROP_NOT_INTERESTING: Statement S produces nothing of interest and does not affect any of the work lists. SSA_PROP_VARYING: The value produced by S cannot be determined at compile time. Further simulation of S is not required. If S is a conditional jump, all the outgoing edges for the block are considered executable and added to the work list. SSA_PROP_INTERESTING: S produces a value that can be computed at compile time. Its result can be propagated into the statements that feed from S. Furthermore, if S is a conditional jump, only the edge known to be taken is added to the work list. Edges that are known not to execute are never simulated. 3- PHI nodes are simulated with a call to SSA_PROP_VISIT_PHI. The return value from SSA_PROP_VISIT_PHI has the same semantics as described in #2. 4- Three work lists are kept. Statements are only added to these lists if they produce one of SSA_PROP_INTERESTING or SSA_PROP_VARYING. CFG_BLOCKS contains the list of blocks to be simulated. Blocks are added to this list if their incoming edges are found executable. VARYING_SSA_EDGES contains the list of statements that feed from statements that produce an SSA_PROP_VARYING result. These are simulated first to speed up processing. INTERESTING_SSA_EDGES contains the list of statements that feed from statements that produce an SSA_PROP_INTERESTING result. 5- Simulation terminates when all three work lists are drained. Before calling ssa_propagate, it is important to clear DONT_SIMULATE_AGAIN for all the statements in the program that should be simulated. This initialization allows an implementation to specify which statements should never be simulated. It is also important to compute def-use information before calling ssa_propagate. References: [1] Constant propagation with conditional branches, Wegman and Zadeck, ACM TOPLAS 13(2):181-210. [2] Building an Optimizing Compiler, Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9. [3] Advanced Compiler Design and Implementation, Steven Muchnick, Morgan Kaufmann, 1997, Section 12.6 */ /* Function pointers used to parameterize the propagation engine. */ static ssa_prop_visit_stmt_fn ssa_prop_visit_stmt; static ssa_prop_visit_phi_fn ssa_prop_visit_phi; /* Use the TREE_DEPRECATED bitflag to mark statements that have been added to one of the SSA edges worklists. This flag is used to avoid visiting statements unnecessarily when draining an SSA edge worklist. If while simulating a basic block, we find a statement with STMT_IN_SSA_EDGE_WORKLIST set, we clear it to prevent SSA edge processing from visiting it again. */ #define STMT_IN_SSA_EDGE_WORKLIST(T) TREE_DEPRECATED (T) /* A bitmap to keep track of executable blocks in the CFG. */ static sbitmap executable_blocks; /* Array of control flow edges on the worklist. */ static GTY(()) varray_type cfg_blocks = NULL; static unsigned int cfg_blocks_num = 0; static int cfg_blocks_tail; static int cfg_blocks_head; static sbitmap bb_in_list; /* Worklist of SSA edges which will need reexamination as their definition has changed. SSA edges are def-use edges in the SSA web. For each D-U edge, we store the target statement or PHI node U. */ static GTY(()) VEC(tree) *interesting_ssa_edges; /* Identical to INTERESTING_SSA_EDGES. For performance reasons, the list of SSA edges is split into two. One contains all SSA edges who need to be reexamined because their lattice value changed to varying (this worklist), and the other contains all other SSA edges to be reexamined (INTERESTING_SSA_EDGES). Since most values in the program are VARYING, the ideal situation is to move them to that lattice value as quickly as possible. Thus, it doesn't make sense to process any other type of lattice value until all VARYING values are propagated fully, which is one thing using the VARYING worklist achieves. In addition, if we don't use a separate worklist for VARYING edges, we end up with situations where lattice values move from UNDEFINED->INTERESTING->VARYING instead of UNDEFINED->VARYING. */ static GTY(()) VEC(tree) *varying_ssa_edges; /* Return true if the block worklist empty. */ static inline bool cfg_blocks_empty_p (void) { return (cfg_blocks_num == 0); } /* Add a basic block to the worklist. The block must not be already in the worklist, and it must not be the ENTRY or EXIT block. */ static void cfg_blocks_add (basic_block bb) { gcc_assert (bb != ENTRY_BLOCK_PTR && bb != EXIT_BLOCK_PTR); gcc_assert (!TEST_BIT (bb_in_list, bb->index)); if (cfg_blocks_empty_p ()) { cfg_blocks_tail = cfg_blocks_head = 0; cfg_blocks_num = 1; } else { cfg_blocks_num++; if (cfg_blocks_num > VARRAY_SIZE (cfg_blocks)) { /* We have to grow the array now. Adjust to queue to occupy the full space of the original array. */ cfg_blocks_tail = VARRAY_SIZE (cfg_blocks); cfg_blocks_head = 0; VARRAY_GROW (cfg_blocks, 2 * VARRAY_SIZE (cfg_blocks)); } else cfg_blocks_tail = (cfg_blocks_tail + 1) % VARRAY_SIZE (cfg_blocks); } VARRAY_BB (cfg_blocks, cfg_blocks_tail) = bb; SET_BIT (bb_in_list, bb->index); } /* Remove a block from the worklist. */ static basic_block cfg_blocks_get (void) { basic_block bb; bb = VARRAY_BB (cfg_blocks, cfg_blocks_head); gcc_assert (!cfg_blocks_empty_p ()); gcc_assert (bb); cfg_blocks_head = (cfg_blocks_head + 1) % VARRAY_SIZE (cfg_blocks); --cfg_blocks_num; RESET_BIT (bb_in_list, bb->index); return bb; } /* We have just defined a new value for VAR. If IS_VARYING is true, add all immediate uses of VAR to VARYING_SSA_EDGES, otherwise add them to INTERESTING_SSA_EDGES. */ static void add_ssa_edge (tree var, bool is_varying) { tree stmt = SSA_NAME_DEF_STMT (var); dataflow_t df = get_immediate_uses (stmt); int num_uses = num_immediate_uses (df); int i; for (i = 0; i < num_uses; i++) { tree use_stmt = immediate_use (df, i); if (!DONT_SIMULATE_AGAIN (use_stmt) && !STMT_IN_SSA_EDGE_WORKLIST (use_stmt)) { STMT_IN_SSA_EDGE_WORKLIST (use_stmt) = 1; if (is_varying) VEC_safe_push (tree, varying_ssa_edges, use_stmt); else VEC_safe_push (tree, interesting_ssa_edges, use_stmt); } } } /* Add edge E to the control flow worklist. */ static void add_control_edge (edge e) { basic_block bb = e->dest; if (bb == EXIT_BLOCK_PTR) return; /* If the edge had already been executed, skip it. */ if (e->flags & EDGE_EXECUTABLE) return; e->flags |= EDGE_EXECUTABLE; /* If the block is already in the list, we're done. */ if (TEST_BIT (bb_in_list, bb->index)) return; cfg_blocks_add (bb); if (dump_file && (dump_flags & TDF_DETAILS)) fprintf (dump_file, "Adding Destination of edge (%d -> %d) to worklist\n\n", e->src->index, e->dest->index); } /* Simulate the execution of STMT and update the work lists accordingly. */ static void simulate_stmt (tree stmt) { enum ssa_prop_result val = SSA_PROP_NOT_INTERESTING; edge taken_edge = NULL; tree output_name = NULL_TREE; /* Don't bother visiting statements that are already considered varying by the propagator. */ if (DONT_SIMULATE_AGAIN (stmt)) return; if (TREE_CODE (stmt) == PHI_NODE) { val = ssa_prop_visit_phi (stmt); output_name = PHI_RESULT (stmt); } else val = ssa_prop_visit_stmt (stmt, &taken_edge, &output_name); if (val == SSA_PROP_VARYING) { DONT_SIMULATE_AGAIN (stmt) = 1; /* If the statement produced a new varying value, add the SSA edges coming out of OUTPUT_NAME. */ if (output_name) add_ssa_edge (output_name, true); /* If STMT transfers control out of its basic block, add all outgoing edges to the work list. */ if (stmt_ends_bb_p (stmt)) { edge e; edge_iterator ei; basic_block bb = bb_for_stmt (stmt); FOR_EACH_EDGE (e, ei, bb->succs) add_control_edge (e); } } else if (val == SSA_PROP_INTERESTING) { /* If the statement produced new value, add the SSA edges coming out of OUTPUT_NAME. */ if (output_name) add_ssa_edge (output_name, false); /* If we know which edge is going to be taken out of this block, add it to the CFG work list. */ if (taken_edge) add_control_edge (taken_edge); } } /* Process an SSA edge worklist. WORKLIST is the SSA edge worklist to drain. This pops statements off the given WORKLIST and processes them until there are no more statements on WORKLIST. We take a pointer to WORKLIST because it may be reallocated when an SSA edge is added to it in simulate_stmt. */ static void process_ssa_edge_worklist (VEC(tree) **worklist) { /* Drain the entire worklist. */ while (VEC_length (tree, *worklist) > 0) { basic_block bb; /* Pull the statement to simulate off the worklist. */ tree stmt = VEC_pop (tree, *worklist); /* If this statement was already visited by simulate_block, then we don't need to visit it again here. */ if (!STMT_IN_SSA_EDGE_WORKLIST (stmt)) continue; /* STMT is no longer in a worklist. */ STMT_IN_SSA_EDGE_WORKLIST (stmt) = 0; if (dump_file && (dump_flags & TDF_DETAILS)) { fprintf (dump_file, "\nSimulating statement (from ssa_edges): "); print_generic_stmt (dump_file, stmt, dump_flags); } bb = bb_for_stmt (stmt); /* PHI nodes are always visited, regardless of whether or not the destination block is executable. Otherwise, visit the statement only if its block is marked executable. */ if (TREE_CODE (stmt) == PHI_NODE || TEST_BIT (executable_blocks, bb->index)) simulate_stmt (stmt); } } /* Simulate the execution of BLOCK. Evaluate the statement associated with each variable reference inside the block. */ static void simulate_block (basic_block block) { tree phi; /* There is nothing to do for the exit block. */ if (block == EXIT_BLOCK_PTR) return; if (dump_file && (dump_flags & TDF_DETAILS)) fprintf (dump_file, "\nSimulating block %d\n", block->index); /* Always simulate PHI nodes, even if we have simulated this block before. */ for (phi = phi_nodes (block); phi; phi = PHI_CHAIN (phi)) simulate_stmt (phi); /* If this is the first time we've simulated this block, then we must simulate each of its statements. */ if (!TEST_BIT (executable_blocks, block->index)) { block_stmt_iterator j; unsigned int normal_edge_count; edge e, normal_edge; edge_iterator ei; /* Note that we have simulated this block. */ SET_BIT (executable_blocks, block->index); for (j = bsi_start (block); !bsi_end_p (j); bsi_next (&j)) { tree stmt = bsi_stmt (j); /* If this statement is already in the worklist then "cancel" it. The reevaluation implied by the worklist entry will produce the same value we generate here and thus reevaluating it again from the worklist is pointless. */ if (STMT_IN_SSA_EDGE_WORKLIST (stmt)) STMT_IN_SSA_EDGE_WORKLIST (stmt) = 0; simulate_stmt (stmt); } /* We can not predict when abnormal edges will be executed, so once a block is considered executable, we consider any outgoing abnormal edges as executable. At the same time, if this block has only one successor that is reached by non-abnormal edges, then add that successor to the worklist. */ normal_edge_count = 0; normal_edge = NULL; FOR_EACH_EDGE (e, ei, block->succs) { if (e->flags & EDGE_ABNORMAL) add_control_edge (e); else { normal_edge_count++; normal_edge = e; } } if (normal_edge_count == 1) add_control_edge (normal_edge); } } /* Initialize local data structures and work lists. */ static void ssa_prop_init (void) { edge e; edge_iterator ei; basic_block bb; /* Worklists of SSA edges. */ interesting_ssa_edges = VEC_alloc (tree, 20); varying_ssa_edges = VEC_alloc (tree, 20); executable_blocks = sbitmap_alloc (last_basic_block); sbitmap_zero (executable_blocks); bb_in_list = sbitmap_alloc (last_basic_block); sbitmap_zero (bb_in_list); if (dump_file && (dump_flags & TDF_DETAILS)) dump_immediate_uses (dump_file); VARRAY_BB_INIT (cfg_blocks, 20, "cfg_blocks"); /* Initially assume that every edge in the CFG is not executable (including the edges coming out of ENTRY_BLOCK_PTR). */ FOR_ALL_BB (bb) { block_stmt_iterator si; for (si = bsi_start (bb); !bsi_end_p (si); bsi_next (&si)) STMT_IN_SSA_EDGE_WORKLIST (bsi_stmt (si)) = 0; FOR_EACH_EDGE (e, ei, bb->succs) e->flags &= ~EDGE_EXECUTABLE; } /* Seed the algorithm by adding the successors of the entry block to the edge worklist. */ FOR_EACH_EDGE (e, ei, ENTRY_BLOCK_PTR->succs) add_control_edge (e); } /* Free allocated storage. */ static void ssa_prop_fini (void) { VEC_free (tree, interesting_ssa_edges); VEC_free (tree, varying_ssa_edges); cfg_blocks = NULL; sbitmap_free (bb_in_list); sbitmap_free (executable_blocks); free_df (); } /* Get the main expression from statement STMT. */ tree get_rhs (tree stmt) { enum tree_code code = TREE_CODE (stmt); switch (code) { case RETURN_EXPR: stmt = TREE_OPERAND (stmt, 0); if (!stmt || TREE_CODE (stmt) != MODIFY_EXPR) return stmt; /* FALLTHRU */ case MODIFY_EXPR: stmt = TREE_OPERAND (stmt, 1); if (TREE_CODE (stmt) == WITH_SIZE_EXPR) return TREE_OPERAND (stmt, 0); else return stmt; case COND_EXPR: return COND_EXPR_COND (stmt); case SWITCH_EXPR: return SWITCH_COND (stmt); case GOTO_EXPR: return GOTO_DESTINATION (stmt); case LABEL_EXPR: return LABEL_EXPR_LABEL (stmt); default: return stmt; } } /* Set the main expression of *STMT_P to EXPR. If EXPR is not a valid GIMPLE expression no changes are done and the function returns false. */ bool set_rhs (tree *stmt_p, tree expr) { tree stmt = *stmt_p, op; enum tree_code code = TREE_CODE (expr); stmt_ann_t ann; tree var; ssa_op_iter iter; /* Verify the constant folded result is valid gimple. */ if (TREE_CODE_CLASS (code) == tcc_binary) { if (!is_gimple_val (TREE_OPERAND (expr, 0)) || !is_gimple_val (TREE_OPERAND (expr, 1))) return false; } else if (TREE_CODE_CLASS (code) == tcc_unary) { if (!is_gimple_val (TREE_OPERAND (expr, 0))) return false; } else if (code == COMPOUND_EXPR) return false; switch (TREE_CODE (stmt)) { case RETURN_EXPR: op = TREE_OPERAND (stmt, 0); if (TREE_CODE (op) != MODIFY_EXPR) { TREE_OPERAND (stmt, 0) = expr; break; } stmt = op; /* FALLTHRU */ case MODIFY_EXPR: op = TREE_OPERAND (stmt, 1); if (TREE_CODE (op) == WITH_SIZE_EXPR) stmt = op; TREE_OPERAND (stmt, 1) = expr; break; case COND_EXPR: COND_EXPR_COND (stmt) = expr; break; case SWITCH_EXPR: SWITCH_COND (stmt) = expr; break; case GOTO_EXPR: GOTO_DESTINATION (stmt) = expr; break; case LABEL_EXPR: LABEL_EXPR_LABEL (stmt) = expr; break; default: /* Replace the whole statement with EXPR. If EXPR has no side effects, then replace *STMT_P with an empty statement. */ ann = stmt_ann (stmt); *stmt_p = TREE_SIDE_EFFECTS (expr) ? expr : build_empty_stmt (); (*stmt_p)->common.ann = (tree_ann_t) ann; if (TREE_SIDE_EFFECTS (expr)) { /* Fix all the SSA_NAMEs created by *STMT_P to point to its new replacement. */ FOR_EACH_SSA_TREE_OPERAND (var, stmt, iter, SSA_OP_ALL_DEFS) { if (TREE_CODE (var) == SSA_NAME) SSA_NAME_DEF_STMT (var) = *stmt_p; } } break; } return true; } /* Entry point to the propagation engine. VISIT_STMT is called for every statement visited. VISIT_PHI is called for every PHI node visited. */ void ssa_propagate (ssa_prop_visit_stmt_fn visit_stmt, ssa_prop_visit_phi_fn visit_phi) { ssa_prop_visit_stmt = visit_stmt; ssa_prop_visit_phi = visit_phi; ssa_prop_init (); /* Iterate until the worklists are empty. */ while (!cfg_blocks_empty_p () || VEC_length (tree, interesting_ssa_edges) > 0 || VEC_length (tree, varying_ssa_edges) > 0) { if (!cfg_blocks_empty_p ()) { /* Pull the next block to simulate off the worklist. */ basic_block dest_block = cfg_blocks_get (); simulate_block (dest_block); } /* In order to move things to varying as quickly as possible,process the VARYING_SSA_EDGES worklist first. */ process_ssa_edge_worklist (&varying_ssa_edges); /* Now process the INTERESTING_SSA_EDGES worklist. */ process_ssa_edge_worklist (&interesting_ssa_edges); } ssa_prop_fini (); } #include "gt-tree-ssa-propagate.h"