/* Dead code elimination pass for the GNU compiler. Copyright (C) 2002, 2003, 2004 Free Software Foundation, Inc. Contributed by Ben Elliston and Andrew MacLeod Adapted to use control dependence by Steven Bosscher, SUSE Labs. 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. */ /* Dead code elimination. References: Building an Optimizing Compiler, Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9. Advanced Compiler Design and Implementation, Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10. Dead-code elimination is the removal of statements which have no impact on the program's output. "Dead statements" have no impact on the program's output, while "necessary statements" may have impact on the output. The algorithm consists of three phases: 1. Marking as necessary all statements known to be necessary, e.g. most function calls, writing a value to memory, etc; 2. Propagating necessary statements, e.g., the statements giving values to operands in necessary statements; and 3. Removing dead statements. */ #include "config.h" #include "system.h" #include "coretypes.h" #include "tm.h" #include "errors.h" #include "ggc.h" /* These RTL headers are needed for basic-block.h. */ #include "rtl.h" #include "tm_p.h" #include "hard-reg-set.h" #include "obstack.h" #include "basic-block.h" #include "tree.h" #include "diagnostic.h" #include "tree-flow.h" #include "tree-gimple.h" #include "tree-dump.h" #include "tree-pass.h" #include "timevar.h" #include "flags.h" static struct stmt_stats { int total; int total_phis; int removed; int removed_phis; } stats; static varray_type worklist; /* Vector indicating an SSA name has already been processed and marked as necessary. */ static sbitmap processed; /* Vector indicating that last_stmt if a basic block has already been marked as necessary. */ static sbitmap last_stmt_necessary; /* Before we can determine whether a control branch is dead, we need to compute which blocks are control dependent on which edges. We expect each block to be control dependent on very few edges so we use a bitmap for each block recording its edges. An array holds the bitmap. The Ith bit in the bitmap is set if that block is dependent on the Ith edge. */ bitmap *control_dependence_map; /* Execute CODE for each edge (given number EDGE_NUMBER within the CODE) for which the block with index N is control dependent. */ #define EXECUTE_IF_CONTROL_DEPENDENT(N, EDGE_NUMBER, CODE) \ { \ bitmap_iterator bi; \ \ EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[N], 0, EDGE_NUMBER, bi) \ { \ CODE; \ } \ } /* Local function prototypes. */ static inline void set_control_dependence_map_bit (basic_block, int); static inline void clear_control_dependence_bitmap (basic_block); static void find_all_control_dependences (struct edge_list *); static void find_control_dependence (struct edge_list *, int); static inline basic_block find_pdom (basic_block); static inline void mark_stmt_necessary (tree, bool); static inline void mark_operand_necessary (tree, bool); static void mark_stmt_if_obviously_necessary (tree, bool); static void find_obviously_necessary_stmts (struct edge_list *); static void mark_control_dependent_edges_necessary (basic_block, struct edge_list *); static void propagate_necessity (struct edge_list *); static void eliminate_unnecessary_stmts (void); static void remove_dead_phis (basic_block); static void remove_dead_stmt (block_stmt_iterator *, basic_block); static void print_stats (void); static void tree_dce_init (bool); static void tree_dce_done (bool); /* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */ static inline void set_control_dependence_map_bit (basic_block bb, int edge_index) { if (bb == ENTRY_BLOCK_PTR) return; gcc_assert (bb != EXIT_BLOCK_PTR); bitmap_set_bit (control_dependence_map[bb->index], edge_index); } /* Clear all control dependences for block BB. */ static inline void clear_control_dependence_bitmap (basic_block bb) { bitmap_clear (control_dependence_map[bb->index]); } /* Record all blocks' control dependences on all edges in the edge list EL, ala Morgan, Section 3.6. */ static void find_all_control_dependences (struct edge_list *el) { int i; for (i = 0; i < NUM_EDGES (el); ++i) find_control_dependence (el, i); } /* Determine all blocks' control dependences on the given edge with edge_list EL index EDGE_INDEX, ala Morgan, Section 3.6. */ static void find_control_dependence (struct edge_list *el, int edge_index) { basic_block current_block; basic_block ending_block; gcc_assert (INDEX_EDGE_PRED_BB (el, edge_index) != EXIT_BLOCK_PTR); if (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR) ending_block = ENTRY_BLOCK_PTR->next_bb; else ending_block = find_pdom (INDEX_EDGE_PRED_BB (el, edge_index)); for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index); current_block != ending_block && current_block != EXIT_BLOCK_PTR; current_block = find_pdom (current_block)) { edge e = INDEX_EDGE (el, edge_index); /* For abnormal edges, we don't make current_block control dependent because instructions that throw are always necessary anyway. */ if (e->flags & EDGE_ABNORMAL) continue; set_control_dependence_map_bit (current_block, edge_index); } } /* Find the immediate postdominator PDOM of the specified basic block BLOCK. This function is necessary because some blocks have negative numbers. */ static inline basic_block find_pdom (basic_block block) { gcc_assert (block != ENTRY_BLOCK_PTR); if (block == EXIT_BLOCK_PTR) return EXIT_BLOCK_PTR; else { basic_block bb = get_immediate_dominator (CDI_POST_DOMINATORS, block); if (! bb) return EXIT_BLOCK_PTR; return bb; } } #define NECESSARY(stmt) stmt->common.asm_written_flag /* If STMT is not already marked necessary, mark it, and add it to the worklist if ADD_TO_WORKLIST is true. */ static inline void mark_stmt_necessary (tree stmt, bool add_to_worklist) { gcc_assert (stmt); gcc_assert (stmt != error_mark_node); gcc_assert (!DECL_P (stmt)); if (NECESSARY (stmt)) return; if (dump_file && (dump_flags & TDF_DETAILS)) { fprintf (dump_file, "Marking useful stmt: "); print_generic_stmt (dump_file, stmt, TDF_SLIM); fprintf (dump_file, "\n"); } NECESSARY (stmt) = 1; if (add_to_worklist) VARRAY_PUSH_TREE (worklist, stmt); } /* Mark the statement defining operand OP as necessary. PHIONLY is true if we should only mark it necessary if it is a phi node. */ static inline void mark_operand_necessary (tree op, bool phionly) { tree stmt; int ver; gcc_assert (op); ver = SSA_NAME_VERSION (op); if (TEST_BIT (processed, ver)) return; SET_BIT (processed, ver); stmt = SSA_NAME_DEF_STMT (op); gcc_assert (stmt); if (NECESSARY (stmt) || IS_EMPTY_STMT (stmt) || (phionly && TREE_CODE (stmt) != PHI_NODE)) return; NECESSARY (stmt) = 1; VARRAY_PUSH_TREE (worklist, stmt); } /* Mark STMT as necessary if it obviously is. Add it to the worklist if it can make other statements necessary. If AGGRESSIVE is false, control statements are conservatively marked as necessary. */ static void mark_stmt_if_obviously_necessary (tree stmt, bool aggressive) { v_may_def_optype v_may_defs; v_must_def_optype v_must_defs; stmt_ann_t ann; tree op, def; ssa_op_iter iter; /* Statements that are implicitly live. Most function calls, asm and return statements are required. Labels and BIND_EXPR nodes are kept because they are control flow, and we have no way of knowing whether they can be removed. DCE can eliminate all the other statements in a block, and CFG can then remove the block and labels. */ switch (TREE_CODE (stmt)) { case BIND_EXPR: case LABEL_EXPR: case CASE_LABEL_EXPR: mark_stmt_necessary (stmt, false); return; case ASM_EXPR: case RESX_EXPR: case RETURN_EXPR: mark_stmt_necessary (stmt, true); return; case CALL_EXPR: /* Most, but not all function calls are required. Function calls that produce no result and have no side effects (i.e. const pure functions) are unnecessary. */ if (TREE_SIDE_EFFECTS (stmt)) mark_stmt_necessary (stmt, true); return; case MODIFY_EXPR: op = get_call_expr_in (stmt); if (op && TREE_SIDE_EFFECTS (op)) { mark_stmt_necessary (stmt, true); return; } /* These values are mildly magic bits of the EH runtime. We can't see the entire lifetime of these values until landing pads are generated. */ if (TREE_CODE (TREE_OPERAND (stmt, 0)) == EXC_PTR_EXPR || TREE_CODE (TREE_OPERAND (stmt, 0)) == FILTER_EXPR) { mark_stmt_necessary (stmt, true); return; } break; case GOTO_EXPR: gcc_assert (!simple_goto_p (stmt)); mark_stmt_necessary (stmt, true); return; case COND_EXPR: gcc_assert (EDGE_COUNT (bb_for_stmt (stmt)->succs) == 2); /* Fall through. */ case SWITCH_EXPR: if (! aggressive) mark_stmt_necessary (stmt, true); break; default: break; } ann = stmt_ann (stmt); /* If the statement has volatile operands, it needs to be preserved. Same for statements that can alter control flow in unpredictable ways. */ if (ann->has_volatile_ops || is_ctrl_altering_stmt (stmt)) { mark_stmt_necessary (stmt, true); return; } get_stmt_operands (stmt); FOR_EACH_SSA_TREE_OPERAND (def, stmt, iter, SSA_OP_DEF) { if (is_global_var (SSA_NAME_VAR (def))) { mark_stmt_necessary (stmt, true); return; } } /* Check virtual definitions. If we get here, the only virtual definitions we should see are those generated by assignment statements. */ v_may_defs = V_MAY_DEF_OPS (ann); v_must_defs = V_MUST_DEF_OPS (ann); if (NUM_V_MAY_DEFS (v_may_defs) > 0 || NUM_V_MUST_DEFS (v_must_defs) > 0) { tree lhs; gcc_assert (TREE_CODE (stmt) == MODIFY_EXPR); /* Note that we must not check the individual virtual operands here. In particular, if this is an aliased store, we could end up with something like the following (SSA notation redacted for brevity): foo (int *p, int i) { int x; p_1 = (i_2 > 3) ? &x : p_1; # x_4 = V_MAY_DEF *p_1 = 5; return 2; } Notice that the store to '*p_1' should be preserved, if we were to check the virtual definitions in that store, we would not mark it needed. This is because 'x' is not a global variable. Therefore, we check the base address of the LHS. If the address is a pointer, we check if its name tag or type tag is a global variable. Otherwise, we check if the base variable is a global. */ lhs = TREE_OPERAND (stmt, 0); if (REFERENCE_CLASS_P (lhs)) lhs = get_base_address (lhs); if (lhs == NULL_TREE) { /* If LHS is NULL, it means that we couldn't get the base address of the reference. In which case, we should not remove this store. */ mark_stmt_necessary (stmt, true); } else if (DECL_P (lhs)) { /* If the store is to a global symbol, we need to keep it. */ if (is_global_var (lhs)) mark_stmt_necessary (stmt, true); } else if (INDIRECT_REF_P (lhs)) { tree ptr = TREE_OPERAND (lhs, 0); struct ptr_info_def *pi = SSA_NAME_PTR_INFO (ptr); tree nmt = (pi) ? pi->name_mem_tag : NULL_TREE; tree tmt = var_ann (SSA_NAME_VAR (ptr))->type_mem_tag; /* If either the name tag or the type tag for PTR is a global variable, then the store is necessary. */ if ((nmt && is_global_var (nmt)) || (tmt && is_global_var (tmt))) { mark_stmt_necessary (stmt, true); return; } } else gcc_unreachable (); } return; } /* Find obviously necessary statements. These are things like most function calls, and stores to file level variables. If EL is NULL, control statements are conservatively marked as necessary. Otherwise it contains the list of edges used by control dependence analysis. */ static void find_obviously_necessary_stmts (struct edge_list *el) { basic_block bb; block_stmt_iterator i; edge e; FOR_EACH_BB (bb) { tree phi; /* Check any PHI nodes in the block. */ for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi)) { NECESSARY (phi) = 0; /* PHIs for virtual variables do not directly affect code generation and need not be considered inherently necessary regardless of the bits set in their decl. Thus, we only need to mark PHIs for real variables which need their result preserved as being inherently necessary. */ if (is_gimple_reg (PHI_RESULT (phi)) && is_global_var (SSA_NAME_VAR (PHI_RESULT (phi)))) mark_stmt_necessary (phi, true); } /* Check all statements in the block. */ for (i = bsi_start (bb); ! bsi_end_p (i); bsi_next (&i)) { tree stmt = bsi_stmt (i); NECESSARY (stmt) = 0; mark_stmt_if_obviously_necessary (stmt, el != NULL); } /* Mark this basic block as `not visited'. A block will be marked visited when the edges that it is control dependent on have been marked. */ bb->flags &= ~BB_VISITED; } if (el) { /* Prevent the loops from being removed. We must keep the infinite loops, and we currently do not have a means to recognize the finite ones. */ FOR_EACH_BB (bb) { edge_iterator ei; FOR_EACH_EDGE (e, ei, bb->succs) if (e->flags & EDGE_DFS_BACK) mark_control_dependent_edges_necessary (e->dest, el); } } } /* Make corresponding control dependent edges necessary. We only have to do this once for each basic block, so we clear the bitmap after we're done. */ static void mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el) { unsigned edge_number; gcc_assert (bb != EXIT_BLOCK_PTR); if (bb == ENTRY_BLOCK_PTR) return; EXECUTE_IF_CONTROL_DEPENDENT (bb->index, edge_number, { tree t; basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number); if (TEST_BIT (last_stmt_necessary, cd_bb->index)) continue; SET_BIT (last_stmt_necessary, cd_bb->index); t = last_stmt (cd_bb); if (t && is_ctrl_stmt (t)) mark_stmt_necessary (t, true); }); } /* Propagate necessity using the operands of necessary statements. Process the uses on each statement in the worklist, and add all feeding statements which contribute to the calculation of this value to the worklist. In conservative mode, EL is NULL. */ static void propagate_necessity (struct edge_list *el) { tree i; bool aggressive = (el ? true : false); if (dump_file && (dump_flags & TDF_DETAILS)) fprintf (dump_file, "\nProcessing worklist:\n"); while (VARRAY_ACTIVE_SIZE (worklist) > 0) { /* Take `i' from worklist. */ i = VARRAY_TOP_TREE (worklist); VARRAY_POP (worklist); if (dump_file && (dump_flags & TDF_DETAILS)) { fprintf (dump_file, "processing: "); print_generic_stmt (dump_file, i, TDF_SLIM); fprintf (dump_file, "\n"); } if (aggressive) { /* Mark the last statements of the basic blocks that the block containing `i' is control dependent on, but only if we haven't already done so. */ basic_block bb = bb_for_stmt (i); if (! (bb->flags & BB_VISITED)) { bb->flags |= BB_VISITED; mark_control_dependent_edges_necessary (bb, el); } } if (TREE_CODE (i) == PHI_NODE) { /* PHI nodes are somewhat special in that each PHI alternative has data and control dependencies. All the statements feeding the PHI node's arguments are always necessary. In aggressive mode, we also consider the control dependent edges leading to the predecessor block associated with each PHI alternative as necessary. */ int k; for (k = 0; k < PHI_NUM_ARGS (i); k++) { tree arg = PHI_ARG_DEF (i, k); if (TREE_CODE (arg) == SSA_NAME) mark_operand_necessary (arg, false); } if (aggressive) { for (k = 0; k < PHI_NUM_ARGS (i); k++) { basic_block arg_bb = PHI_ARG_EDGE (i, k)->src; if (! (arg_bb->flags & BB_VISITED)) { arg_bb->flags |= BB_VISITED; mark_control_dependent_edges_necessary (arg_bb, el); } } } } else { /* Propagate through the operands. Examine all the USE, VUSE and V_MAY_DEF operands in this statement. Mark all the statements which feed this statement's uses as necessary. */ ssa_op_iter iter; tree use; get_stmt_operands (i); /* The operands of V_MAY_DEF expressions are also needed as they represent potential definitions that may reach this statement (V_MAY_DEF operands allow us to follow def-def links). */ FOR_EACH_SSA_TREE_OPERAND (use, i, iter, SSA_OP_ALL_USES) mark_operand_necessary (use, false); } } } /* Propagate necessity around virtual phi nodes used in kill operands. The reason this isn't done during propagate_necessity is because we don't want to keep phis around that are just there for must-defs, unless we absolutely have to. After we've rewritten the reaching definitions to be correct in the previous part of the fixup routine, we can simply propagate around the information about which of these virtual phi nodes are really used, and set the NECESSARY flag accordingly. Note that we do the minimum here to ensure that we keep alive the phis that are actually used in the corrected SSA form. In particular, some of these phis may now have all of the same operand, and will be deleted by some other pass. */ static void mark_really_necessary_kill_operand_phis (void) { basic_block bb; int i; /* Seed the worklist with the new virtual phi arguments and virtual uses */ FOR_EACH_BB (bb) { block_stmt_iterator bsi; tree phi; for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi)) { if (!is_gimple_reg (PHI_RESULT (phi)) && NECESSARY (phi)) { for (i = 0; i < PHI_NUM_ARGS (phi); i++) mark_operand_necessary (PHI_ARG_DEF (phi, i), true); } } for (bsi = bsi_last (bb); !bsi_end_p (bsi); bsi_prev (&bsi)) { tree stmt = bsi_stmt (bsi); if (NECESSARY (stmt)) { use_operand_p use_p; ssa_op_iter iter; FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_VIRTUAL_USES | SSA_OP_VIRTUAL_KILLS) { tree use = USE_FROM_PTR (use_p); mark_operand_necessary (use, true); } } } } /* Mark all virtual phis still in use as necessary, and all of their arguments that are phis as necessary. */ while (VARRAY_ACTIVE_SIZE (worklist) > 0) { tree use = VARRAY_TOP_TREE (worklist); VARRAY_POP (worklist); for (i = 0; i < PHI_NUM_ARGS (use); i++) mark_operand_necessary (PHI_ARG_DEF (use, i), true); } } /* Eliminate unnecessary statements. Any instruction not marked as necessary contributes nothing to the program, and can be deleted. */ static void eliminate_unnecessary_stmts (void) { basic_block bb; block_stmt_iterator i; if (dump_file && (dump_flags & TDF_DETAILS)) fprintf (dump_file, "\nEliminating unnecessary statements:\n"); clear_special_calls (); FOR_EACH_BB (bb) { /* Remove dead PHI nodes. */ remove_dead_phis (bb); /* Remove dead statements. */ for (i = bsi_start (bb); ! bsi_end_p (i) ; ) { tree t = bsi_stmt (i); stats.total++; /* If `i' is not necessary then remove it. */ if (! NECESSARY (t)) remove_dead_stmt (&i, bb); else { tree call = get_call_expr_in (t); if (call) notice_special_calls (call); bsi_next (&i); } } } } /* Remove dead PHI nodes from block BB. */ static void remove_dead_phis (basic_block bb) { tree prev, phi; prev = NULL_TREE; phi = phi_nodes (bb); while (phi) { stats.total_phis++; if (! NECESSARY (phi)) { tree next = PHI_CHAIN (phi); if (dump_file && (dump_flags & TDF_DETAILS)) { fprintf (dump_file, "Deleting : "); print_generic_stmt (dump_file, phi, TDF_SLIM); fprintf (dump_file, "\n"); } remove_phi_node (phi, prev, bb); stats.removed_phis++; phi = next; } else { prev = phi; phi = PHI_CHAIN (phi); } } } /* Remove dead statement pointed by iterator I. Receives the basic block BB containing I so that we don't have to look it up. */ static void remove_dead_stmt (block_stmt_iterator *i, basic_block bb) { tree t = bsi_stmt (*i); def_operand_p def_p; ssa_op_iter iter; if (dump_file && (dump_flags & TDF_DETAILS)) { fprintf (dump_file, "Deleting : "); print_generic_stmt (dump_file, t, TDF_SLIM); fprintf (dump_file, "\n"); } stats.removed++; /* If we have determined that a conditional branch statement contributes nothing to the program, then we not only remove it, but we also change the flow graph so that the current block will simply fall-thru to its immediate post-dominator. The blocks we are circumventing will be removed by cleaup_cfg if this change in the flow graph makes them unreachable. */ if (is_ctrl_stmt (t)) { basic_block post_dom_bb; /* The post dominance info has to be up-to-date. */ gcc_assert (dom_computed[CDI_POST_DOMINATORS] == DOM_OK); /* Get the immediate post dominator of bb. */ post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb); /* Some blocks don't have an immediate post dominator. This can happen for example with infinite loops. Removing an infinite loop is an inappropriate transformation anyway... */ if (! post_dom_bb) { bsi_next (i); return; } /* Redirect the first edge out of BB to reach POST_DOM_BB. */ redirect_edge_and_branch (EDGE_SUCC (bb, 0), post_dom_bb); PENDING_STMT (EDGE_SUCC (bb, 0)) = NULL; EDGE_SUCC (bb, 0)->probability = REG_BR_PROB_BASE; EDGE_SUCC (bb, 0)->count = bb->count; /* The edge is no longer associated with a conditional, so it does not have TRUE/FALSE flags. */ EDGE_SUCC (bb, 0)->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE); /* If the edge reaches any block other than the exit, then it is a fallthru edge; if it reaches the exit, then it is not a fallthru edge. */ if (post_dom_bb != EXIT_BLOCK_PTR) EDGE_SUCC (bb, 0)->flags |= EDGE_FALLTHRU; else EDGE_SUCC (bb, 0)->flags &= ~EDGE_FALLTHRU; /* Remove the remaining the outgoing edges. */ while (EDGE_COUNT (bb->succs) != 1) remove_edge (EDGE_SUCC (bb, 1)); } FOR_EACH_SSA_DEF_OPERAND (def_p, t, iter, SSA_OP_VIRTUAL_DEFS | SSA_OP_VIRTUAL_KILLS) { tree def = DEF_FROM_PTR (def_p); bitmap_set_bit (vars_to_rename, var_ann (SSA_NAME_VAR (def))->uid); } bsi_remove (i); release_defs (t); } /* Print out removed statement statistics. */ static void print_stats (void) { if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS))) { float percg; percg = ((float) stats.removed / (float) stats.total) * 100; fprintf (dump_file, "Removed %d of %d statements (%d%%)\n", stats.removed, stats.total, (int) percg); if (stats.total_phis == 0) percg = 0; else percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100; fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n", stats.removed_phis, stats.total_phis, (int) percg); } } /* Initialization for this pass. Set up the used data structures. */ static void tree_dce_init (bool aggressive) { memset ((void *) &stats, 0, sizeof (stats)); if (aggressive) { int i; control_dependence_map = xmalloc (last_basic_block * sizeof (bitmap)); for (i = 0; i < last_basic_block; ++i) control_dependence_map[i] = BITMAP_XMALLOC (); last_stmt_necessary = sbitmap_alloc (last_basic_block); sbitmap_zero (last_stmt_necessary); } processed = sbitmap_alloc (num_ssa_names + 1); sbitmap_zero (processed); VARRAY_TREE_INIT (worklist, 64, "work list"); } /* Cleanup after this pass. */ static void tree_dce_done (bool aggressive) { if (aggressive) { int i; for (i = 0; i < last_basic_block; ++i) BITMAP_XFREE (control_dependence_map[i]); free (control_dependence_map); sbitmap_free (last_stmt_necessary); } sbitmap_free (processed); } /* Main routine to eliminate dead code. AGGRESSIVE controls the aggressiveness of the algorithm. In conservative mode, we ignore control dependence and simply declare all but the most trivially dead branches necessary. This mode is fast. In aggressive mode, control dependences are taken into account, which results in more dead code elimination, but at the cost of some time. FIXME: Aggressive mode before PRE doesn't work currently because the dominance info is not invalidated after DCE1. This is not an issue right now because we only run aggressive DCE as the last tree SSA pass, but keep this in mind when you start experimenting with pass ordering. */ static void perform_tree_ssa_dce (bool aggressive) { struct edge_list *el = NULL; tree_dce_init (aggressive); if (aggressive) { /* Compute control dependence. */ timevar_push (TV_CONTROL_DEPENDENCES); calculate_dominance_info (CDI_POST_DOMINATORS); el = create_edge_list (); find_all_control_dependences (el); timevar_pop (TV_CONTROL_DEPENDENCES); mark_dfs_back_edges (); } find_obviously_necessary_stmts (el); propagate_necessity (el); mark_really_necessary_kill_operand_phis (); eliminate_unnecessary_stmts (); if (aggressive) free_dominance_info (CDI_POST_DOMINATORS); /* Debugging dumps. */ if (dump_file) print_stats (); tree_dce_done (aggressive); free_edge_list (el); } /* Pass entry points. */ static void tree_ssa_dce (void) { perform_tree_ssa_dce (/*aggressive=*/false); } static void tree_ssa_cd_dce (void) { perform_tree_ssa_dce (/*aggressive=*/optimize >= 2); } static bool gate_dce (void) { return flag_tree_dce != 0; } struct tree_opt_pass pass_dce = { "dce", /* name */ gate_dce, /* gate */ tree_ssa_dce, /* execute */ NULL, /* sub */ NULL, /* next */ 0, /* static_pass_number */ TV_TREE_DCE, /* tv_id */ PROP_cfg | PROP_ssa | PROP_alias, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ TODO_dump_func | TODO_fix_def_def_chains | TODO_cleanup_cfg | TODO_ggc_collect | TODO_verify_ssa, /* todo_flags_finish */ 0 /* letter */ }; struct tree_opt_pass pass_cd_dce = { "cddce", /* name */ gate_dce, /* gate */ tree_ssa_cd_dce, /* execute */ NULL, /* sub */ NULL, /* next */ 0, /* static_pass_number */ TV_TREE_CD_DCE, /* tv_id */ PROP_cfg | PROP_ssa | PROP_alias, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ TODO_dump_func | TODO_fix_def_def_chains | TODO_cleanup_cfg | TODO_ggc_collect | TODO_verify_ssa | TODO_verify_flow, /* todo_flags_finish */ 0 /* letter */ };