/* Dead store elimination Copyright (C) 2004-2013 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 . */ #include "config.h" #include "system.h" #include "coretypes.h" #include "tm.h" #include "ggc.h" #include "tree.h" #include "tm_p.h" #include "basic-block.h" #include "gimple-pretty-print.h" #include "tree-flow.h" #include "tree-pass.h" #include "domwalk.h" #include "flags.h" #include "langhooks.h" /* This file implements dead store elimination. A dead store is a store into a memory location which will later be overwritten by another store without any intervening loads. In this case the earlier store can be deleted. In our SSA + virtual operand world we use immediate uses of virtual operands to detect dead stores. If a store's virtual definition is used precisely once by a later store to the same location which post dominates the first store, then the first store is dead. The single use of the store's virtual definition ensures that there are no intervening aliased loads and the requirement that the second load post dominate the first ensures that if the earlier store executes, then the later stores will execute before the function exits. It may help to think of this as first moving the earlier store to the point immediately before the later store. Again, the single use of the virtual definition and the post-dominance relationship ensure that such movement would be safe. Clearly if there are back to back stores, then the second is redundant. Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler" may also help in understanding this code since it discusses the relationship between dead store and redundant load elimination. In fact, they are the same transformation applied to different views of the CFG. */ /* Bitmap of blocks that have had EH statements cleaned. We should remove their dead edges eventually. */ static bitmap need_eh_cleanup; static bool gate_dse (void); static unsigned int tree_ssa_dse (void); static void dse_enter_block (struct dom_walk_data *, basic_block); /* A helper of dse_optimize_stmt. Given a GIMPLE_ASSIGN in STMT, find a candidate statement *USE_STMT that may prove STMT to be dead. Return TRUE if the above conditions are met, otherwise FALSE. */ static bool dse_possible_dead_store_p (gimple stmt, gimple *use_stmt) { gimple temp; unsigned cnt = 0; *use_stmt = NULL; /* Self-assignments are zombies. */ if (operand_equal_p (gimple_assign_rhs1 (stmt), gimple_assign_lhs (stmt), 0)) { *use_stmt = stmt; return true; } /* Find the first dominated statement that clobbers (part of) the memory stmt stores to with no intermediate statement that may use part of the memory stmt stores. That is, find a store that may prove stmt to be a dead store. */ temp = stmt; do { gimple use_stmt, defvar_def; imm_use_iterator ui; bool fail = false; tree defvar; /* Limit stmt walking to be linear in the number of possibly dead stores. */ if (++cnt > 256) return false; if (gimple_code (temp) == GIMPLE_PHI) defvar = PHI_RESULT (temp); else defvar = gimple_vdef (temp); defvar_def = temp; temp = NULL; FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar) { cnt++; /* If we ever reach our DSE candidate stmt again fail. We cannot handle dead stores in loops. */ if (use_stmt == stmt) { fail = true; BREAK_FROM_IMM_USE_STMT (ui); } /* In simple cases we can look through PHI nodes, but we have to be careful with loops and with memory references containing operands that are also operands of PHI nodes. See gcc.c-torture/execute/20051110-*.c. */ else if (gimple_code (use_stmt) == GIMPLE_PHI) { if (temp /* Make sure we are not in a loop latch block. */ || gimple_bb (stmt) == gimple_bb (use_stmt) || dominated_by_p (CDI_DOMINATORS, gimple_bb (stmt), gimple_bb (use_stmt)) /* We can look through PHIs to regions post-dominating the DSE candidate stmt. */ || !dominated_by_p (CDI_POST_DOMINATORS, gimple_bb (stmt), gimple_bb (use_stmt))) { fail = true; BREAK_FROM_IMM_USE_STMT (ui); } /* Do not consider the PHI as use if it dominates the stmt defining the virtual operand we are processing, we have processed it already in this case. */ if (gimple_bb (defvar_def) != gimple_bb (use_stmt) && !dominated_by_p (CDI_DOMINATORS, gimple_bb (defvar_def), gimple_bb (use_stmt))) temp = use_stmt; } /* If the statement is a use the store is not dead. */ else if (ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt))) { fail = true; BREAK_FROM_IMM_USE_STMT (ui); } /* If this is a store, remember it or bail out if we have multiple ones (the will be in different CFG parts then). */ else if (gimple_vdef (use_stmt)) { if (temp) { fail = true; BREAK_FROM_IMM_USE_STMT (ui); } temp = use_stmt; } } if (fail) return false; /* If we didn't find any definition this means the store is dead if it isn't a store to global reachable memory. In this case just pretend the stmt makes itself dead. Otherwise fail. */ if (!temp) { if (stmt_may_clobber_global_p (stmt)) return false; temp = stmt; break; } } /* We deliberately stop on clobbering statements and not only on killing ones to make walking cheaper. Otherwise we can just continue walking until both stores have equal reference trees. */ while (!stmt_may_clobber_ref_p (temp, gimple_assign_lhs (stmt))); *use_stmt = temp; return true; } /* Attempt to eliminate dead stores in the statement referenced by BSI. A dead store is a store into a memory location which will later be overwritten by another store without any intervening loads. In this case the earlier store can be deleted. In our SSA + virtual operand world we use immediate uses of virtual operands to detect dead stores. If a store's virtual definition is used precisely once by a later store to the same location which post dominates the first store, then the first store is dead. */ static void dse_optimize_stmt (gimple_stmt_iterator *gsi) { gimple stmt = gsi_stmt (*gsi); /* If this statement has no virtual defs, then there is nothing to do. */ if (!gimple_vdef (stmt)) return; /* We know we have virtual definitions. If this is a GIMPLE_ASSIGN that's not also a function call, then record it into our table. */ if (is_gimple_call (stmt) && gimple_call_fndecl (stmt)) return; /* Don't return early on *this_2(D) ={v} {CLOBBER}. */ if (gimple_has_volatile_ops (stmt) && (!gimple_clobber_p (stmt) || TREE_CODE (gimple_assign_lhs (stmt)) != MEM_REF)) return; if (is_gimple_assign (stmt)) { gimple use_stmt; if (!dse_possible_dead_store_p (stmt, &use_stmt)) return; /* But only remove *this_2(D) ={v} {CLOBBER} if killed by another clobber stmt. */ if (gimple_clobber_p (stmt) && !gimple_clobber_p (use_stmt)) return; /* If we have precisely one immediate use at this point and the stores are to the same memory location or there is a chain of virtual uses from stmt and the stmt which stores to that same memory location, then we may have found redundant store. */ if ((gimple_has_lhs (use_stmt) && (operand_equal_p (gimple_assign_lhs (stmt), gimple_get_lhs (use_stmt), 0))) || stmt_kills_ref_p (use_stmt, gimple_assign_lhs (stmt))) { basic_block bb; /* If use_stmt is or might be a nop assignment, e.g. for struct { ... } S a, b, *p; ... b = a; b = b; or b = a; b = *p; where p might be &b, or *p = a; *p = b; where p might be &b, or *p = *u; *p = *v; where p might be v, then USE_STMT acts as a use as well as definition, so store in STMT is not dead. */ if (stmt != use_stmt && ref_maybe_used_by_stmt_p (use_stmt, gimple_assign_lhs (stmt))) return; if (dump_file && (dump_flags & TDF_DETAILS)) { fprintf (dump_file, " Deleted dead store '"); print_gimple_stmt (dump_file, gsi_stmt (*gsi), dump_flags, 0); fprintf (dump_file, "'\n"); } /* Then we need to fix the operand of the consuming stmt. */ unlink_stmt_vdef (stmt); /* Remove the dead store. */ bb = gimple_bb (stmt); if (gsi_remove (gsi, true)) bitmap_set_bit (need_eh_cleanup, bb->index); /* And release any SSA_NAMEs set in this statement back to the SSA_NAME manager. */ release_defs (stmt); } } } static void dse_enter_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED, basic_block bb) { gimple_stmt_iterator gsi; for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi);) { dse_optimize_stmt (&gsi); if (gsi_end_p (gsi)) gsi = gsi_last_bb (bb); else gsi_prev (&gsi); } } /* Main entry point. */ static unsigned int tree_ssa_dse (void) { struct dom_walk_data walk_data; need_eh_cleanup = BITMAP_ALLOC (NULL); renumber_gimple_stmt_uids (); /* We might consider making this a property of each pass so that it can be [re]computed on an as-needed basis. Particularly since this pass could be seen as an extension of DCE which needs post dominators. */ calculate_dominance_info (CDI_POST_DOMINATORS); calculate_dominance_info (CDI_DOMINATORS); /* Dead store elimination is fundamentally a walk of the post-dominator tree and a backwards walk of statements within each block. */ walk_data.dom_direction = CDI_POST_DOMINATORS; walk_data.initialize_block_local_data = NULL; walk_data.before_dom_children = dse_enter_block; walk_data.after_dom_children = NULL; walk_data.block_local_data_size = 0; walk_data.global_data = NULL; /* Initialize the dominator walker. */ init_walk_dominator_tree (&walk_data); /* Recursively walk the dominator tree. */ walk_dominator_tree (&walk_data, EXIT_BLOCK_PTR); /* Finalize the dominator walker. */ fini_walk_dominator_tree (&walk_data); /* Removal of stores may make some EH edges dead. Purge such edges from the CFG as needed. */ if (!bitmap_empty_p (need_eh_cleanup)) { gimple_purge_all_dead_eh_edges (need_eh_cleanup); cleanup_tree_cfg (); } BITMAP_FREE (need_eh_cleanup); /* For now, just wipe the post-dominator information. */ free_dominance_info (CDI_POST_DOMINATORS); return 0; } static bool gate_dse (void) { return flag_tree_dse != 0; } struct gimple_opt_pass pass_dse = { { GIMPLE_PASS, "dse", /* name */ OPTGROUP_NONE, /* optinfo_flags */ gate_dse, /* gate */ tree_ssa_dse, /* execute */ NULL, /* sub */ NULL, /* next */ 0, /* static_pass_number */ TV_TREE_DSE, /* tv_id */ PROP_cfg | PROP_ssa, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ TODO_verify_ssa /* todo_flags_finish */ } };