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+/* Dead store elimination
+ Copyright (C) 2004, 2005 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 "ggc.h"
+#include "tree.h"
+#include "rtl.h"
+#include "tm_p.h"
+#include "basic-block.h"
+#include "timevar.h"
+#include "diagnostic.h"
+#include "tree-flow.h"
+#include "tree-pass.h"
+#include "tree-dump.h"
+#include "domwalk.h"
+#include "flags.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. */
+
+
+struct dse_global_data
+{
+ /* This is the global bitmap for store statements.
+
+ Each statement has a unique ID. When we encounter a store statement
+ that we want to record, set the bit corresponding to the statement's
+ unique ID in this bitmap. */
+ bitmap stores;
+};
+
+/* We allocate a bitmap-per-block for stores which are encountered
+ during the scan of that block. This allows us to restore the
+ global bitmap of stores when we finish processing a block. */
+struct dse_block_local_data
+{
+ bitmap stores;
+};
+
+/* Basic blocks of the potentially dead store and the following
+ store, for memory_address_same. */
+struct address_walk_data
+{
+ basic_block store1_bb, store2_bb;
+};
+
+static bool gate_dse (void);
+static unsigned int tree_ssa_dse (void);
+static void dse_initialize_block_local_data (struct dom_walk_data *,
+ basic_block,
+ bool);
+static void dse_optimize_stmt (struct dom_walk_data *,
+ basic_block,
+ block_stmt_iterator);
+static void dse_record_phis (struct dom_walk_data *, basic_block);
+static void dse_finalize_block (struct dom_walk_data *, basic_block);
+static void record_voperand_set (bitmap, bitmap *, unsigned int);
+
+static unsigned max_stmt_uid; /* Maximal uid of a statement. Uids to phi
+ nodes are assigned using the versions of
+ ssa names they define. */
+
+/* Returns uid of statement STMT. */
+
+static unsigned
+get_stmt_uid (tree stmt)
+{
+ if (TREE_CODE (stmt) == PHI_NODE)
+ return SSA_NAME_VERSION (PHI_RESULT (stmt)) + max_stmt_uid;
+
+ return stmt_ann (stmt)->uid;
+}
+
+/* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed. */
+
+static void
+record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
+{
+ /* Lazily allocate the bitmap. Note that we do not get a notification
+ when the block local data structures die, so we allocate the local
+ bitmap backed by the GC system. */
+ if (*local == NULL)
+ *local = BITMAP_GGC_ALLOC ();
+
+ /* Set the bit in the local and global bitmaps. */
+ bitmap_set_bit (*local, uid);
+ bitmap_set_bit (global, uid);
+}
+
+/* Initialize block local data structures. */
+
+static void
+dse_initialize_block_local_data (struct dom_walk_data *walk_data,
+ basic_block bb ATTRIBUTE_UNUSED,
+ bool recycled)
+{
+ struct dse_block_local_data *bd
+ = VEC_last (void_p, walk_data->block_data_stack);
+
+ /* If we are given a recycled block local data structure, ensure any
+ bitmap associated with the block is cleared. */
+ if (recycled)
+ {
+ if (bd->stores)
+ bitmap_clear (bd->stores);
+ }
+}
+
+/* Helper function for memory_address_same via walk_tree. Returns
+ non-NULL if it finds an SSA_NAME which is part of the address,
+ such that the definition of the SSA_NAME post-dominates the store
+ we want to delete but not the store that we believe makes it
+ redundant. This indicates that the address may change between
+ the two stores. */
+
+static tree
+memory_ssa_name_same (tree *expr_p, int *walk_subtrees ATTRIBUTE_UNUSED,
+ void *data)
+{
+ struct address_walk_data *walk_data = data;
+ tree expr = *expr_p;
+ tree def_stmt;
+ basic_block def_bb;
+
+ if (TREE_CODE (expr) != SSA_NAME)
+ return NULL_TREE;
+
+ /* If we've found a default definition, then there's no problem. Both
+ stores will post-dominate it. And def_bb will be NULL. */
+ if (expr == default_def (SSA_NAME_VAR (expr)))
+ return NULL_TREE;
+
+ def_stmt = SSA_NAME_DEF_STMT (expr);
+ def_bb = bb_for_stmt (def_stmt);
+
+ /* DEF_STMT must dominate both stores. So if it is in the same
+ basic block as one, it does not post-dominate that store. */
+ if (walk_data->store1_bb != def_bb
+ && dominated_by_p (CDI_POST_DOMINATORS, walk_data->store1_bb, def_bb))
+ {
+ if (walk_data->store2_bb == def_bb
+ || !dominated_by_p (CDI_POST_DOMINATORS, walk_data->store2_bb,
+ def_bb))
+ /* Return non-NULL to stop the walk. */
+ return def_stmt;
+ }
+
+ return NULL_TREE;
+}
+
+/* Return TRUE if the destination memory address in STORE1 and STORE2
+ might be modified after STORE1, before control reaches STORE2. */
+
+static bool
+memory_address_same (tree store1, tree store2)
+{
+ struct address_walk_data walk_data;
+
+ walk_data.store1_bb = bb_for_stmt (store1);
+ walk_data.store2_bb = bb_for_stmt (store2);
+
+ return (walk_tree (&TREE_OPERAND (store1, 0), memory_ssa_name_same,
+ &walk_data, NULL)
+ == NULL);
+}
+
+/* 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 (struct dom_walk_data *walk_data,
+ basic_block bb ATTRIBUTE_UNUSED,
+ block_stmt_iterator bsi)
+{
+ struct dse_block_local_data *bd
+ = VEC_last (void_p, walk_data->block_data_stack);
+ struct dse_global_data *dse_gd = walk_data->global_data;
+ tree stmt = bsi_stmt (bsi);
+ stmt_ann_t ann = stmt_ann (stmt);
+
+ /* If this statement has no virtual defs, then there is nothing
+ to do. */
+ if (ZERO_SSA_OPERANDS (stmt, (SSA_OP_VMAYDEF|SSA_OP_VMUSTDEF)))
+ return;
+
+ /* We know we have virtual definitions. If this is a MODIFY_EXPR that's
+ not also a function call, then record it into our table. */
+ if (get_call_expr_in (stmt))
+ return;
+
+ if (ann->has_volatile_ops)
+ return;
+
+ if (TREE_CODE (stmt) == MODIFY_EXPR)
+ {
+ use_operand_p first_use_p = NULL_USE_OPERAND_P;
+ use_operand_p use_p = NULL;
+ tree use_stmt, temp;
+ tree defvar = NULL_TREE, usevar = NULL_TREE;
+ bool fail = false;
+ use_operand_p var2;
+ def_operand_p var1;
+ ssa_op_iter op_iter;
+
+ /* We want to verify that each virtual definition in STMT has
+ precisely one use and that all the virtual definitions are
+ used by the same single statement. When complete, we
+ want USE_STMT to refer to the one statement which uses
+ all of the virtual definitions from STMT. */
+ use_stmt = NULL;
+ FOR_EACH_SSA_MUST_AND_MAY_DEF_OPERAND (var1, var2, stmt, op_iter)
+ {
+ defvar = DEF_FROM_PTR (var1);
+ usevar = USE_FROM_PTR (var2);
+
+ /* If this virtual def does not have precisely one use, then
+ we will not be able to eliminate STMT. */
+ if (! has_single_use (defvar))
+ {
+ fail = true;
+ break;
+ }
+
+ /* Get the one and only immediate use of DEFVAR. */
+ single_imm_use (defvar, &use_p, &temp);
+ gcc_assert (use_p != NULL_USE_OPERAND_P);
+ first_use_p = use_p;
+
+ /* If the immediate use of DEF_VAR is not the same as the
+ previously find immediate uses, then we will not be able
+ to eliminate STMT. */
+ if (use_stmt == NULL)
+ use_stmt = temp;
+ else if (temp != use_stmt)
+ {
+ fail = true;
+ break;
+ }
+ }
+
+ if (fail)
+ {
+ record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
+ return;
+ }
+
+ /* Skip through any PHI nodes we have already seen if the PHI
+ represents the only use of this store.
+
+ Note this does not handle the case where the store has
+ multiple V_{MAY,MUST}_DEFs which all reach a set of PHI nodes in the
+ same block. */
+ while (use_p != NULL_USE_OPERAND_P
+ && TREE_CODE (use_stmt) == PHI_NODE
+ && bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt)))
+ {
+ /* A PHI node can both define and use the same SSA_NAME if
+ the PHI is at the top of a loop and the PHI_RESULT is
+ a loop invariant and copies have not been fully propagated.
+
+ The safe thing to do is exit assuming no optimization is
+ possible. */
+ if (SSA_NAME_DEF_STMT (PHI_RESULT (use_stmt)) == use_stmt)
+ return;
+
+ /* Skip past this PHI and loop again in case we had a PHI
+ chain. */
+ single_imm_use (PHI_RESULT (use_stmt), &use_p, &use_stmt);
+ }
+
+ /* If we have precisely one immediate use at this point, then we may
+ have found redundant store. Make sure that the stores are to
+ the same memory location. This includes checking that any
+ SSA-form variables in the address will have the same values. */
+ if (use_p != NULL_USE_OPERAND_P
+ && bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
+ && operand_equal_p (TREE_OPERAND (stmt, 0),
+ TREE_OPERAND (use_stmt, 0), 0)
+ && memory_address_same (stmt, use_stmt))
+ {
+ /* Make sure we propagate the ABNORMAL bit setting. */
+ if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (USE_FROM_PTR (first_use_p)))
+ SSA_NAME_OCCURS_IN_ABNORMAL_PHI (usevar) = 1;
+
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ {
+ fprintf (dump_file, " Deleted dead store '");
+ print_generic_expr (dump_file, bsi_stmt (bsi), dump_flags);
+ fprintf (dump_file, "'\n");
+ }
+ /* Then we need to fix the operand of the consuming stmt. */
+ FOR_EACH_SSA_MUST_AND_MAY_DEF_OPERAND (var1, var2, stmt, op_iter)
+ {
+ single_imm_use (DEF_FROM_PTR (var1), &use_p, &temp);
+ SET_USE (use_p, USE_FROM_PTR (var2));
+ }
+ /* Remove the dead store. */
+ bsi_remove (&bsi, true);
+
+ /* And release any SSA_NAMEs set in this statement back to the
+ SSA_NAME manager. */
+ release_defs (stmt);
+ }
+
+ record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
+ }
+}
+
+/* Record that we have seen the PHIs at the start of BB which correspond
+ to virtual operands. */
+static void
+dse_record_phis (struct dom_walk_data *walk_data, basic_block bb)
+{
+ struct dse_block_local_data *bd
+ = VEC_last (void_p, walk_data->block_data_stack);
+ struct dse_global_data *dse_gd = walk_data->global_data;
+ tree phi;
+
+ for (phi = phi_nodes (bb); phi; phi = PHI_CHAIN (phi))
+ if (!is_gimple_reg (PHI_RESULT (phi)))
+ record_voperand_set (dse_gd->stores,
+ &bd->stores,
+ get_stmt_uid (phi));
+}
+
+static void
+dse_finalize_block (struct dom_walk_data *walk_data,
+ basic_block bb ATTRIBUTE_UNUSED)
+{
+ struct dse_block_local_data *bd
+ = VEC_last (void_p, walk_data->block_data_stack);
+ struct dse_global_data *dse_gd = walk_data->global_data;
+ bitmap stores = dse_gd->stores;
+ unsigned int i;
+ bitmap_iterator bi;
+
+ /* Unwind the stores noted in this basic block. */
+ if (bd->stores)
+ EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bi)
+ {
+ bitmap_clear_bit (stores, i);
+ }
+}
+
+static unsigned int
+tree_ssa_dse (void)
+{
+ struct dom_walk_data walk_data;
+ struct dse_global_data dse_gd;
+ basic_block bb;
+
+ /* Create a UID for each statement in the function. Ordering of the
+ UIDs is not important for this pass. */
+ max_stmt_uid = 0;
+ FOR_EACH_BB (bb)
+ {
+ block_stmt_iterator bsi;
+
+ for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
+ stmt_ann (bsi_stmt (bsi))->uid = max_stmt_uid++;
+ }
+
+ /* 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);
+
+ /* Dead store elimination is fundamentally a walk of the post-dominator
+ tree and a backwards walk of statements within each block. */
+ walk_data.walk_stmts_backward = true;
+ walk_data.dom_direction = CDI_POST_DOMINATORS;
+ walk_data.initialize_block_local_data = dse_initialize_block_local_data;
+ walk_data.before_dom_children_before_stmts = NULL;
+ walk_data.before_dom_children_walk_stmts = dse_optimize_stmt;
+ walk_data.before_dom_children_after_stmts = dse_record_phis;
+ walk_data.after_dom_children_before_stmts = NULL;
+ walk_data.after_dom_children_walk_stmts = NULL;
+ walk_data.after_dom_children_after_stmts = dse_finalize_block;
+ walk_data.interesting_blocks = NULL;
+
+ walk_data.block_local_data_size = sizeof (struct dse_block_local_data);
+
+ /* This is the main hash table for the dead store elimination pass. */
+ dse_gd.stores = BITMAP_ALLOC (NULL);
+ walk_data.global_data = &dse_gd;
+
+ /* 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);
+
+ /* Release the main bitmap. */
+ BITMAP_FREE (dse_gd.stores);
+
+ /* 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 tree_opt_pass pass_dse = {
+ "dse", /* name */
+ gate_dse, /* gate */
+ tree_ssa_dse, /* execute */
+ NULL, /* sub */
+ NULL, /* next */
+ 0, /* static_pass_number */
+ TV_TREE_DSE, /* tv_id */
+ PROP_cfg
+ | PROP_ssa
+ | PROP_alias, /* properties_required */
+ 0, /* properties_provided */
+ 0, /* properties_destroyed */
+ 0, /* todo_flags_start */
+ TODO_dump_func
+ | TODO_ggc_collect
+ | TODO_verify_ssa, /* todo_flags_finish */
+ 0 /* letter */
+};