484 lines
15 KiB
C
484 lines
15 KiB
C
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/* Dead store elimination
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Copyright (C) 2004, 2005 Free Software Foundation, Inc.
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This file is part of GCC.
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GCC is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GCC is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GCC; see the file COPYING. If not, write to
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the Free Software Foundation, 51 Franklin Street, Fifth Floor,
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Boston, MA 02110-1301, USA. */
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#include "config.h"
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#include "system.h"
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#include "coretypes.h"
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#include "tm.h"
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#include "ggc.h"
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#include "tree.h"
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#include "rtl.h"
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#include "tm_p.h"
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#include "basic-block.h"
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#include "timevar.h"
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#include "diagnostic.h"
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#include "tree-flow.h"
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#include "tree-pass.h"
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#include "tree-dump.h"
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#include "domwalk.h"
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#include "flags.h"
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/* This file implements dead store elimination.
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A dead store is a store into a memory location which will later be
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overwritten by another store without any intervening loads. In this
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case the earlier store can be deleted.
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In our SSA + virtual operand world we use immediate uses of virtual
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operands to detect dead stores. If a store's virtual definition
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is used precisely once by a later store to the same location which
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post dominates the first store, then the first store is dead.
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The single use of the store's virtual definition ensures that
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there are no intervening aliased loads and the requirement that
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the second load post dominate the first ensures that if the earlier
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store executes, then the later stores will execute before the function
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exits.
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It may help to think of this as first moving the earlier store to
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the point immediately before the later store. Again, the single
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use of the virtual definition and the post-dominance relationship
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ensure that such movement would be safe. Clearly if there are
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back to back stores, then the second is redundant.
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Reviewing section 10.7.2 in Morgan's "Building an Optimizing Compiler"
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may also help in understanding this code since it discusses the
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relationship between dead store and redundant load elimination. In
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fact, they are the same transformation applied to different views of
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the CFG. */
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struct dse_global_data
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{
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/* This is the global bitmap for store statements.
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Each statement has a unique ID. When we encounter a store statement
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that we want to record, set the bit corresponding to the statement's
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unique ID in this bitmap. */
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bitmap stores;
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};
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/* We allocate a bitmap-per-block for stores which are encountered
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during the scan of that block. This allows us to restore the
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global bitmap of stores when we finish processing a block. */
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struct dse_block_local_data
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{
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bitmap stores;
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};
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/* Basic blocks of the potentially dead store and the following
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store, for memory_address_same. */
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struct address_walk_data
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{
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basic_block store1_bb, store2_bb;
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};
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static bool gate_dse (void);
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static unsigned int tree_ssa_dse (void);
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static void dse_initialize_block_local_data (struct dom_walk_data *,
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basic_block,
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bool);
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static void dse_optimize_stmt (struct dom_walk_data *,
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basic_block,
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block_stmt_iterator);
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static void dse_record_phis (struct dom_walk_data *, basic_block);
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static void dse_finalize_block (struct dom_walk_data *, basic_block);
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static void record_voperand_set (bitmap, bitmap *, unsigned int);
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static unsigned max_stmt_uid; /* Maximal uid of a statement. Uids to phi
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nodes are assigned using the versions of
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ssa names they define. */
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/* Returns uid of statement STMT. */
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static unsigned
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get_stmt_uid (tree stmt)
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{
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if (TREE_CODE (stmt) == PHI_NODE)
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return SSA_NAME_VERSION (PHI_RESULT (stmt)) + max_stmt_uid;
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return stmt_ann (stmt)->uid;
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}
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/* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed. */
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static void
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record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
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{
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/* Lazily allocate the bitmap. Note that we do not get a notification
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when the block local data structures die, so we allocate the local
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bitmap backed by the GC system. */
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if (*local == NULL)
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*local = BITMAP_GGC_ALLOC ();
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/* Set the bit in the local and global bitmaps. */
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bitmap_set_bit (*local, uid);
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bitmap_set_bit (global, uid);
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}
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/* Initialize block local data structures. */
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static void
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dse_initialize_block_local_data (struct dom_walk_data *walk_data,
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basic_block bb ATTRIBUTE_UNUSED,
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bool recycled)
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{
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struct dse_block_local_data *bd
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= VEC_last (void_p, walk_data->block_data_stack);
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/* If we are given a recycled block local data structure, ensure any
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bitmap associated with the block is cleared. */
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if (recycled)
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{
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if (bd->stores)
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bitmap_clear (bd->stores);
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}
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}
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/* Helper function for memory_address_same via walk_tree. Returns
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non-NULL if it finds an SSA_NAME which is part of the address,
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such that the definition of the SSA_NAME post-dominates the store
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we want to delete but not the store that we believe makes it
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redundant. This indicates that the address may change between
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the two stores. */
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static tree
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memory_ssa_name_same (tree *expr_p, int *walk_subtrees ATTRIBUTE_UNUSED,
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void *data)
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{
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struct address_walk_data *walk_data = data;
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tree expr = *expr_p;
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tree def_stmt;
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basic_block def_bb;
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if (TREE_CODE (expr) != SSA_NAME)
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return NULL_TREE;
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/* If we've found a default definition, then there's no problem. Both
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stores will post-dominate it. And def_bb will be NULL. */
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if (expr == default_def (SSA_NAME_VAR (expr)))
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return NULL_TREE;
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def_stmt = SSA_NAME_DEF_STMT (expr);
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def_bb = bb_for_stmt (def_stmt);
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/* DEF_STMT must dominate both stores. So if it is in the same
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basic block as one, it does not post-dominate that store. */
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if (walk_data->store1_bb != def_bb
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&& dominated_by_p (CDI_POST_DOMINATORS, walk_data->store1_bb, def_bb))
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{
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if (walk_data->store2_bb == def_bb
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|| !dominated_by_p (CDI_POST_DOMINATORS, walk_data->store2_bb,
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def_bb))
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/* Return non-NULL to stop the walk. */
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return def_stmt;
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}
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return NULL_TREE;
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}
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/* Return TRUE if the destination memory address in STORE1 and STORE2
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might be modified after STORE1, before control reaches STORE2. */
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static bool
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memory_address_same (tree store1, tree store2)
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{
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struct address_walk_data walk_data;
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walk_data.store1_bb = bb_for_stmt (store1);
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walk_data.store2_bb = bb_for_stmt (store2);
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return (walk_tree (&TREE_OPERAND (store1, 0), memory_ssa_name_same,
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&walk_data, NULL)
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== NULL);
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}
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/* Attempt to eliminate dead stores in the statement referenced by BSI.
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A dead store is a store into a memory location which will later be
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overwritten by another store without any intervening loads. In this
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case the earlier store can be deleted.
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In our SSA + virtual operand world we use immediate uses of virtual
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operands to detect dead stores. If a store's virtual definition
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is used precisely once by a later store to the same location which
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post dominates the first store, then the first store is dead. */
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static void
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dse_optimize_stmt (struct dom_walk_data *walk_data,
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basic_block bb ATTRIBUTE_UNUSED,
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block_stmt_iterator bsi)
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{
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struct dse_block_local_data *bd
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= VEC_last (void_p, walk_data->block_data_stack);
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struct dse_global_data *dse_gd = walk_data->global_data;
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tree stmt = bsi_stmt (bsi);
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stmt_ann_t ann = stmt_ann (stmt);
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/* If this statement has no virtual defs, then there is nothing
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to do. */
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if (ZERO_SSA_OPERANDS (stmt, (SSA_OP_VMAYDEF|SSA_OP_VMUSTDEF)))
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return;
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/* We know we have virtual definitions. If this is a MODIFY_EXPR that's
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not also a function call, then record it into our table. */
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if (get_call_expr_in (stmt))
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return;
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if (ann->has_volatile_ops)
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return;
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if (TREE_CODE (stmt) == MODIFY_EXPR)
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{
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use_operand_p first_use_p = NULL_USE_OPERAND_P;
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use_operand_p use_p = NULL;
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tree use_stmt, temp;
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tree defvar = NULL_TREE, usevar = NULL_TREE;
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bool fail = false;
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use_operand_p var2;
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def_operand_p var1;
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ssa_op_iter op_iter;
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/* We want to verify that each virtual definition in STMT has
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precisely one use and that all the virtual definitions are
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used by the same single statement. When complete, we
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want USE_STMT to refer to the one statement which uses
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all of the virtual definitions from STMT. */
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use_stmt = NULL;
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FOR_EACH_SSA_MUST_AND_MAY_DEF_OPERAND (var1, var2, stmt, op_iter)
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{
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defvar = DEF_FROM_PTR (var1);
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usevar = USE_FROM_PTR (var2);
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/* If this virtual def does not have precisely one use, then
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we will not be able to eliminate STMT. */
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if (! has_single_use (defvar))
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{
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fail = true;
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break;
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}
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/* Get the one and only immediate use of DEFVAR. */
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single_imm_use (defvar, &use_p, &temp);
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gcc_assert (use_p != NULL_USE_OPERAND_P);
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first_use_p = use_p;
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/* If the immediate use of DEF_VAR is not the same as the
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previously find immediate uses, then we will not be able
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to eliminate STMT. */
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if (use_stmt == NULL)
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use_stmt = temp;
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else if (temp != use_stmt)
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{
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fail = true;
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break;
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}
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}
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if (fail)
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{
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record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
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return;
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}
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/* Skip through any PHI nodes we have already seen if the PHI
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represents the only use of this store.
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Note this does not handle the case where the store has
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multiple V_{MAY,MUST}_DEFs which all reach a set of PHI nodes in the
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same block. */
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while (use_p != NULL_USE_OPERAND_P
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&& TREE_CODE (use_stmt) == PHI_NODE
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&& bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt)))
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{
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/* A PHI node can both define and use the same SSA_NAME if
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the PHI is at the top of a loop and the PHI_RESULT is
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a loop invariant and copies have not been fully propagated.
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The safe thing to do is exit assuming no optimization is
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possible. */
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if (SSA_NAME_DEF_STMT (PHI_RESULT (use_stmt)) == use_stmt)
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return;
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/* Skip past this PHI and loop again in case we had a PHI
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chain. */
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single_imm_use (PHI_RESULT (use_stmt), &use_p, &use_stmt);
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}
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/* If we have precisely one immediate use at this point, then we may
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have found redundant store. Make sure that the stores are to
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the same memory location. This includes checking that any
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SSA-form variables in the address will have the same values. */
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if (use_p != NULL_USE_OPERAND_P
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&& bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
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&& operand_equal_p (TREE_OPERAND (stmt, 0),
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TREE_OPERAND (use_stmt, 0), 0)
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&& memory_address_same (stmt, use_stmt))
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{
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/* Make sure we propagate the ABNORMAL bit setting. */
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if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (USE_FROM_PTR (first_use_p)))
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SSA_NAME_OCCURS_IN_ABNORMAL_PHI (usevar) = 1;
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if (dump_file && (dump_flags & TDF_DETAILS))
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{
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fprintf (dump_file, " Deleted dead store '");
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print_generic_expr (dump_file, bsi_stmt (bsi), dump_flags);
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fprintf (dump_file, "'\n");
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}
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/* Then we need to fix the operand of the consuming stmt. */
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FOR_EACH_SSA_MUST_AND_MAY_DEF_OPERAND (var1, var2, stmt, op_iter)
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{
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single_imm_use (DEF_FROM_PTR (var1), &use_p, &temp);
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SET_USE (use_p, USE_FROM_PTR (var2));
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}
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/* Remove the dead store. */
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bsi_remove (&bsi, true);
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/* And release any SSA_NAMEs set in this statement back to the
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SSA_NAME manager. */
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release_defs (stmt);
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}
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record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
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}
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}
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/* 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 */
|
||
|
|
};
|