/* Dead store elimination
- Copyright (C) 2004, 2005 Free Software Foundation, Inc.
+ Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010
+ 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)
+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,
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. */
+along with GCC; see the file COPYING3. If not see
+<http://www.gnu.org/licenses/>. */
#include "config.h"
#include "system.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 "gimple-pretty-print.h"
#include "tree-flow.h"
#include "tree-pass.h"
#include "tree-dump.h"
#include "domwalk.h"
#include "flags.h"
+#include "langhooks.h"
/* This file implements dead store elimination.
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.
+ 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
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
+ 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"
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;
-};
+/* 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_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;
+static void dse_enter_block (struct dom_walk_data *, basic_block);
- return stmt_ann (stmt)->uid;
-}
-/* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed. */
+/* 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 void
-record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
+static bool
+dse_possible_dead_store_p (gimple stmt, gimple *use_stmt)
{
- /* 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. */
+ gimple temp;
+ unsigned cnt = 0;
-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);
+ *use_stmt = NULL;
- /* If we are given a recycled block local data structure, ensure any
- bitmap associated with the block is cleared. */
- if (recycled)
+ /* 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
{
- 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. */
+ gimple use_stmt;
+ 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);
+ temp = NULL;
+ FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
+ {
+ cnt++;
-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;
- }
+ /* 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);
+ }
+ 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;
+ }
+ }
- return NULL_TREE;
-}
+ if (fail)
+ return false;
-/* Return TRUE if the destination memory address in STORE1 and STORE2
- might be modified after STORE1, before control reaches STORE2. */
+ /* 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 (is_hidden_global_store (stmt))
+ return false;
-static bool
-memory_address_same (tree store1, tree store2)
-{
- struct address_walk_data walk_data;
+ 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)));
- walk_data.store1_bb = bb_for_stmt (store1);
- walk_data.store2_bb = bb_for_stmt (store2);
+ *use_stmt = temp;
- return (walk_tree (&TREE_OPERAND (store1, 0), memory_ssa_name_same,
- &walk_data, NULL)
- == NULL);
+ 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
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)
+dse_optimize_stmt (gimple_stmt_iterator gsi)
{
- 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);
+ gimple stmt = gsi_stmt (gsi);
/* If this statement has no virtual defs, then there is nothing
to do. */
- if (ZERO_SSA_OPERANDS (stmt, (SSA_OP_VMAYDEF|SSA_OP_VMUSTDEF)))
+ if (!gimple_vdef (stmt))
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))
+ /* 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;
- if (ann->has_volatile_ops)
+ if (gimple_has_volatile_ops (stmt))
return;
- if (TREE_CODE (stmt) == MODIFY_EXPR)
+ if (is_gimple_assign (stmt))
{
- use_operand_p first_use_p = NULL_USE_OPERAND_P;
- use_operand_p use_p = NULL;
- tree use, 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;
- use = USE_FROM_PTR (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)
+ gimple use_stmt;
+
+ if (!dse_possible_dead_store_p (stmt, &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)))
{
- 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)
+ /* 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;
- /* Skip past this PHI and loop again in case we had a PHI
- chain. */
- if (single_imm_use (PHI_RESULT (use_stmt), &use_p, &use_stmt))
- use = USE_FROM_PTR (use_p);
- }
-
- /* 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);
+ 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. */
- 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));
- }
+ unlink_stmt_vdef (stmt);
+
+ bitmap_set_bit (need_eh_cleanup, gimple_bb (stmt)->index);
+
/* Remove the dead store. */
- bsi_remove (&bsi, true);
+ gsi_remove (&gsi, 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)
+dse_enter_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED,
+ 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));
-}
+ gimple_stmt_iterator gsi;
-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);
- }
+ for (gsi = gsi_last (bb_seq (bb)); !gsi_end_p (gsi); gsi_prev (&gsi))
+ dse_optimize_stmt (gsi);
}
+/* Main entry point. */
+
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;
+ need_eh_cleanup = BITMAP_ALLOC (NULL);
- for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
- stmt_ann (bsi_stmt (bsi))->uid = max_stmt_uid++;
- }
+ 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.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);
+ walk_data.initialize_block_local_data = NULL;
+ walk_data.before_dom_children = dse_enter_block;
+ walk_data.after_dom_children = NULL;
- /* This is the main hash table for the dead store elimination pass. */
- dse_gd.stores = BITMAP_ALLOC (NULL);
- walk_data.global_data = &dse_gd;
+ walk_data.block_local_data_size = 0;
+ walk_data.global_data = NULL;
/* Initialize the dominator walker. */
init_walk_dominator_tree (&walk_data);
/* Finalize the dominator walker. */
fini_walk_dominator_tree (&walk_data);
- /* Release the main bitmap. */
- BITMAP_FREE (dse_gd.stores);
+ /* 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;
return flag_tree_dse != 0;
}
-struct tree_opt_pass pass_dse = {
+struct gimple_opt_pass pass_dse =
+{
+ {
+ GIMPLE_PASS,
"dse", /* name */
gate_dse, /* gate */
tree_ssa_dse, /* execute */
NULL, /* next */
0, /* static_pass_number */
TV_TREE_DSE, /* tv_id */
- PROP_cfg
- | PROP_ssa
- | PROP_alias, /* properties_required */
+ PROP_cfg | PROP_ssa, /* 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 */
+ TODO_ggc_collect
+ | TODO_verify_ssa /* todo_flags_finish */
+ }
};
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