/* Dead store elimination
- Copyright (C) 2004 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, 59 Temple Place - Suite 330,
-Boston, MA 02111-1307, USA. */
+along with GCC; see the file COPYING3. If not see
+<http://www.gnu.org/licenses/>. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
-#include "errors.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 defintion and the post-dominance relationship
- ensure that such movement would be safe. Clearly if there are
+ 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"
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
{
};
/* We allocate a bitmap-per-block for stores which are encountered
- during the scan of that block. This allows us to restore the
+ 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
{
};
static bool gate_dse (void);
-static void tree_ssa_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 fix_phi_uses (tree, tree);
-static void fix_stmt_vdefs (tree, tree);
+static void dse_enter_block (struct dom_walk_data *, basic_block);
+static void dse_leave_block (struct dom_walk_data *, basic_block);
static void record_voperand_set (bitmap, bitmap *, unsigned int);
-/* Function indicating whether we ought to include information for 'var'
- when calculating immediate uses. For this pass we only want use
- information for virtual variables. */
+/* Returns uid of statement STMT. */
-static bool
-need_imm_uses_for (tree var)
+static unsigned
+get_stmt_uid (gimple stmt)
{
- return !is_gimple_reg (var);
-}
-
-
-/* Replace uses in PHI which match VDEF_RESULTs in STMT with the
- corresponding VDEF_OP in STMT. */
-
-static void
-fix_phi_uses (tree phi, tree stmt)
-{
- stmt_ann_t ann = stmt_ann (stmt);
- vdef_optype vdefs;
- unsigned int i;
- int j;
-
- get_stmt_operands (stmt);
- vdefs = VDEF_OPS (ann);
-
- /* Walk each VDEF in STMT. */
- for (i = 0; i < NUM_VDEFS (vdefs); i++)
- {
- tree vdef = VDEF_RESULT (vdefs, i);
+ if (gimple_code (stmt) == GIMPLE_PHI)
+ return SSA_NAME_VERSION (gimple_phi_result (stmt))
+ + gimple_stmt_max_uid (cfun);
- /* Find any uses in the PHI which match VDEF and replace
- them with the appropriate VDEF_OP. */
- for (j = 0; j < PHI_NUM_ARGS (phi); j++)
- if (vdef == PHI_ARG_DEF (phi, j))
- PHI_ARG_DEF (phi, j) = VDEF_OP (vdefs, i);
- }
+ return gimple_uid (stmt);
}
-/* Replace the VDEF_OPs in STMT1 which match VDEF_RESULTs in STMT2 with
- the appropriate VDEF_OPs from STMT2. */
-
-static void
-fix_stmt_vdefs (tree stmt1, tree stmt2)
-{
- stmt_ann_t ann1 = stmt_ann (stmt1);
- stmt_ann_t ann2 = stmt_ann (stmt2);
- vdef_optype vdefs1;
- vdef_optype vdefs2;
- unsigned int i, j;
-
- get_stmt_operands (stmt1);
- get_stmt_operands (stmt2);
- vdefs1 = VDEF_OPS (ann1);
- vdefs2 = VDEF_OPS (ann2);
-
- /* Walk each VDEF_OP in stmt1. */
- for (i = 0; i < NUM_VDEFS (vdefs1); i++)
- {
- tree vdef1 = VDEF_OP (vdefs1, i);
-
- /* Find the appropriate VDEF_RESULT in STMT2. */
- for (j = 0; j < NUM_VDEFS (vdefs2); j++)
- {
- if (vdef1 == VDEF_RESULT (vdefs2, j))
- {
- /* Update. */
- *VDEF_OP_PTR (vdefs1, i) = VDEF_OP (vdefs2, j);
- break;
- }
- }
-
-#ifdef ENABLE_CHECKING
- /* If we did not find a corresponding VDEF_RESULT, then something
- has gone terribly wrong. */
- if (j == NUM_VDEFS (vdefs2))
- abort ();
-#endif
-
- }
-}
-
-
/* Set bit UID in bitmaps GLOBAL and *LOCAL, creating *LOCAL as needed. */
+
static void
record_voperand_set (bitmap global, bitmap *local, unsigned int uid)
{
bitmap_set_bit (*local, uid);
bitmap_set_bit (global, uid);
}
+
/* Initialize block local data structures. */
static void
bool recycled)
{
struct dse_block_local_data *bd
- = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
+ = (struct dse_block_local_data *)
+ 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. */
}
}
+/* 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;
+
+ /* 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;
+ 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++;
+
+ /* 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;
+ }
+ }
+
+ 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 (is_hidden_global_store (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)));
+
+ if (!is_gimple_assign (temp))
+ return false;
+
+ *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
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 (struct dse_global_data *dse_gd,
+ struct dse_block_local_data *bd,
+ gimple_stmt_iterator gsi)
{
- struct dse_block_local_data *bd
- = VARRAY_TOP_GENERIC_PTR (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);
- vdef_optype vdefs;
+ gimple stmt = gsi_stmt (gsi);
- get_stmt_operands (stmt);
- vdefs = VDEF_OPS (ann);
-
- /* If this statement has no virtual uses, then there is nothing
+ /* If this statement has no virtual defs, then there is nothing
to do. */
- if (NUM_VDEFS (vdefs) == 0)
+ if (!gimple_vdef (stmt))
return;
- /* We know we have virtual definitions. If this is a MODIFY_EXPR, then
- record it into our table. */
- if (TREE_CODE (stmt) == MODIFY_EXPR
- && TREE_CODE (TREE_OPERAND (stmt, 1)) != CALL_EXPR)
- {
- dataflow_t df = get_immediate_uses (stmt);
- unsigned int num_uses = num_immediate_uses (df);
- tree use;
- tree skipped_phi;
-
+ /* 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 there are no uses then there is nothing left to do. */
- if (num_uses == 0)
- {
- record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
- return;
- }
+ if (gimple_has_volatile_ops (stmt))
+ return;
- use = immediate_use (df, 0);
- skipped_phi = NULL;
+ if (is_gimple_assign (stmt))
+ {
+ gimple use_stmt;
- /* Skip through any PHI nodes we have already seen if the PHI
- represents the only use of this store.
+ record_voperand_set (dse_gd->stores, &bd->stores, gimple_uid (stmt));
- Note this does not handle the case where the store has
- multiple VDEFs which all reach a set of PHI nodes in the
- same block. */
- while (num_uses == 1
- && TREE_CODE (use) == PHI_NODE
- && bitmap_bit_p (dse_gd->stores, stmt_ann (use)->uid))
- {
- /* Record the first PHI we skip so that we can fix its
- uses if we find that STMT is a dead store. */
- if (!skipped_phi)
- skipped_phi = use;
-
- /* Skip past this PHI and loop again in case we had a PHI
- chain. */
- df = get_immediate_uses (use);
- num_uses = num_immediate_uses (df);
- use = immediate_use (df, 0);
- }
+ if (!dse_possible_dead_store_p (stmt, &use_stmt))
+ return;
- /* If we have precisely one immediate use at this point, then we may
- have found redundant store. */
- if (num_uses == 1
- && bitmap_bit_p (dse_gd->stores, stmt_ann (use)->uid)
- && operand_equal_p (TREE_OPERAND (stmt, 0),
- TREE_OPERAND (use, 0), 0))
+ /* 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 (bitmap_bit_p (dse_gd->stores, get_stmt_uid (use_stmt))
+ && operand_equal_p (gimple_assign_lhs (stmt),
+ gimple_assign_lhs (use_stmt), 0))
{
- /* We need to fix the operands if either the first PHI we
- skipped, or the store which we are not deleting if we did
- not skip any PHIs. */
- if (skipped_phi)
- fix_phi_uses (skipped_phi, stmt);
- else
- fix_stmt_vdefs (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
+ && !is_gimple_reg (gimple_assign_rhs1 (use_stmt))
+ && !is_gimple_min_invariant (gimple_assign_rhs1 (use_stmt))
+ /* ??? Should {} be invariant? */
+ && gimple_assign_rhs_code (use_stmt) != CONSTRUCTOR
+ && refs_may_alias_p (gimple_assign_lhs (use_stmt),
+ gimple_assign_rhs1 (use_stmt)))
+ return;
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");
}
- /* Any immediate uses which reference STMT need to instead
- reference the new consumer, either SKIPPED_PHI or USE.
- This allows us to cascade dead stores. */
- redirect_immediate_uses (stmt, skipped_phi ? skipped_phi : use);
+ /* Then we need to fix the operand of the consuming stmt. */
+ unlink_stmt_vdef (stmt);
- /* Finally remove the dead store. */
- bsi_remove (&bsi);
- }
+ /* Remove the dead store. */
+ gsi_remove (&gsi, true);
- record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
+ /* And release any SSA_NAMEs set in this statement back to the
+ SSA_NAME manager. */
+ release_defs (stmt);
+ }
}
}
/* 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_record_phi (struct dse_global_data *dse_gd,
+ struct dse_block_local_data *bd,
+ gimple phi)
+{
+ if (!is_gimple_reg (gimple_phi_result (phi)))
+ record_voperand_set (dse_gd->stores, &bd->stores, get_stmt_uid (phi));
+}
+
+static void
+dse_enter_block (struct dom_walk_data *walk_data, basic_block bb)
{
struct dse_block_local_data *bd
- = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
- struct dse_global_data *dse_gd = walk_data->global_data;
- tree phi;
-
- for (phi = phi_nodes (bb); phi; phi = TREE_CHAIN (phi))
- if (need_imm_uses_for (PHI_RESULT (phi)))
- record_voperand_set (dse_gd->stores,
- &bd->stores,
- get_stmt_ann (phi)->uid);
+ = (struct dse_block_local_data *)
+ VEC_last (void_p, walk_data->block_data_stack);
+ struct dse_global_data *dse_gd
+ = (struct dse_global_data *) walk_data->global_data;
+ gimple_stmt_iterator gsi;
+
+ for (gsi = gsi_last (bb_seq (bb)); !gsi_end_p (gsi); gsi_prev (&gsi))
+ dse_optimize_stmt (dse_gd, bd, gsi);
+ for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
+ dse_record_phi (dse_gd, bd, gsi_stmt (gsi));
}
static void
-dse_finalize_block (struct dom_walk_data *walk_data,
- basic_block bb ATTRIBUTE_UNUSED)
+dse_leave_block (struct dom_walk_data *walk_data,
+ basic_block bb ATTRIBUTE_UNUSED)
{
struct dse_block_local_data *bd
- = VARRAY_TOP_GENERIC_PTR (walk_data->block_data_stack);
- struct dse_global_data *dse_gd = walk_data->global_data;
+ = (struct dse_block_local_data *)
+ VEC_last (void_p, walk_data->block_data_stack);
+ struct dse_global_data *dse_gd
+ = (struct dse_global_data *) 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, bitmap_clear_bit (stores, i););
+ EXECUTE_IF_SET_IN_BITMAP (bd->stores, 0, i, bi)
+ {
+ bitmap_clear_bit (stores, i);
+ }
}
-static void
+/* Main entry point. */
+
+static unsigned int
tree_ssa_dse (void)
{
struct dom_walk_data walk_data;
struct dse_global_data dse_gd;
- unsigned int uid = 0;
- basic_block bb;
-
- /* Create a UID for each statement in the function. Ordering of the
- UIDs is not important for this pass. */
- FOR_EACH_BB (bb)
- {
- block_stmt_iterator bsi;
- tree phi;
-
- for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi))
- stmt_ann (bsi_stmt (bsi))->uid = uid++;
- for (phi = phi_nodes (bb); phi; phi = TREE_CHAIN (phi))
- stmt_ann (phi)->uid = 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);
-
- /* We also need immediate use information for virtual operands. */
- compute_immediate_uses (TDFA_USE_VOPS, need_imm_uses_for);
+ 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.before_dom_children = dse_enter_block;
+ walk_data.after_dom_children = dse_leave_block;
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_XMALLOC ();
+ dse_gd.stores = BITMAP_ALLOC (NULL);
walk_data.global_data = &dse_gd;
/* Initialize the dominator walker. */
fini_walk_dominator_tree (&walk_data);
/* Release the main bitmap. */
- BITMAP_XFREE (dse_gd.stores);
-
- /* Free dataflow information. It's probably out of date now anyway. */
- free_df ();
+ BITMAP_FREE (dse_gd.stores);
/* For now, just wipe the post-dominator information. */
free_dominance_info (CDI_POST_DOMINATORS);
+ return 0;
}
static bool
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 */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
- TODO_dump_func | TODO_ggc_collect /* todo_flags_finish */
- | TODO_verify_ssa
+ TODO_dump_func
+ | TODO_ggc_collect
+ | TODO_verify_ssa /* todo_flags_finish */
+ }
};
+
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