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rt_gccstream/gcc/tree-ssa-dce.c

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/* Dead code elimination pass for the GNU compiler.
Copyright (C) 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010
Free Software Foundation, Inc.
Contributed by Ben Elliston <bje@redhat.com>
and Andrew MacLeod <amacleod@redhat.com>
Adapted to use control dependence by Steven Bosscher, SUSE Labs.
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
<http://www.gnu.org/licenses/>. */
/* Dead code elimination.
References:
Building an Optimizing Compiler,
Robert Morgan, Butterworth-Heinemann, 1998, Section 8.9.
Advanced Compiler Design and Implementation,
Steven Muchnick, Morgan Kaufmann, 1997, Section 18.10.
Dead-code elimination is the removal of statements which have no
impact on the program's output. "Dead statements" have no impact
on the program's output, while "necessary statements" may have
impact on the output.
The algorithm consists of three phases:
1. Marking as necessary all statements known to be necessary,
e.g. most function calls, writing a value to memory, etc;
2. Propagating necessary statements, e.g., the statements
giving values to operands in necessary statements; and
3. Removing dead statements. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "ggc.h"
/* These RTL headers are needed for basic-block.h. */
#include "rtl.h"
#include "tm_p.h"
#include "hard-reg-set.h"
#include "obstack.h"
#include "basic-block.h"
#include "tree.h"
#include "diagnostic.h"
#include "tree-flow.h"
#include "gimple.h"
#include "tree-dump.h"
#include "tree-pass.h"
#include "timevar.h"
#include "flags.h"
#include "cfgloop.h"
#include "tree-scalar-evolution.h"
static struct stmt_stats
{
int total;
int total_phis;
int removed;
int removed_phis;
} stats;
#define STMT_NECESSARY GF_PLF_1
static VEC(gimple,heap) *worklist;
/* Vector indicating an SSA name has already been processed and marked
as necessary. */
static sbitmap processed;
/* Vector indicating that last_stmt if a basic block has already been
marked as necessary. */
static sbitmap last_stmt_necessary;
/* Vector indicating that BB contains statements that are live. */
static sbitmap bb_contains_live_stmts;
/* Before we can determine whether a control branch is dead, we need to
compute which blocks are control dependent on which edges.
We expect each block to be control dependent on very few edges so we
use a bitmap for each block recording its edges. An array holds the
bitmap. The Ith bit in the bitmap is set if that block is dependent
on the Ith edge. */
static bitmap *control_dependence_map;
/* Vector indicating that a basic block has already had all the edges
processed that it is control dependent on. */
static sbitmap visited_control_parents;
/* TRUE if this pass alters the CFG (by removing control statements).
FALSE otherwise.
If this pass alters the CFG, then it will arrange for the dominators
to be recomputed. */
static bool cfg_altered;
/* Execute code that follows the macro for each edge (given number
EDGE_NUMBER within the CODE) for which the block with index N is
control dependent. */
#define EXECUTE_IF_CONTROL_DEPENDENT(BI, N, EDGE_NUMBER) \
EXECUTE_IF_SET_IN_BITMAP (control_dependence_map[(N)], 0, \
(EDGE_NUMBER), (BI))
/* Indicate block BB is control dependent on an edge with index EDGE_INDEX. */
static inline void
set_control_dependence_map_bit (basic_block bb, int edge_index)
{
if (bb == ENTRY_BLOCK_PTR)
return;
gcc_assert (bb != EXIT_BLOCK_PTR);
bitmap_set_bit (control_dependence_map[bb->index], edge_index);
}
/* Clear all control dependences for block BB. */
static inline void
clear_control_dependence_bitmap (basic_block bb)
{
bitmap_clear (control_dependence_map[bb->index]);
}
/* Find the immediate postdominator PDOM of the specified basic block BLOCK.
This function is necessary because some blocks have negative numbers. */
static inline basic_block
find_pdom (basic_block block)
{
gcc_assert (block != ENTRY_BLOCK_PTR);
if (block == EXIT_BLOCK_PTR)
return EXIT_BLOCK_PTR;
else
{
basic_block bb = get_immediate_dominator (CDI_POST_DOMINATORS, block);
if (! bb)
return EXIT_BLOCK_PTR;
return bb;
}
}
/* Determine all blocks' control dependences on the given edge with edge_list
EL index EDGE_INDEX, ala Morgan, Section 3.6. */
static void
find_control_dependence (struct edge_list *el, int edge_index)
{
basic_block current_block;
basic_block ending_block;
gcc_assert (INDEX_EDGE_PRED_BB (el, edge_index) != EXIT_BLOCK_PTR);
if (INDEX_EDGE_PRED_BB (el, edge_index) == ENTRY_BLOCK_PTR)
ending_block = single_succ (ENTRY_BLOCK_PTR);
else
ending_block = find_pdom (INDEX_EDGE_PRED_BB (el, edge_index));
for (current_block = INDEX_EDGE_SUCC_BB (el, edge_index);
current_block != ending_block && current_block != EXIT_BLOCK_PTR;
current_block = find_pdom (current_block))
{
edge e = INDEX_EDGE (el, edge_index);
/* For abnormal edges, we don't make current_block control
dependent because instructions that throw are always necessary
anyway. */
if (e->flags & EDGE_ABNORMAL)
continue;
set_control_dependence_map_bit (current_block, edge_index);
}
}
/* Record all blocks' control dependences on all edges in the edge
list EL, ala Morgan, Section 3.6. */
static void
find_all_control_dependences (struct edge_list *el)
{
int i;
for (i = 0; i < NUM_EDGES (el); ++i)
find_control_dependence (el, i);
}
/* If STMT is not already marked necessary, mark it, and add it to the
worklist if ADD_TO_WORKLIST is true. */
static inline void
mark_stmt_necessary (gimple stmt, bool add_to_worklist)
{
gcc_assert (stmt);
if (gimple_plf (stmt, STMT_NECESSARY))
return;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Marking useful stmt: ");
print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
fprintf (dump_file, "\n");
}
gimple_set_plf (stmt, STMT_NECESSARY, true);
if (add_to_worklist)
VEC_safe_push (gimple, heap, worklist, stmt);
if (bb_contains_live_stmts && !is_gimple_debug (stmt))
SET_BIT (bb_contains_live_stmts, gimple_bb (stmt)->index);
}
/* Mark the statement defining operand OP as necessary. */
static inline void
mark_operand_necessary (tree op)
{
gimple stmt;
int ver;
gcc_assert (op);
ver = SSA_NAME_VERSION (op);
if (TEST_BIT (processed, ver))
{
stmt = SSA_NAME_DEF_STMT (op);
gcc_assert (gimple_nop_p (stmt)
|| gimple_plf (stmt, STMT_NECESSARY));
return;
}
SET_BIT (processed, ver);
stmt = SSA_NAME_DEF_STMT (op);
gcc_assert (stmt);
if (gimple_plf (stmt, STMT_NECESSARY) || gimple_nop_p (stmt))
return;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "marking necessary through ");
print_generic_expr (dump_file, op, 0);
fprintf (dump_file, " stmt ");
print_gimple_stmt (dump_file, stmt, 0, 0);
}
gimple_set_plf (stmt, STMT_NECESSARY, true);
if (bb_contains_live_stmts)
SET_BIT (bb_contains_live_stmts, gimple_bb (stmt)->index);
VEC_safe_push (gimple, heap, worklist, stmt);
}
/* Mark STMT as necessary if it obviously is. Add it to the worklist if
it can make other statements necessary.
If AGGRESSIVE is false, control statements are conservatively marked as
necessary. */
static void
mark_stmt_if_obviously_necessary (gimple stmt, bool aggressive)
{
tree lhs = NULL_TREE;
/* With non-call exceptions, we have to assume that all statements could
throw. If a statement may throw, it is inherently necessary. */
if (flag_non_call_exceptions
&& stmt_could_throw_p (stmt))
{
mark_stmt_necessary (stmt, true);
return;
}
/* Statements that are implicitly live. Most function calls, asm
and return statements are required. Labels and GIMPLE_BIND nodes
are kept because they are control flow, and we have no way of
knowing whether they can be removed. DCE can eliminate all the
other statements in a block, and CFG can then remove the block
and labels. */
switch (gimple_code (stmt))
{
case GIMPLE_PREDICT:
case GIMPLE_LABEL:
mark_stmt_necessary (stmt, false);
return;
case GIMPLE_ASM:
case GIMPLE_RESX:
case GIMPLE_RETURN:
mark_stmt_necessary (stmt, true);
return;
case GIMPLE_CALL:
/* Most, but not all function calls are required. Function calls that
produce no result and have no side effects (i.e. const pure
functions) are unnecessary. */
if (gimple_has_side_effects (stmt))
{
mark_stmt_necessary (stmt, true);
return;
}
if (!gimple_call_lhs (stmt))
return;
lhs = gimple_call_lhs (stmt);
/* Fall through */
case GIMPLE_ASSIGN:
if (!lhs)
lhs = gimple_assign_lhs (stmt);
break;
case GIMPLE_DEBUG:
/* Debug temps without a value are not useful. ??? If we could
easily locate the debug temp bind stmt for a use thereof,
would could refrain from marking all debug temps here, and
mark them only if they're used. */
if (gimple_debug_bind_has_value_p (stmt)
|| TREE_CODE (gimple_debug_bind_get_var (stmt)) != DEBUG_EXPR_DECL)
mark_stmt_necessary (stmt, false);
return;
case GIMPLE_GOTO:
gcc_assert (!simple_goto_p (stmt));
mark_stmt_necessary (stmt, true);
return;
case GIMPLE_COND:
gcc_assert (EDGE_COUNT (gimple_bb (stmt)->succs) == 2);
/* Fall through. */
case GIMPLE_SWITCH:
if (! aggressive)
mark_stmt_necessary (stmt, true);
break;
default:
break;
}
/* If the statement has volatile operands, it needs to be preserved.
Same for statements that can alter control flow in unpredictable
ways. */
if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))
{
mark_stmt_necessary (stmt, true);
return;
}
if (is_hidden_global_store (stmt))
{
mark_stmt_necessary (stmt, true);
return;
}
return;
}
/* Make corresponding control dependent edges necessary. We only
have to do this once for each basic block, so we clear the bitmap
after we're done.
When IGNORE_SELF it true, ignore BB from the list of control dependences. */
static void
mark_control_dependent_edges_necessary (basic_block bb, struct edge_list *el, bool ignore_self)
{
bitmap_iterator bi;
unsigned edge_number;
bool skipped = false;
gcc_assert (bb != EXIT_BLOCK_PTR);
if (bb == ENTRY_BLOCK_PTR)
return;
EXECUTE_IF_CONTROL_DEPENDENT (bi, bb->index, edge_number)
{
gimple stmt;
basic_block cd_bb = INDEX_EDGE_PRED_BB (el, edge_number);
if (ignore_self && cd_bb == bb)
{
skipped = true;
continue;
}
if (TEST_BIT (last_stmt_necessary, cd_bb->index))
continue;
SET_BIT (last_stmt_necessary, cd_bb->index);
SET_BIT (bb_contains_live_stmts, cd_bb->index);
stmt = last_stmt (cd_bb);
if (stmt && is_ctrl_stmt (stmt))
mark_stmt_necessary (stmt, true);
}
if (!skipped)
SET_BIT (visited_control_parents, bb->index);
}
/* Find obviously necessary statements. These are things like most function
calls, and stores to file level variables.
If EL is NULL, control statements are conservatively marked as
necessary. Otherwise it contains the list of edges used by control
dependence analysis. */
static void
find_obviously_necessary_stmts (struct edge_list *el)
{
basic_block bb;
gimple_stmt_iterator gsi;
edge e;
gimple phi, stmt;
FOR_EACH_BB (bb)
{
/* PHI nodes are never inherently necessary. */
for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
{
phi = gsi_stmt (gsi);
gimple_set_plf (phi, STMT_NECESSARY, false);
}
/* Check all statements in the block. */
for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
{
stmt = gsi_stmt (gsi);
gimple_set_plf (stmt, STMT_NECESSARY, false);
mark_stmt_if_obviously_necessary (stmt, el != NULL);
}
}
/* Pure and const functions are finite and thus have no infinite loops in
them. */
if ((TREE_READONLY (current_function_decl)
|| DECL_PURE_P (current_function_decl))
&& !DECL_LOOPING_CONST_OR_PURE_P (current_function_decl))
return;
/* Prevent the empty possibly infinite loops from being removed. */
if (el)
{
loop_iterator li;
struct loop *loop;
scev_initialize ();
if (mark_irreducible_loops ())
FOR_EACH_BB (bb)
{
edge_iterator ei;
FOR_EACH_EDGE (e, ei, bb->succs)
if ((e->flags & EDGE_DFS_BACK)
&& (e->flags & EDGE_IRREDUCIBLE_LOOP))
{
if (dump_file)
fprintf (dump_file, "Marking back edge of irreducible loop %i->%i\n",
e->src->index, e->dest->index);
mark_control_dependent_edges_necessary (e->dest, el, false);
}
}
FOR_EACH_LOOP (li, loop, 0)
if (!finite_loop_p (loop))
{
if (dump_file)
fprintf (dump_file, "can not prove finiteness of loop %i\n", loop->num);
mark_control_dependent_edges_necessary (loop->latch, el, false);
}
scev_finalize ();
}
}
/* Return true if REF is based on an aliased base, otherwise false. */
static bool
ref_may_be_aliased (tree ref)
{
while (handled_component_p (ref))
ref = TREE_OPERAND (ref, 0);
return !(DECL_P (ref)
&& !may_be_aliased (ref));
}
static bitmap visited = NULL;
static unsigned int longest_chain = 0;
static unsigned int total_chain = 0;
static unsigned int nr_walks = 0;
static bool chain_ovfl = false;
/* Worker for the walker that marks reaching definitions of REF,
which is based on a non-aliased decl, necessary. It returns
true whenever the defining statement of the current VDEF is
a kill for REF, as no dominating may-defs are necessary for REF
anymore. DATA points to the basic-block that contains the
stmt that refers to REF. */
static bool
mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)
{
gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
/* All stmts we visit are necessary. */
mark_operand_necessary (vdef);
/* If the stmt lhs kills ref, then we can stop walking. */
if (gimple_has_lhs (def_stmt)
&& TREE_CODE (gimple_get_lhs (def_stmt)) != SSA_NAME)
{
tree base, lhs = gimple_get_lhs (def_stmt);
HOST_WIDE_INT size, offset, max_size;
ao_ref_base (ref);
base = get_ref_base_and_extent (lhs, &offset, &size, &max_size);
/* We can get MEM[symbol: sZ, index: D.8862_1] here,
so base == refd->base does not always hold. */
if (base == ref->base)
{
/* For a must-alias check we need to be able to constrain
the accesses properly. */
if (size != -1 && size == max_size
&& ref->max_size != -1)
{
if (offset <= ref->offset
&& offset + size >= ref->offset + ref->max_size)
return true;
}
/* Or they need to be exactly the same. */
else if (ref->ref
/* Make sure there is no induction variable involved
in the references (gcc.c-torture/execute/pr42142.c).
The simplest way is to check if the kill dominates
the use. */
&& dominated_by_p (CDI_DOMINATORS, (basic_block) data,
gimple_bb (def_stmt))
&& operand_equal_p (ref->ref, lhs, 0))
return true;
}
}
/* Otherwise keep walking. */
return false;
}
static void
mark_aliased_reaching_defs_necessary (gimple stmt, tree ref)
{
unsigned int chain;
ao_ref refd;
gcc_assert (!chain_ovfl);
ao_ref_init (&refd, ref);
chain = walk_aliased_vdefs (&refd, gimple_vuse (stmt),
mark_aliased_reaching_defs_necessary_1,
gimple_bb (stmt), NULL);
if (chain > longest_chain)
longest_chain = chain;
total_chain += chain;
nr_walks++;
}
/* Worker for the walker that marks reaching definitions of REF, which
is not based on a non-aliased decl. For simplicity we need to end
up marking all may-defs necessary that are not based on a non-aliased
decl. The only job of this walker is to skip may-defs based on
a non-aliased decl. */
static bool
mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,
tree vdef, void *data ATTRIBUTE_UNUSED)
{
gimple def_stmt = SSA_NAME_DEF_STMT (vdef);
/* We have to skip already visited (and thus necessary) statements
to make the chaining work after we dropped back to simple mode. */
if (chain_ovfl
&& TEST_BIT (processed, SSA_NAME_VERSION (vdef)))
{
gcc_assert (gimple_nop_p (def_stmt)
|| gimple_plf (def_stmt, STMT_NECESSARY));
return false;
}
/* We want to skip stores to non-aliased variables. */
if (!chain_ovfl
&& gimple_assign_single_p (def_stmt))
{
tree lhs = gimple_assign_lhs (def_stmt);
if (!ref_may_be_aliased (lhs))
return false;
}
mark_operand_necessary (vdef);
return false;
}
static void
mark_all_reaching_defs_necessary (gimple stmt)
{
walk_aliased_vdefs (NULL, gimple_vuse (stmt),
mark_all_reaching_defs_necessary_1, NULL, &visited);
}
/* Return true for PHI nodes with one or identical arguments
can be removed. */
static bool
degenerate_phi_p (gimple phi)
{
unsigned int i;
tree op = gimple_phi_arg_def (phi, 0);
for (i = 1; i < gimple_phi_num_args (phi); i++)
if (gimple_phi_arg_def (phi, i) != op)
return false;
return true;
}
/* Propagate necessity using the operands of necessary statements.
Process the uses on each statement in the worklist, and add all
feeding statements which contribute to the calculation of this
value to the worklist.
In conservative mode, EL is NULL. */
static void
propagate_necessity (struct edge_list *el)
{
gimple stmt;
bool aggressive = (el ? true : false);
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, "\nProcessing worklist:\n");
while (VEC_length (gimple, worklist) > 0)
{
/* Take STMT from worklist. */
stmt = VEC_pop (gimple, worklist);
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "processing: ");
print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
fprintf (dump_file, "\n");
}
if (aggressive)
{
/* Mark the last statements of the basic blocks that the block
containing STMT is control dependent on, but only if we haven't
already done so. */
basic_block bb = gimple_bb (stmt);
if (bb != ENTRY_BLOCK_PTR
&& ! TEST_BIT (visited_control_parents, bb->index))
mark_control_dependent_edges_necessary (bb, el, false);
}
if (gimple_code (stmt) == GIMPLE_PHI
/* We do not process virtual PHI nodes nor do we track their
necessity. */
&& is_gimple_reg (gimple_phi_result (stmt)))
{
/* PHI nodes are somewhat special in that each PHI alternative has
data and control dependencies. All the statements feeding the
PHI node's arguments are always necessary. In aggressive mode,
we also consider the control dependent edges leading to the
predecessor block associated with each PHI alternative as
necessary. */
size_t k;
for (k = 0; k < gimple_phi_num_args (stmt); k++)
{
tree arg = PHI_ARG_DEF (stmt, k);
if (TREE_CODE (arg) == SSA_NAME)
mark_operand_necessary (arg);
}
/* For PHI operands it matters from where the control flow arrives
to the BB. Consider the following example:
a=exp1;
b=exp2;
if (test)
;
else
;
c=PHI(a,b)
We need to mark control dependence of the empty basic blocks, since they
contains computation of PHI operands.
Doing so is too restrictive in the case the predecestor block is in
the loop. Consider:
if (b)
{
int i;
for (i = 0; i<1000; ++i)
;
j = 0;
}
return j;
There is PHI for J in the BB containing return statement.
In this case the control dependence of predecestor block (that is
within the empty loop) also contains the block determining number
of iterations of the block that would prevent removing of empty
loop in this case.
This scenario can be avoided by splitting critical edges.
To save the critical edge splitting pass we identify how the control
dependence would look like if the edge was split.
Consider the modified CFG created from current CFG by splitting
edge B->C. In the postdominance tree of modified CFG, C' is
always child of C. There are two cases how chlids of C' can look
like:
1) C' is leaf
In this case the only basic block C' is control dependent on is B.
2) C' has single child that is B
In this case control dependence of C' is same as control
dependence of B in original CFG except for block B itself.
(since C' postdominate B in modified CFG)
Now how to decide what case happens? There are two basic options:
a) C postdominate B. Then C immediately postdominate B and
case 2 happens iff there is no other way from B to C except
the edge B->C.
There is other way from B to C iff there is succesor of B that
is not postdominated by B. Testing this condition is somewhat
expensive, because we need to iterate all succesors of B.
We are safe to assume that this does not happen: we will mark B
as needed when processing the other path from B to C that is
conrol dependent on B and marking control dependencies of B
itself is harmless because they will be processed anyway after
processing control statement in B.
b) C does not postdominate B. Always case 1 happens since there is
path from C to exit that does not go through B and thus also C'. */
if (aggressive && !degenerate_phi_p (stmt))
{
for (k = 0; k < gimple_phi_num_args (stmt); k++)
{
basic_block arg_bb = gimple_phi_arg_edge (stmt, k)->src;
if (gimple_bb (stmt)
!= get_immediate_dominator (CDI_POST_DOMINATORS, arg_bb))
{
if (!TEST_BIT (last_stmt_necessary, arg_bb->index))
{
gimple stmt2;
SET_BIT (last_stmt_necessary, arg_bb->index);
SET_BIT (bb_contains_live_stmts, arg_bb->index);
stmt2 = last_stmt (arg_bb);
if (stmt2 && is_ctrl_stmt (stmt2))
mark_stmt_necessary (stmt2, true);
}
}
else if (arg_bb != ENTRY_BLOCK_PTR
&& ! TEST_BIT (visited_control_parents, arg_bb->index))
mark_control_dependent_edges_necessary (arg_bb, el, true);
}
}
}
else
{
/* Propagate through the operands. Examine all the USE, VUSE and
VDEF operands in this statement. Mark all the statements
which feed this statement's uses as necessary. */
ssa_op_iter iter;
tree use;
FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
mark_operand_necessary (use);
use = gimple_vuse (stmt);
if (!use)
continue;
/* If we dropped to simple mode make all immediately
reachable definitions necessary. */
if (chain_ovfl)
{
mark_all_reaching_defs_necessary (stmt);
continue;
}
/* For statements that may load from memory (have a VUSE) we
have to mark all reaching (may-)definitions as necessary.
We partition this task into two cases:
1) explicit loads based on decls that are not aliased
2) implicit loads (like calls) and explicit loads not
based on decls that are not aliased (like indirect
references or loads from globals)
For 1) we mark all reaching may-defs as necessary, stopping
at dominating kills. For 2) we want to mark all dominating
references necessary, but non-aliased ones which we handle
in 1). By keeping a global visited bitmap for references
we walk for 2) we avoid quadratic behavior for those. */
if (is_gimple_call (stmt))
{
tree callee = gimple_call_fndecl (stmt);
unsigned i;
/* Calls to functions that are merely acting as barriers
or that only store to memory do not make any previous
stores necessary. */
if (callee != NULL_TREE
&& DECL_BUILT_IN_CLASS (callee) == BUILT_IN_NORMAL
&& (DECL_FUNCTION_CODE (callee) == BUILT_IN_MEMSET
|| DECL_FUNCTION_CODE (callee) == BUILT_IN_MALLOC
|| DECL_FUNCTION_CODE (callee) == BUILT_IN_FREE))
continue;
/* Calls implicitly load from memory, their arguments
in addition may explicitly perform memory loads. */
mark_all_reaching_defs_necessary (stmt);
for (i = 0; i < gimple_call_num_args (stmt); ++i)
{
tree arg = gimple_call_arg (stmt, i);
if (TREE_CODE (arg) == SSA_NAME
|| is_gimple_min_invariant (arg))
continue;
if (!ref_may_be_aliased (arg))
mark_aliased_reaching_defs_necessary (stmt, arg);
}
}
else if (gimple_assign_single_p (stmt))
{
tree rhs;
bool rhs_aliased = false;
/* If this is a load mark things necessary. */
rhs = gimple_assign_rhs1 (stmt);
if (TREE_CODE (rhs) != SSA_NAME
&& !is_gimple_min_invariant (rhs))
{
if (!ref_may_be_aliased (rhs))
mark_aliased_reaching_defs_necessary (stmt, rhs);
else
rhs_aliased = true;
}
if (rhs_aliased)
mark_all_reaching_defs_necessary (stmt);
}
else if (gimple_code (stmt) == GIMPLE_RETURN)
{
tree rhs = gimple_return_retval (stmt);
/* A return statement may perform a load. */
if (TREE_CODE (rhs) != SSA_NAME
&& !is_gimple_min_invariant (rhs))
{
if (!ref_may_be_aliased (rhs))
mark_aliased_reaching_defs_necessary (stmt, rhs);
else
mark_all_reaching_defs_necessary (stmt);
}
}
else if (gimple_code (stmt) == GIMPLE_ASM)
{
unsigned i;
mark_all_reaching_defs_necessary (stmt);
/* Inputs may perform loads. */
for (i = 0; i < gimple_asm_ninputs (stmt); ++i)
{
tree op = TREE_VALUE (gimple_asm_input_op (stmt, i));
if (TREE_CODE (op) != SSA_NAME
&& !is_gimple_min_invariant (op)
&& !ref_may_be_aliased (op))
mark_aliased_reaching_defs_necessary (stmt, op);
}
}
else
gcc_unreachable ();
/* If we over-used our alias oracle budget drop to simple
mode. The cost metric allows quadratic behavior
(number of uses times number of may-defs queries) up to
a constant maximal number of queries and after that falls back to
super-linear complexity. */
if (/* Constant but quadratic for small functions. */
total_chain > 128 * 128
/* Linear in the number of may-defs. */
&& total_chain > 32 * longest_chain
/* Linear in the number of uses. */
&& total_chain > nr_walks * 32)
{
chain_ovfl = true;
if (visited)
bitmap_clear (visited);
}
}
}
}
/* Replace all uses of result of PHI by underlying variable and mark it
for renaming. */
void
mark_virtual_phi_result_for_renaming (gimple phi)
{
bool used = false;
imm_use_iterator iter;
use_operand_p use_p;
gimple stmt;
tree result_ssa, result_var;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Marking result for renaming : ");
print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
fprintf (dump_file, "\n");
}
result_ssa = gimple_phi_result (phi);
result_var = SSA_NAME_VAR (result_ssa);
FOR_EACH_IMM_USE_STMT (stmt, iter, result_ssa)
{
FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
SET_USE (use_p, result_var);
update_stmt (stmt);
used = true;
}
if (used)
mark_sym_for_renaming (result_var);
}
/* Remove dead PHI nodes from block BB. */
static bool
remove_dead_phis (basic_block bb)
{
bool something_changed = false;
gimple_seq phis;
gimple phi;
gimple_stmt_iterator gsi;
phis = phi_nodes (bb);
for (gsi = gsi_start (phis); !gsi_end_p (gsi);)
{
stats.total_phis++;
phi = gsi_stmt (gsi);
/* We do not track necessity of virtual PHI nodes. Instead do
very simple dead PHI removal here. */
if (!is_gimple_reg (gimple_phi_result (phi)))
{
/* Virtual PHI nodes with one or identical arguments
can be removed. */
if (degenerate_phi_p (phi))
{
tree vdef = gimple_phi_result (phi);
tree vuse = gimple_phi_arg_def (phi, 0);
use_operand_p use_p;
imm_use_iterator iter;
gimple use_stmt;
FOR_EACH_IMM_USE_STMT (use_stmt, iter, vdef)
FOR_EACH_IMM_USE_ON_STMT (use_p, iter)
SET_USE (use_p, vuse);
if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vdef)
&& TREE_CODE (vuse) == SSA_NAME)
SSA_NAME_OCCURS_IN_ABNORMAL_PHI (vuse) = 1;
}
else
gimple_set_plf (phi, STMT_NECESSARY, true);
}
if (!gimple_plf (phi, STMT_NECESSARY))
{
something_changed = true;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Deleting : ");
print_gimple_stmt (dump_file, phi, 0, TDF_SLIM);
fprintf (dump_file, "\n");
}
remove_phi_node (&gsi, true);
stats.removed_phis++;
continue;
}
gsi_next (&gsi);
}
return something_changed;
}
/* Forward edge E to respective POST_DOM_BB and update PHIs. */
static edge
forward_edge_to_pdom (edge e, basic_block post_dom_bb)
{
gimple_stmt_iterator gsi;
edge e2 = NULL;
edge_iterator ei;
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, "Redirecting edge %i->%i to %i\n", e->src->index,
e->dest->index, post_dom_bb->index);
e2 = redirect_edge_and_branch (e, post_dom_bb);
cfg_altered = true;
/* If edge was already around, no updating is neccesary. */
if (e2 != e)
return e2;
if (!gimple_seq_empty_p (phi_nodes (post_dom_bb)))
{
/* We are sure that for every live PHI we are seeing control dependent BB.
This means that we can pick any edge to duplicate PHI args from. */
FOR_EACH_EDGE (e2, ei, post_dom_bb->preds)
if (e2 != e)
break;
for (gsi = gsi_start_phis (post_dom_bb); !gsi_end_p (gsi);)
{
gimple phi = gsi_stmt (gsi);
tree op;
source_location locus;
/* PHIs for virtuals have no control dependency relation on them.
We are lost here and must force renaming of the symbol. */
if (!is_gimple_reg (gimple_phi_result (phi)))
{
mark_virtual_phi_result_for_renaming (phi);
remove_phi_node (&gsi, true);
continue;
}
/* Dead PHI do not imply control dependency. */
if (!gimple_plf (phi, STMT_NECESSARY))
{
gsi_next (&gsi);
continue;
}
op = gimple_phi_arg_def (phi, e2->dest_idx);
locus = gimple_phi_arg_location (phi, e2->dest_idx);
add_phi_arg (phi, op, e, locus);
/* The resulting PHI if not dead can only be degenerate. */
gcc_assert (degenerate_phi_p (phi));
gsi_next (&gsi);
}
}
return e;
}
/* Remove dead statement pointed to by iterator I. Receives the basic block BB
containing I so that we don't have to look it up. */
static void
remove_dead_stmt (gimple_stmt_iterator *i, basic_block bb)
{
gimple stmt = gsi_stmt (*i);
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Deleting : ");
print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
fprintf (dump_file, "\n");
}
stats.removed++;
/* If we have determined that a conditional branch statement contributes
nothing to the program, then we not only remove it, but we also change
the flow graph so that the current block will simply fall-thru to its
immediate post-dominator. The blocks we are circumventing will be
removed by cleanup_tree_cfg if this change in the flow graph makes them
unreachable. */
if (is_ctrl_stmt (stmt))
{
basic_block post_dom_bb;
edge e, e2;
edge_iterator ei;
post_dom_bb = get_immediate_dominator (CDI_POST_DOMINATORS, bb);
e = find_edge (bb, post_dom_bb);
/* If edge is already there, try to use it. This avoids need to update
PHI nodes. Also watch for cases where post dominator does not exists
or is exit block. These can happen for infinite loops as we create
fake edges in the dominator tree. */
if (e)
;
else if (! post_dom_bb || post_dom_bb == EXIT_BLOCK_PTR)
e = EDGE_SUCC (bb, 0);
else
e = forward_edge_to_pdom (EDGE_SUCC (bb, 0), post_dom_bb);
gcc_assert (e);
e->probability = REG_BR_PROB_BASE;
e->count = bb->count;
/* The edge is no longer associated with a conditional, so it does
not have TRUE/FALSE flags. */
e->flags &= ~(EDGE_TRUE_VALUE | EDGE_FALSE_VALUE);
/* The lone outgoing edge from BB will be a fallthru edge. */
e->flags |= EDGE_FALLTHRU;
/* Remove the remaining outgoing edges. */
for (ei = ei_start (bb->succs); (e2 = ei_safe_edge (ei)); )
if (e != e2)
{
cfg_altered = true;
remove_edge (e2);
}
else
ei_next (&ei);
}
unlink_stmt_vdef (stmt);
gsi_remove (i, true);
release_defs (stmt);
}
/* Eliminate unnecessary statements. Any instruction not marked as necessary
contributes nothing to the program, and can be deleted. */
static bool
eliminate_unnecessary_stmts (void)
{
bool something_changed = false;
basic_block bb;
gimple_stmt_iterator gsi, psi;
gimple stmt;
tree call;
VEC (basic_block, heap) *h;
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, "\nEliminating unnecessary statements:\n");
clear_special_calls ();
/* Walking basic blocks and statements in reverse order avoids
releasing SSA names before any other DEFs that refer to them are
released. This helps avoid loss of debug information, as we get
a chance to propagate all RHSs of removed SSAs into debug uses,
rather than only the latest ones. E.g., consider:
x_3 = y_1 + z_2;
a_5 = x_3 - b_4;
# DEBUG a => a_5
If we were to release x_3 before a_5, when we reached a_5 and
tried to substitute it into the debug stmt, we'd see x_3 there,
but x_3's DEF, type, etc would have already been disconnected.
By going backwards, the debug stmt first changes to:
# DEBUG a => x_3 - b_4
and then to:
# DEBUG a => y_1 + z_2 - b_4
as desired. */
gcc_assert (dom_info_available_p (CDI_DOMINATORS));
h = get_all_dominated_blocks (CDI_DOMINATORS, single_succ (ENTRY_BLOCK_PTR));
while (VEC_length (basic_block, h))
{
bb = VEC_pop (basic_block, h);
/* Remove dead statements. */
for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi = psi)
{
stmt = gsi_stmt (gsi);
psi = gsi;
gsi_prev (&psi);
stats.total++;
/* If GSI is not necessary then remove it. */
if (!gimple_plf (stmt, STMT_NECESSARY))
{
if (!is_gimple_debug (stmt))
something_changed = true;
remove_dead_stmt (&gsi, bb);
}
else if (is_gimple_call (stmt))
{
call = gimple_call_fndecl (stmt);
if (call)
{
tree name;
/* When LHS of var = call (); is dead, simplify it into
call (); saving one operand. */
name = gimple_call_lhs (stmt);
if (name && TREE_CODE (name) == SSA_NAME
&& !TEST_BIT (processed, SSA_NAME_VERSION (name)))
{
something_changed = true;
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Deleting LHS of call: ");
print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM);
fprintf (dump_file, "\n");
}
gimple_call_set_lhs (stmt, NULL_TREE);
maybe_clean_or_replace_eh_stmt (stmt, stmt);
update_stmt (stmt);
release_ssa_name (name);
}
notice_special_calls (stmt);
}
}
}
}
VEC_free (basic_block, heap, h);
/* Since we don't track liveness of virtual PHI nodes, it is possible that we
rendered some PHI nodes unreachable while they are still in use.
Mark them for renaming. */
if (cfg_altered)
{
basic_block prev_bb;
find_unreachable_blocks ();
/* Delete all unreachable basic blocks in reverse dominator order. */
for (bb = EXIT_BLOCK_PTR->prev_bb; bb != ENTRY_BLOCK_PTR; bb = prev_bb)
{
prev_bb = bb->prev_bb;
if (!TEST_BIT (bb_contains_live_stmts, bb->index)
|| !(bb->flags & BB_REACHABLE))
{
for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
if (!is_gimple_reg (gimple_phi_result (gsi_stmt (gsi))))
{
bool found = false;
imm_use_iterator iter;
FOR_EACH_IMM_USE_STMT (stmt, iter, gimple_phi_result (gsi_stmt (gsi)))
{
if (!(gimple_bb (stmt)->flags & BB_REACHABLE))
continue;
if (gimple_code (stmt) == GIMPLE_PHI
|| gimple_plf (stmt, STMT_NECESSARY))
{
found = true;
BREAK_FROM_IMM_USE_STMT (iter);
}
}
if (found)
mark_virtual_phi_result_for_renaming (gsi_stmt (gsi));
}
if (!(bb->flags & BB_REACHABLE))
{
/* Speed up the removal of blocks that don't
dominate others. Walking backwards, this should
be the common case. ??? Do we need to recompute
dominators because of cfg_altered? */
if (!MAY_HAVE_DEBUG_STMTS
|| !first_dom_son (CDI_DOMINATORS, bb))
delete_basic_block (bb);
else
{
h = get_all_dominated_blocks (CDI_DOMINATORS, bb);
while (VEC_length (basic_block, h))
{
bb = VEC_pop (basic_block, h);
prev_bb = bb->prev_bb;
/* Rearrangements to the CFG may have failed
to update the dominators tree, so that
formerly-dominated blocks are now
otherwise reachable. */
if (!!(bb->flags & BB_REACHABLE))
continue;
delete_basic_block (bb);
}
VEC_free (basic_block, heap, h);
}
}
}
}
}
FOR_EACH_BB (bb)
{
/* Remove dead PHI nodes. */
something_changed |= remove_dead_phis (bb);
}
return something_changed;
}
/* Print out removed statement statistics. */
static void
print_stats (void)
{
float percg;
percg = ((float) stats.removed / (float) stats.total) * 100;
fprintf (dump_file, "Removed %d of %d statements (%d%%)\n",
stats.removed, stats.total, (int) percg);
if (stats.total_phis == 0)
percg = 0;
else
percg = ((float) stats.removed_phis / (float) stats.total_phis) * 100;
fprintf (dump_file, "Removed %d of %d PHI nodes (%d%%)\n",
stats.removed_phis, stats.total_phis, (int) percg);
}
/* Initialization for this pass. Set up the used data structures. */
static void
tree_dce_init (bool aggressive)
{
memset ((void *) &stats, 0, sizeof (stats));
if (aggressive)
{
int i;
control_dependence_map = XNEWVEC (bitmap, last_basic_block);
for (i = 0; i < last_basic_block; ++i)
control_dependence_map[i] = BITMAP_ALLOC (NULL);
last_stmt_necessary = sbitmap_alloc (last_basic_block);
sbitmap_zero (last_stmt_necessary);
bb_contains_live_stmts = sbitmap_alloc (last_basic_block);
sbitmap_zero (bb_contains_live_stmts);
}
processed = sbitmap_alloc (num_ssa_names + 1);
sbitmap_zero (processed);
worklist = VEC_alloc (gimple, heap, 64);
cfg_altered = false;
}
/* Cleanup after this pass. */
static void
tree_dce_done (bool aggressive)
{
if (aggressive)
{
int i;
for (i = 0; i < last_basic_block; ++i)
BITMAP_FREE (control_dependence_map[i]);
free (control_dependence_map);
sbitmap_free (visited_control_parents);
sbitmap_free (last_stmt_necessary);
sbitmap_free (bb_contains_live_stmts);
bb_contains_live_stmts = NULL;
}
sbitmap_free (processed);
VEC_free (gimple, heap, worklist);
}
/* Main routine to eliminate dead code.
AGGRESSIVE controls the aggressiveness of the algorithm.
In conservative mode, we ignore control dependence and simply declare
all but the most trivially dead branches necessary. This mode is fast.
In aggressive mode, control dependences are taken into account, which
results in more dead code elimination, but at the cost of some time.
FIXME: Aggressive mode before PRE doesn't work currently because
the dominance info is not invalidated after DCE1. This is
not an issue right now because we only run aggressive DCE
as the last tree SSA pass, but keep this in mind when you
start experimenting with pass ordering. */
static unsigned int
perform_tree_ssa_dce (bool aggressive)
{
struct edge_list *el = NULL;
bool something_changed = 0;
/* Preheaders are needed for SCEV to work.
Simple lateches and recorded exits improve chances that loop will
proved to be finite in testcases such as in loop-15.c and loop-24.c */
if (aggressive)
loop_optimizer_init (LOOPS_NORMAL
| LOOPS_HAVE_RECORDED_EXITS);
tree_dce_init (aggressive);
if (aggressive)
{
/* Compute control dependence. */
timevar_push (TV_CONTROL_DEPENDENCES);
calculate_dominance_info (CDI_POST_DOMINATORS);
el = create_edge_list ();
find_all_control_dependences (el);
timevar_pop (TV_CONTROL_DEPENDENCES);
visited_control_parents = sbitmap_alloc (last_basic_block);
sbitmap_zero (visited_control_parents);
mark_dfs_back_edges ();
}
find_obviously_necessary_stmts (el);
if (aggressive)
loop_optimizer_finalize ();
longest_chain = 0;
total_chain = 0;
nr_walks = 0;
chain_ovfl = false;
visited = BITMAP_ALLOC (NULL);
propagate_necessity (el);
BITMAP_FREE (visited);
something_changed |= eliminate_unnecessary_stmts ();
something_changed |= cfg_altered;
/* We do not update postdominators, so free them unconditionally. */
free_dominance_info (CDI_POST_DOMINATORS);
/* If we removed paths in the CFG, then we need to update
dominators as well. I haven't investigated the possibility
of incrementally updating dominators. */
if (cfg_altered)
free_dominance_info (CDI_DOMINATORS);
statistics_counter_event (cfun, "Statements deleted", stats.removed);
statistics_counter_event (cfun, "PHI nodes deleted", stats.removed_phis);
/* Debugging dumps. */
if (dump_file && (dump_flags & (TDF_STATS|TDF_DETAILS)))
print_stats ();
tree_dce_done (aggressive);
free_edge_list (el);
if (something_changed)
return (TODO_update_ssa | TODO_cleanup_cfg | TODO_ggc_collect
| TODO_remove_unused_locals);
else
return 0;
}
/* Pass entry points. */
static unsigned int
tree_ssa_dce (void)
{
return perform_tree_ssa_dce (/*aggressive=*/false);
}
static unsigned int
tree_ssa_dce_loop (void)
{
unsigned int todo;
todo = perform_tree_ssa_dce (/*aggressive=*/false);
if (todo)
{
free_numbers_of_iterations_estimates ();
scev_reset ();
}
return todo;
}
static unsigned int
tree_ssa_cd_dce (void)
{
return perform_tree_ssa_dce (/*aggressive=*/optimize >= 2);
}
static bool
gate_dce (void)
{
return flag_tree_dce != 0;
}
struct gimple_opt_pass pass_dce =
{
{
GIMPLE_PASS,
"dce", /* name */
gate_dce, /* gate */
tree_ssa_dce, /* execute */
NULL, /* sub */
NULL, /* next */
0, /* static_pass_number */
TV_TREE_DCE, /* tv_id */
PROP_cfg | PROP_ssa, /* properties_required */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
TODO_dump_func | TODO_verify_ssa /* todo_flags_finish */
}
};
struct gimple_opt_pass pass_dce_loop =
{
{
GIMPLE_PASS,
"dceloop", /* name */
gate_dce, /* gate */
tree_ssa_dce_loop, /* execute */
NULL, /* sub */
NULL, /* next */
0, /* static_pass_number */
TV_TREE_DCE, /* tv_id */
PROP_cfg | PROP_ssa, /* properties_required */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
TODO_dump_func | TODO_verify_ssa /* todo_flags_finish */
}
};
struct gimple_opt_pass pass_cd_dce =
{
{
GIMPLE_PASS,
"cddce", /* name */
gate_dce, /* gate */
tree_ssa_cd_dce, /* execute */
NULL, /* sub */
NULL, /* next */
0, /* static_pass_number */
TV_TREE_CD_DCE, /* tv_id */
PROP_cfg | PROP_ssa, /* properties_required */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
TODO_dump_func | TODO_verify_ssa
| TODO_verify_flow /* todo_flags_finish */
}
};
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