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/* Backward propagation of indirect loads through PHIs.
Copyright (C) 2007, 2008 Free Software Foundation, Inc.
Contributed by Richard Guenther <>
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
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
<>. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "ggc.h"
#include "tree.h"
#include "rtl.h"
#include "tm_p.h"
#include "basic-block.h"
#include "timevar.h"
#include "diagnostic.h"
#include "tree-flow.h"
#include "tree-pass.h"
#include "tree-dump.h"
#include "langhooks.h"
#include "flags.h"
/* This pass propagates indirect loads through the PHI node for its
address to make the load source possibly non-addressable and to
allow for PHI optimization to trigger.
For example the pass changes
# addr_1 = PHI <&a, &b>
tmp_1 = *addr_1;
# tmp_1 = PHI <a, b>
but also handles more complex scenarios like
D.2077_2 = &this_1(D)->a1;
# b_12 = PHI <&c(2), D.2077_2(3)>
D.2114_13 = *b_12;
# b_15 = PHI <b_12(4), &b(5)>
D.2080_5 = &this_1(D)->a0;
# b_18 = PHI <D.2080_5(6), &c(7)>
# b_21 = PHI <b_15(8), b_18(9)>
D.2076_8 = *b_21;
where the addresses loaded are defined by PHIs itself.
The above happens for
std::max(std::min(a0, c), std::min(std::max(a1, c), b))
where this pass transforms it to a form later PHI optimization
recognizes and transforms it to the simple
D.2109_10 = this_1(D)->a1;
D.2110_11 = c;
D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>;
D.2115_14 = b;
D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>;
D.2119_16 = this_1(D)->a0;
D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>;
D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;
The pass does a dominator walk processing loads using a basic-block
local analysis and stores the result for use by transformations on
dominated basic-blocks. */
/* Structure to keep track of the value of a dereferenced PHI result
and the set of virtual operands used for that dereference. */
struct phiprop_d
tree value;
gimple vop_stmt;
/* Verify if the value recorded for NAME in PHIVN is still valid at
the start of basic block BB. */
static bool
phivn_valid_p (struct phiprop_d *phivn, tree name, basic_block bb)
gimple vop_stmt = phivn[SSA_NAME_VERSION (name)].vop_stmt;
ssa_op_iter ui;
tree vuse;
/* The def stmts of all virtual uses need to be post-dominated
by bb. */
gimple use_stmt;
imm_use_iterator ui2;
bool ok = true;
FOR_EACH_IMM_USE_STMT (use_stmt, ui2, vuse)
/* If BB does not dominate a VDEF, the value is invalid. */
|| gimple_code (use_stmt) == GIMPLE_PHI)
&& !dominated_by_p (CDI_DOMINATORS, gimple_bb (use_stmt), bb))
ok = false;
if (!ok)
return false;
return true;
/* Insert a new phi node for the dereference of PHI at basic_block
BB with the virtual operands from USE_STMT. */
static tree
phiprop_insert_phi (basic_block bb, gimple phi, gimple use_stmt,
struct phiprop_d *phivn, size_t n)
tree res;
gimple new_phi;
edge_iterator ei;
edge e;
gcc_assert (is_gimple_assign (use_stmt)
&& gimple_assign_rhs_code (use_stmt) == INDIRECT_REF);
/* Build a new PHI node to replace the definition of
the indirect reference lhs. */
res = gimple_assign_lhs (use_stmt);
SSA_NAME_DEF_STMT (res) = new_phi = create_phi_node (res, bb);
/* Add PHI arguments for each edge inserting loads of the
addressable operands. */
FOR_EACH_EDGE (e, ei, bb->preds)
tree old_arg, new_var;
gimple tmp;
old_arg = PHI_ARG_DEF_FROM_EDGE (phi, e);
while (TREE_CODE (old_arg) == SSA_NAME
&& (SSA_NAME_VERSION (old_arg) >= n
|| phivn[SSA_NAME_VERSION (old_arg)].value == NULL_TREE))
gimple def_stmt = SSA_NAME_DEF_STMT (old_arg);
old_arg = gimple_assign_rhs1 (def_stmt);
if (TREE_CODE (old_arg) == SSA_NAME)
/* Reuse a formerly created dereference. */
new_var = phivn[SSA_NAME_VERSION (old_arg)].value;
gcc_assert (TREE_CODE (old_arg) == ADDR_EXPR);
old_arg = TREE_OPERAND (old_arg, 0);
new_var = create_tmp_var (TREE_TYPE (old_arg), NULL);
tmp = gimple_build_assign (new_var, unshare_expr (old_arg));
DECL_GIMPLE_REG_P (new_var) = 1;
gcc_assert (is_gimple_reg (new_var));
add_referenced_var (new_var);
new_var = make_ssa_name (new_var, tmp);
gimple_assign_set_lhs (tmp, new_var);
gsi_insert_on_edge (e, tmp);
update_stmt (tmp);
mark_symbols_for_renaming (tmp);
add_phi_arg (new_phi, new_var, e);
update_stmt (new_phi);
return res;
/* Propagate between the phi node arguments of PHI in BB and phi result
users. For now this matches
# p_2 = PHI <&x, &y>
p_3 = p_2;
z_2 = *p_3;
and converts it to
# z_2 = PHI <x, y>
Returns true if a transformation was done and edge insertions
need to be committed. Global data PHIVN and N is used to track
past transformation results. We need to be especially careful here
with aliasing issues as we are moving memory reads. */
static bool
propagate_with_phi (basic_block bb, gimple phi, struct phiprop_d *phivn,
size_t n)
tree ptr = PHI_RESULT (phi);
gimple use_stmt;
tree res = NULL_TREE;
gimple_stmt_iterator gsi;
imm_use_iterator ui;
use_operand_p arg_p, use;
ssa_op_iter i;
bool phi_inserted;
if (MTAG_P (SSA_NAME_VAR (ptr))
|| !is_gimple_reg_type (TREE_TYPE (TREE_TYPE (ptr))))
return false;
/* Check if we can "cheaply" dereference all phi arguments. */
FOR_EACH_PHI_ARG (arg_p, phi, i, SSA_OP_USE)
tree arg = USE_FROM_PTR (arg_p);
/* Walk the ssa chain until we reach a ssa name we already
created a value for or we reach a definition of the form
ssa_name_n = &var; */
while (TREE_CODE (arg) == SSA_NAME
&& (SSA_NAME_VERSION (arg) >= n
|| phivn[SSA_NAME_VERSION (arg)].value == NULL_TREE))
gimple def_stmt = SSA_NAME_DEF_STMT (arg);
if (!gimple_assign_single_p (def_stmt))
return false;
arg = gimple_assign_rhs1 (def_stmt);
if ((TREE_CODE (arg) != ADDR_EXPR
/* Avoid to have to decay *&a to a[0] later. */
|| !is_gimple_reg_type (TREE_TYPE (TREE_OPERAND (arg, 0))))
&& !(TREE_CODE (arg) == SSA_NAME
&& phivn[SSA_NAME_VERSION (arg)].value != NULL_TREE
&& phivn_valid_p (phivn, arg, bb)))
return false;
/* Find a dereferencing use. First follow (single use) ssa
copy chains for ptr. */
while (single_imm_use (ptr, &use, &use_stmt)
&& gimple_assign_ssa_name_copy_p (use_stmt))
ptr = gimple_assign_lhs (use_stmt);
/* Replace the first dereference of *ptr if there is one and if we
can move the loads to the place of the ptr phi node. */
phi_inserted = false;
FOR_EACH_IMM_USE_STMT (use_stmt, ui, ptr)
ssa_op_iter ui2;
tree vuse;
/* Check whether this is a load of *ptr. */
if (!(is_gimple_assign (use_stmt)
&& TREE_CODE (gimple_assign_lhs (use_stmt)) == SSA_NAME
&& gimple_assign_rhs_code (use_stmt) == INDIRECT_REF
&& TREE_OPERAND (gimple_assign_rhs1 (use_stmt), 0) == ptr
/* We cannot replace a load that may throw or is volatile. */
&& !stmt_can_throw_internal (use_stmt)))
/* Check if we can move the loads. The def stmts of all virtual uses
need to be post-dominated by bb. */
FOR_EACH_SSA_TREE_OPERAND (vuse, use_stmt, ui2, SSA_OP_VUSE)
gimple def_stmt = SSA_NAME_DEF_STMT (vuse);
&& (gimple_bb (def_stmt) == bb
|| !dominated_by_p (CDI_DOMINATORS,
bb, gimple_bb (def_stmt))))
goto next;
/* Found a proper dereference. Insert a phi node if this
is the first load transformation. */
if (!phi_inserted)
res = phiprop_insert_phi (bb, phi, use_stmt, phivn, n);
/* Remember the value we created for *ptr. */
phivn[SSA_NAME_VERSION (ptr)].value = res;
phivn[SSA_NAME_VERSION (ptr)].vop_stmt = use_stmt;
/* Remove old stmt. The phi is taken care of by DCE, if we
want to delete it here we also have to delete all intermediate
copies. */
gsi = gsi_for_stmt (use_stmt);
gsi_remove (&gsi, false);
phi_inserted = true;
/* Further replacements are easy, just make a copy out of the
load. */
gimple_assign_set_rhs1 (use_stmt, res);
update_stmt (use_stmt);
/* Continue searching for a proper dereference. */
return phi_inserted;
/* Helper walking the dominator tree starting from BB and processing
phi nodes with global data PHIVN and N. */
static bool
tree_ssa_phiprop_1 (basic_block bb, struct phiprop_d *phivn, size_t n)
bool did_something = false;
basic_block son;
gimple_stmt_iterator gsi;
for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
did_something |= propagate_with_phi (bb, gsi_stmt (gsi), phivn, n);
for (son = first_dom_son (CDI_DOMINATORS, bb);
son = next_dom_son (CDI_DOMINATORS, son))
did_something |= tree_ssa_phiprop_1 (son, phivn, n);
return did_something;
/* Main entry for phiprop pass. */
static unsigned int
tree_ssa_phiprop (void)
struct phiprop_d *phivn;
calculate_dominance_info (CDI_DOMINATORS);
phivn = XCNEWVEC (struct phiprop_d, num_ssa_names);
if (tree_ssa_phiprop_1 (ENTRY_BLOCK_PTR, phivn, num_ssa_names))
gsi_commit_edge_inserts ();
free (phivn);
return 0;
static bool
gate_phiprop (void)
return 1;
struct gimple_opt_pass pass_phiprop =
"phiprop", /* name */
gate_phiprop, /* gate */
tree_ssa_phiprop, /* execute */
NULL, /* sub */
NULL, /* next */
0, /* static_pass_number */
TV_TREE_PHIPROP, /* tv_id */
PROP_cfg | PROP_ssa, /* properties_required */
0, /* properties_provided */
0, /* properties_destroyed */
0, /* todo_flags_start */
| TODO_ggc_collect
| TODO_update_ssa
| TODO_verify_ssa /* todo_flags_finish */