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tree-optimization/100221 - improve DSE a bit 

When facing multiple PHI defs and one feeding the other we can
postpone processing uses of one and thus can proceed.

2022-05-20  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100221
	* tree-ssa-dse.cc (contains_phi_arg): New function.
	(dse_classify_store): Postpone PHI defs that feed another PHI in defs.

	* gcc.dg/tree-ssa/ssa-dse-44.c: New testcase.
	* gcc.dg/tree-ssa/ssa-dse-45.c: Likewise.
This commit is contained in:
Richard Biener 2022-05-20 12:24:40 +02:00
parent d918faea12
commit 1adf11822b
3 changed files with 84 additions and 5 deletions
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19
gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c Normal file
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@ -0,0 +1,19 @@
/* { dg-do link } */
/* { dg-options "-O -fdump-tree-dse1-details" } */
extern void foo(void);
int a, b;
static int c;
int main()
{
if (c)
foo ();
int *g = &c;
int **h = &g;
int ***h1 = &h;
if (a)
while (b)
b = 0;
}
/* { dg-final { scan-tree-dump "Deleted dead store: g = &c;" "dse1" } } */

24
gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c Normal file
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@ -0,0 +1,24 @@
/* { dg-do link } */
/* { dg-options "-O" } */
extern void foo(void);
int a, b;
static int c;
static void f() {
while (a)
for (; b; b--)
;
}
void i() {
if (c)
foo();
int *g = &c;
{
int **h[1] = {&g};
f();
}
}
int main() {
i();
return 0;
}

46
gcc/tree-ssa-dse.cc
View File

@ -898,6 +898,17 @@ dse_optimize_redundant_stores (gimple *stmt)
}
}
/* Return whether PHI contains ARG as an argument. */
static bool
contains_phi_arg (gphi *phi, tree arg)
{
for (unsigned i = 0; i < gimple_phi_num_args (phi); ++i)
if (gimple_phi_arg_def (phi, i) == arg)
return true;
return false;
}
/* A helper of dse_optimize_stmt.
Given a GIMPLE_ASSIGN in STMT that writes to REF, classify it
according to downstream uses and defs. Sets *BY_CLOBBER_P to true
@ -949,8 +960,8 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
return DSE_STORE_LIVE;
auto_vec<gimple *, 10> defs;
gimple *first_phi_def = NULL;
gimple *last_phi_def = NULL;
gphi *first_phi_def = NULL;
gphi *last_phi_def = NULL;
FOR_EACH_IMM_USE_STMT (use_stmt, ui, defvar)
{
/* Limit stmt walking. */
@ -973,8 +984,8 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
{
defs.safe_push (use_stmt);
if (!first_phi_def)
first_phi_def = use_stmt;
last_phi_def = use_stmt;
first_phi_def = as_a <gphi *> (use_stmt);
last_phi_def = as_a <gphi *> (use_stmt);
}
}
/* If the statement is a use the store is not dead. */
@ -1046,6 +1057,7 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
use_operand_p use_p;
tree vdef = (gimple_code (def) == GIMPLE_PHI
? gimple_phi_result (def) : gimple_vdef (def));
gphi *phi_def;
/* If the path to check starts with a kill we do not need to
process it further.
??? With byte tracking we need only kill the bytes currently
@ -1079,7 +1091,31 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
&& bitmap_bit_p (visited,
SSA_NAME_VERSION
(PHI_RESULT (use_stmt))))))
defs.unordered_remove (i);
{
defs.unordered_remove (i);
if (def == first_phi_def)
first_phi_def = NULL;
else if (def == last_phi_def)
last_phi_def = NULL;
}
/* If def is a PHI and one of its arguments is another PHI node still
in consideration we can defer processing it. */
else if ((phi_def = dyn_cast <gphi *> (def))
&& ((last_phi_def
&& phi_def != last_phi_def
&& contains_phi_arg (phi_def,
gimple_phi_result (last_phi_def)))
|| (first_phi_def
&& phi_def != first_phi_def
&& contains_phi_arg
(phi_def, gimple_phi_result (first_phi_def)))))
{
defs.unordered_remove (i);
if (phi_def == first_phi_def)
first_phi_def = NULL;
else if (phi_def == last_phi_def)
last_phi_def = NULL;
}
else
++i;
}