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.

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c
new file mode 100644 (file)
index 0000000..aaec41d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-44.c
@@ -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" } } */

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c
new file mode 100644 (file)
index 0000000..fd92d7b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-45.c
@@ -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;
+}

diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
index 881a2d0f98dba81ab4c13af357ca7ec6a3916e40..ea50de789b1ebe0917e1b3f4d44d7550c0c5dbcd 100644 (file)
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -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;
        }