From: Richard Biener <rguenther@suse.de>
Date: Fri, 28 Oct 2022 13:03:49 +0000 (+0200)
Subject: tree-optimization/107407 - wrong code with DSE
X-Git-Tag: release-12.2.mpacbti-rel1~221
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e0e37f97e67c99604129eeac2e73ccd3719ccb28;p=thirdparty%2Fgcc.git

tree-optimization/107407 - wrong code with DSE

So what happens is that we elide processing of this check with

          /* In addition to kills we can remove defs whose only use
             is another def in defs.  That can only ever be PHIs of which
             we track two for simplicity reasons, the first and last in
             {first,last}_phi_def (we fail for multiple PHIs anyways).
             We can also ignore defs that feed only into
             already visited PHIs.  */
          else if (single_imm_use (vdef, &use_p, &use_stmt)
                   && (use_stmt == first_phi_def
                       || use_stmt == last_phi_def
                       || (gimple_code (use_stmt) == GIMPLE_PHI
                           && bitmap_bit_p (visited,
                                            SSA_NAME_VERSION
                                              (PHI_RESULT (use_stmt))))))

where we have the last PHI being the outer loop virtual PHI and the first
PHI being the loop exit PHI of the outer loop and we've already processed
the single immediate use of the outer loop PHI, the inner loop PHI.  But
we still have to perform the above check!

It's easiest to perform the check when we visit the PHI node instead of
delaying it to the actual processing loop.

	PR tree-optimization/107407
	* tree-ssa-dse.cc (dse_classify_store): Perform backedge
	varying index check when collecting PHI uses rather than
	after optimizing processing of the candidate defs.

	* gcc.dg/torture/pr107407.c: New testcase.

(cherry picked from commit 031a400e49d8db156c43f9ec0b21ab0c2aee8c6d)
---

diff --git a/gcc/testsuite/gcc.dg/torture/pr107407.c b/gcc/testsuite/gcc.dg/torture/pr107407.c
new file mode 100644
index 000000000000..228fce1e6992
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr107407.c
@@ -0,0 +1,26 @@
+/* { dg-do run } */
+
+int *a;
+int c[4];
+int d;
+
+static int
+f(char k, int j)
+{
+  for (; k <= 3; k++)
+    {
+      a = &c[k];
+      for (; d <= 1; d++)
+        *a = 3;
+    }
+  *a = 0;
+}
+
+int main()
+{
+  int i;
+  f(0, 0);
+  if (c[0] != 3)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
index 881a2d0f98db..e647f0e7368c 100644
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -930,14 +930,6 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 
       if (gimple_code (temp) == GIMPLE_PHI)
 	{
-	  /* If we visit this PHI by following a backedge then we have to
-	     make sure ref->ref only refers to SSA names that are invariant
-	     with respect to the loop represented by this PHI node.  */
-	  if (dominated_by_p (CDI_DOMINATORS, gimple_bb (stmt),
-			      gimple_bb (temp))
-	      && !for_each_index (ref->ref ? &ref->ref : &ref->base,
-				  check_name, gimple_bb (temp)))
-	    return DSE_STORE_LIVE;
 	  defvar = PHI_RESULT (temp);
 	  bitmap_set_bit (visited, SSA_NAME_VERSION (defvar));
 	}
@@ -971,6 +963,15 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 	      if (!bitmap_bit_p (visited,
 				 SSA_NAME_VERSION (PHI_RESULT (use_stmt))))
 		{
+		  /* If we visit this PHI by following a backedge then we have
+		     to make sure ref->ref only refers to SSA names that are
+		     invariant with respect to the loop represented by this
+		     PHI node.  */
+		  if (dominated_by_p (CDI_DOMINATORS, gimple_bb (stmt),
+				      gimple_bb (use_stmt))
+		      && !for_each_index (ref->ref ? &ref->ref : &ref->base,
+					  check_name, gimple_bb (use_stmt)))
+		    return DSE_STORE_LIVE;
 		  defs.safe_push (use_stmt);
 		  if (!first_phi_def)
 		    first_phi_def = use_stmt;