[gcc r13-3546] tree-optimization/107407 - wrong code with DSE

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:031a400e49d8db156c43f9ec0b21ab0c2aee8c6d

commit r13-3546-g031a400e49d8db156c43f9ec0b21ab0c2aee8c6d
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Oct 28 15:03:49 2022 +0200

    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.

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr107407.c | 26 ++++++++++++++++++++++++++
 gcc/tree-ssa-dse.cc                     | 17 +++++++++--------
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/torture/pr107407.c b/gcc/testsuite/gcc.dg/torture/pr107407.c
new file mode 100644
index 00000000000..228fce1e699
--- /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 c14e5e43eb3..82976bdbeb0 100644
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -978,14 +978,6 @@ 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));
 	}
@@ -1019,6 +1011,15 @@ 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 = as_a <gphi *> (use_stmt);