[PATCH] tree-optimization/101278 - handle self-use in DSE analysis

[Richard Biener <rguenther@suse.de>]

DSE store classification short-cuts the to-be classified stmt itself
from chaining but fails to first check whether the store uses itself
which can be the case when it is a call with the LHS also passed by
value as argument.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

2021-07-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101278
	* tree-ssa-dse.c (dse_classify_store): First check for
	uses, then ignore stmt for chaining purposes.

	* gcc.dg/torture/pr101278.c: New testcase.
---
 gcc/testsuite/gcc.dg/torture/pr101278.c | 21 +++++++++++++++++++++
 gcc/tree-ssa-dse.c                      | 10 +++++-----
 2 files changed, 26 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/torture/pr101278.c

diff --git a/gcc/testsuite/gcc.dg/torture/pr101278.c b/gcc/testsuite/gcc.dg/torture/pr101278.c
new file mode 100644
index 00000000000..1d25658f37d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr101278.c
@@ -0,0 +1,21 @@
+/* { dg-do run } */
+
+struct X { int counter; };
+
+struct X __attribute__((noipa)) foo (struct X x)
+{
+  x.counter++;
+  if (x.counter == 5)
+    __builtin_exit (0);
+  return x;
+}
+
+int
+main ()
+{
+  struct X x;
+  x.counter = 0;
+  for (int i = 0; i < 10; ++i)
+    x = foo (x);
+  __builtin_abort ();
+}
diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
index c3939a6417f..98daa8ab24c 100644
--- a/gcc/tree-ssa-dse.c
+++ b/gcc/tree-ssa-dse.c
@@ -813,15 +813,11 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 	      break;
 	    }
 
-	  /* We have visited ourselves already so ignore STMT for the
-	     purpose of chaining.  */
-	  if (use_stmt == stmt)
-	    ;
 	  /* In simple cases we can look through PHI nodes, but we
 	     have to be careful with loops and with memory references
 	     containing operands that are also operands of PHI nodes.
 	     See gcc.c-torture/execute/20051110-*.c.  */
-	  else if (gimple_code (use_stmt) == GIMPLE_PHI)
+	  if (gimple_code (use_stmt) == GIMPLE_PHI)
 	    {
 	      /* If we already visited this PHI ignore it for further
 		 processing.  */
@@ -861,6 +857,10 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 	      fail = true;
 	      break;
 	    }
+	  /* We have visited ourselves already so ignore STMT for the
+	     purpose of chaining.  */
+	  else if (use_stmt == stmt)
+	    ;
 	  /* If this is a store, remember it as we possibly need to walk the
 	     defs uses.  */
 	  else if (gimple_vdef (use_stmt))
-- 
2.26.2