tree-optimization/96349 - avoid abnormal coalescing issues in loop split

This avoids splitting a loop when the entry value of a loop PHI is
involved with abnormal coalescing.

2020-07-28  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96349
	* tree-ssa-loop-split.c (stmt_semi_invariant_p_1): When the
	condition runs into a loop PHI with an abnormal entry value give up.

	* gcc.dg/torture/pr96349.c: New testcase.

gcc/testsuite/gcc.dg/torture/pr96349.c

@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+
+void __attribute__ ((returns_twice))
+gr (void);
+
+void
+ib (void);
+
+void
+zg (void);
+
+void
+yw (int uz)
+{
+  gr ();
+
+  for (;;)
+    if (uz != 0)
+      {
+        uz = 0;
+        ib ();
+      }
+    else
+      zg ();
+}

gcc/tree-ssa-loop-split.c

@@ -1145,6 +1145,16 @@ stmt_semi_invariant_p_1 (struct loop *loop, gimple *stmt,
 
       if (gimple_bb (stmt) == loop->header)
 	{
+	  /* If the entry value is subject to abnormal coalescing
+	     avoid the transform since we're going to duplicate the
+	     loop header and thus likely introduce overlapping life-ranges
+	     between the PHI def and the entry on the path when the
+	     first loop is skipped.  */
+	  tree entry_def
+	    = PHI_ARG_DEF_FROM_EDGE (phi, loop_preheader_edge (loop));
+	  if (TREE_CODE (entry_def) == SSA_NAME
+	      && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (entry_def))
+	    return false;
 	  invar = loop_iter_phi_semi_invariant_p (loop, phi, skip_head);
 	  return invar;
 	}