tree-optimization/96565 - improve DSE with paths ending in noreturn

This improves DSEs stmt walking by not considering a DEF without
uses for further processing (and thus giving up when there's two
paths to follow).

2020-08-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96565
	* tree-ssa-dse.c (dse_classify_store): Remove defs with
	no uses from further processing.

	* gcc.dg/tree-ssa/ssa-dse-40.c: New testcase.
	* gcc.dg/builtin-object-size-4.c: Adjust.

gcc/testsuite/gcc.dg/builtin-object-size-4.c

@@ -170,6 +170,9 @@ test1 (void *q, int x)
   r = (char *) L"abcd\0efg";
   if (__builtin_object_size (r + 2, 3) != sizeof (L"abcd\0efg") - 2)
     abort ();
+  /* Prevent DSE from removing calls that prevent bad combining of
+     addresses and offsets.  */
+  asm volatile ("" : : "g" (&a));
 }
 
 size_t l1 = 1;

gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-40.c

@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-dse1-details" } */
+
+_Bool g(void);
+
+void f(int x)
+{
+  char arr[x];
+
+  arr[0] = 0;
+
+  if (g())
+    __builtin_abort();
+}
+
+/* { dg-final { scan-tree-dump "Deleted dead store" "dse1" } } */

gcc/tree-ssa-dse.c

@@ -898,6 +898,17 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
 		*by_clobber_p = false;
 	      defs.unordered_remove (i);
 	    }
+	  /* If the path ends here we do not need to process it further.
+	     This for example happens with calls to noreturn functions.  */
+	  else if (gimple_code (def) != GIMPLE_PHI
+		   && has_zero_uses (gimple_vdef (def)))
+	    {
+	      /* But if the store is to global memory it is definitely
+		 not dead.  */
+	      if (ref_may_alias_global_p (ref))
+		return DSE_STORE_LIVE;
+	      defs.unordered_remove (i);
+	    }
 	  /* 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 a single for simplicity reasons (we fail for multiple