Improve DSE to handle stores before __builtin_unreachable ()

DSE isn't good at identifying program points that end lifetime
of variables that are not associated with virtual operands.  But
at least for those that end basic-blocks we can handle the simple
case where this ending is in the same basic-block as the definition
we want to elide.  That should catch quite some common cases already.

	* tree-ssa-dse.cc (dse_classify_store): When we found
	no defs and the basic-block with the original definition
	ends in __builtin_unreachable[_trap] the store is dead.

	* gcc.dg/tree-ssa/ssa-dse-47.c: New testcase.
	* c-c++-common/asan/pr106558.c: Avoid undefined behavior
	due to missing return.

3 files changed, 38 insertions, 2 deletions

diff --git a/gcc/testsuite/c-c++-common/asan/pr106558.c b/gcc/testsuite/c-c++-common/asan/pr106558.c
index d82b2dc..c8cefdf 100644
--- a/gcc/testsuite/c-c++-common/asan/pr106558.c
+++ b/gcc/testsuite/c-c++-common/asan/pr106558.c
@@ -8,7 +8,7 @@ int **c = &b;
 int d[1];
 int *e = &d[1];
 
-static int f(int *g) {
+static void f(int *g) {
   *b = e;
   *c = e;
   *b = 2;
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c
new file mode 100644
index 0000000..659f1d0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-47.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-dse1-details" } */
+
+int a;
+int b[3];
+void test()
+{
+  if (a > 0)
+    {
+      b[0] = 0;
+      b[1] = 1;
+      b[2] = 2;
+      __builtin_unreachable ();
+    }
+}
+
+/* { dg-final { scan-tree-dump-times "Deleted dead store" 3 "dse1" } } */
diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
index eabe8ba..3c7a2e9 100644
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -1118,7 +1118,26 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
       if (defs.is_empty ())
 	{
 	  if (ref_may_alias_global_p (ref, false))
-	    return DSE_STORE_LIVE;
+	    {
+	      basic_block def_bb = gimple_bb (SSA_NAME_DEF_STMT (defvar));
+	      /* Assume that BUILT_IN_UNREACHABLE and BUILT_IN_UNREACHABLE_TRAP
+		 do not need to keep (global) memory side-effects live.
+		 We do not have virtual operands on BUILT_IN_UNREACHABLE
+		 but we can do poor mans reachability when the last
+		 definition we want to elide is in the block that ends
+		 in such a call.  */
+	      if (EDGE_COUNT (def_bb->succs) == 0)
+		if (gcall *last = dyn_cast <gcall *> (*gsi_last_bb (def_bb)))
+		  if (gimple_call_builtin_p (last, BUILT_IN_UNREACHABLE)
+		      || gimple_call_builtin_p (last,
+						BUILT_IN_UNREACHABLE_TRAP))
+		    {
+		      if (by_clobber_p)
+			*by_clobber_p = false;
+		      return DSE_STORE_DEAD;
+		    }
+	      return DSE_STORE_LIVE;
+	    }
 
 	  if (by_clobber_p)
 	    *by_clobber_p = false;