[PATCH] Fix PR middle-end/81194, ICE during RTL pass: expand

[Peter Bergner <bergner at vnet dot ibm dot com>]

With the fix to PR51513 and follow on fixes for PR80707, PR80775 and PR80823,
we can now end up with switch statements that contain nothing but a default
case statement.  The expand_case() function contains code that assumes we
have at least one non-default case statement, leading to the ICE reported
in the PR81194.

This patch fixes the bug by expanding switch statements that contain only
a default case statement, as a GOTO to the default case's label.

This passed bootstrap and regtesting on x86_64-linux with no regressions.
Ok for trunk?

Peter

gcc/
	PR middle-end/81194
	* cfgexpand.c (expand_gimple_stmt_1): Handle switch statements
	with only one label.
	* stmt.c (expand_case): Assert NCASES is greater than one.

gcc/testsuite/
	PR middle-end/81194
	* g++.dg/pr81194.C: New test.

Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c	(revision 249747)
+++ gcc/cfgexpand.c	(working copy)
@@ -3566,7 +3566,13 @@
     case GIMPLE_PREDICT:
       break;
     case GIMPLE_SWITCH:
-      expand_case (as_a <gswitch *> (stmt));
+      {
+	gswitch *swtch = as_a <gswitch *> (stmt);
+	if (gimple_switch_num_labels (swtch) == 1)
+	  expand_goto (CASE_LABEL (gimple_switch_default_label (swtch)));
+	else
+	  expand_case (swtch);
+      }
       break;
     case GIMPLE_ASM:
       expand_asm_stmt (as_a <gasm *> (stmt));
Index: gcc/stmt.c
===================================================================
--- gcc/stmt.c	(revision 249747)
+++ gcc/stmt.c	(working copy)
@@ -1142,8 +1142,11 @@
   /* cleanup_tree_cfg removes all SWITCH_EXPR with their index
      expressions being INTEGER_CST.  */
   gcc_assert (TREE_CODE (index_expr) != INTEGER_CST);
-  
 
+  /* Optimization of switch statements with only one label has already
+     occurred, so we should never see them at this point.  */
+  gcc_assert (ncases > 1);
+
   do_pending_stack_adjust ();
 
   /* Find the default case target label.  */
Index: gcc/testsuite/g++.dg/pr81194.C
===================================================================
--- gcc/testsuite/g++.dg/pr81194.C	(nonexistent)
+++ gcc/testsuite/g++.dg/pr81194.C	(working copy)
@@ -0,0 +1,60 @@
+// { dg-do compile }
+// { dg-options "-O2 -std=c++17 -fno-exceptions" }
+
+template <class a> struct b { typedef a *c; };
+class e {};
+template <typename a> class d {
+public:
+  typedef typename b<a>::c c;
+  c begin();
+  c end();
+};
+struct f {
+  enum { g } h;
+};
+struct i {
+  d<f *> j();
+};
+struct l {
+  d<i *> k();
+};
+class ac;
+class o {
+public:
+  o(int *, int *, int *, ac *);
+};
+class ac {
+public:
+  ac(e);
+  virtual o *ae(int *, int *, int *, int *);
+};
+class p {
+  void af(f *m) {
+    switch (m->h)
+    case f::g:
+      ag();
+  }
+
+public:
+  void n() {
+    l ah;
+    for (i *ai : ah.k())
+      for (f *m : ai->j())
+        af(m);
+  }
+  virtual void ag() { __builtin_unreachable(); }
+};
+template <typename = int> class an : o {
+public:
+  an(int *, int *, int *, int *, ac *);
+};
+class q : ac {
+public:
+  q() : ac([]() -> e {}()) {}
+  o *ae(int *ap, int *aq, int *ar, int *as) { an(ap, aq, ar, as, this); }
+};
+template <typename at>
+an<at>::an(int *, int *aq, int *ar, int *as, ac *au) : o(aq, ar, as, au) {
+  p().n();
+}
+void av() { new q; }