[gcc r16-5482] gimplify: Fix ICE in collect_fallthrough_labels [PR122773]

[Jakub Jelinek]

https://gcc.gnu.org/g:3012aad2dc6318ab490c4a2511f5b2e4d30652b9

commit r16-5482-g3012aad2dc6318ab490c4a2511f5b2e4d30652b9
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Nov 21 11:25:27 2025 +0100

    gimplify: Fix ICE in collect_fallthrough_labels [PR122773]
    
    In r16-4212 I had to tweak two spots in the gimplifier to ignore
    gotos jumping to labels with the new VACUOUS_INIT_LABEL_P flag
    (set by C++ FE when implementing goto/case interceptors with
    extra .DEFERRED_INIT calls, so that jumps over vacuous initialization
    are handled properly with the C++26 erroneous behavior requirements).
    Except as the following testcase shows, the checks blindly assumed
    that gimple_goto_dest operand is a LABEL_DECL, which is not the case
    for computed jumps.
    
    The following patch checks that gimple_goto_dest argument is a LABEL_DECL
    before testing VACUOUS_INIT_LABEL_P flag on it.
    
    2025-11-21  Jakub Jelinek  <jakub@redhat.com>
    
            PR middle-end/122773
            * gimplify.cc (collect_fallthrough_labels): Check whether
            gimple_goto_dest is a LABEL_DECL before testing VACUOUS_INIT_LABEL_P.
            (expand_FALLTHROUGH_r): Likewise.
    
            * gcc.dg/pr122773.c: New test.

Diff:
---
 gcc/gimplify.cc                 | 10 ++++++++--
 gcc/testsuite/gcc.dg/pr122773.c | 25 +++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index d8725e4c5e20..d884e8b0d957 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -2673,6 +2673,7 @@ collect_fallthrough_labels (gimple_stmt_iterator *gsi_p,
 	  gsi_next (gsi_p);
 	}
 
+      tree lab;
       /* Remember the last statement.  Skip labels that are of no interest
 	 to us.  */
       if (gimple_code (gsi_stmt (*gsi_p)) == GIMPLE_LABEL)
@@ -2691,7 +2692,9 @@ collect_fallthrough_labels (gimple_stmt_iterator *gsi_p,
 	;
       else if (flag_auto_var_init > AUTO_INIT_UNINITIALIZED
 	       && gimple_code (gsi_stmt (*gsi_p)) == GIMPLE_GOTO
-	       && VACUOUS_INIT_LABEL_P (gimple_goto_dest (gsi_stmt (*gsi_p))))
+	       && (lab = gimple_goto_dest (gsi_stmt (*gsi_p)))
+	       && TREE_CODE (lab) == LABEL_DECL
+	       && VACUOUS_INIT_LABEL_P (lab))
 	;
       else if (!is_gimple_debug (gsi_stmt (*gsi_p)))
 	prev = gsi_stmt (*gsi_p);
@@ -2928,9 +2931,12 @@ expand_FALLTHROUGH_r (gimple_stmt_iterator *gsi_p, bool *handled_ops_p,
 
 	  gimple_stmt_iterator gsi2 = *gsi_p;
 	  stmt = gsi_stmt (gsi2);
+	  tree lab;
 	  if (flag_auto_var_init > AUTO_INIT_UNINITIALIZED
 	      && gimple_code (stmt) == GIMPLE_GOTO
-	      && VACUOUS_INIT_LABEL_P (gimple_goto_dest (stmt)))
+	      && (lab = gimple_goto_dest (stmt))
+	      && TREE_CODE (lab) == LABEL_DECL
+	      && VACUOUS_INIT_LABEL_P (lab))
 	    {
 	      /* Handle for C++ artificial -ftrivial-auto-var-init=
 		 sequences.  Those look like:
diff --git a/gcc/testsuite/gcc.dg/pr122773.c b/gcc/testsuite/gcc.dg/pr122773.c
new file mode 100644
index 000000000000..a3860e9c7a23
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr122773.c
@@ -0,0 +1,25 @@
+/* PR middle-end/122773 */
+/* { dg-do compile } */
+/* { dg-options "-Wimplicit-fallthrough -O2 -ftrivial-auto-var-init=zero" } */
+
+void *l;
+int
+foo (int x)
+{
+  __label__ l1, l2, l3;
+  static void *l[] = { &&l1, &&l2, &&l3 };
+  switch (0)
+    {
+    case 0:
+      while (0)
+        ;
+      goto *l[x];
+    }
+  l1:
+  ++x;
+  l2:
+  ++x;
+  l3:
+  ++x;
+  return x;
+}