[gcc r17-3284] tree-optimization/126861 - ICE with SLP schedule at explicit region start

https://gcc.gnu.org/g:d42b8b15aa9f246293c976f141df1c1936d93dc6

commit r17-3284-gd42b8b15aa9f246293c976f141df1c1936d93dc6
Author: Richard Biener <[email protected]>
Date:   Fri Aug 14 10:26:00 2026 +0200

    tree-optimization/126861 - ICE with SLP schedule at explicit region start
    
    We cannot use a random stmt at region start as marker as that might
    eventually be a control stmt.  Instead use NULL to mark region
    start as documented.
    
            PR tree-optimization/126861
            * tree-vect-slp.cc (vect_schedule_slp_node): Use NULL for
            scheduling at region start.
    
            * g++.dg/vect/slp-pr126861.cc: New testcase.

Diff:
---
 gcc/testsuite/g++.dg/vect/slp-pr126861.cc | 11 +++++++++++
 gcc/tree-vect-slp.cc                      |  3 +--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/g++.dg/vect/slp-pr126861.cc b/gcc/testsuite/g++.dg/vect/slp-pr126861.cc
new file mode 100644
index 000000000000..d9e5f4ea0756
--- /dev/null
+++ b/gcc/testsuite/g++.dg/vect/slp-pr126861.cc
@@ -0,0 +1,11 @@
+// { dg-do compile }
+
+struct Guard { ~Guard(); };
+void init();
+
+int f (unsigned n1, unsigned n2)
+{
+  Guard g;
+  init();
+  return (n1 != 0) + (n2 != 0);
+}
diff --git a/gcc/tree-vect-slp.cc b/gcc/tree-vect-slp.cc
index bda7ae679e55..985aa4281b31 100644
--- a/gcc/tree-vect-slp.cc
+++ b/gcc/tree-vect-slp.cc
@@ -12279,8 +12279,7 @@ vect_schedule_slp_node (vec_info *vinfo,
       else if (!last_stmt)
 	{
 	  si = gsi_after_labels (vinfo->bbs[0]);
-	  /* ???  last_stmt can be NULL if the block is empty.  */
-	  last_stmt = gsi_stmt (si);
+	  /* last_stmt NULL marks the region start.  */
 	}
       else if (is_a <gphi *> (last_stmt))
 	si = gsi_after_labels (gimple_bb (last_stmt));