expand: Fix up edge splitting for ENTRY block during expansion if there are any PHIs...

Andrew ran some extra ranger checking during bootstrap and found one more
case (though much rarer than the GIMPLE_COND case).

Seems on fold-const.cc (native_encode_expr) we end up with bb 2, ENTRY
bb successor, having PHI nodes (usually there is some bb in between, even if
empty, in the native_encode_expr it is tail recursion but haven't managed
to construct a test with such case by hand).
So, we have in optimized dump
  <bb 2> [local count: 1089340384]:
  # expr_12 = PHI <expr_199(D)(0), part_93(51)>
  # ptr_13 = PHI <ptr_86(D)(0), ptr_13(51)>
  # len_14 = PHI <len_103(D)(0), _198(51)>
  # off_10 = PHI <off_102(D)(0), _207(51)>
  # add_acc_99 = PHI <0(0), add_acc_101(51)>
where there are mostly default defs from the 0->2 edge (and one zero)
and some other values from the other edge.
construct_init_block inserts a BB_RTL basic block with the function start
instructions and similarly to the GIMPLE_COND case it wants to insert that
bb on the edge from ENTRY to its single successor.
Now, without this patch redirect_edge_succ redirects the 0->2 edge to 0->52,
so the 51->2 edge gets moved first by unordered_remove, and
make_single_succ_edge adds a new 52->2 edge.  So we end up with
  # expr_12 = PHI <expr_199(D)(51), part_93(52)>
  # ptr_13 = PHI <ptr_86(D)(51), ptr_13(52)>
  # len_14 = PHI <len_103(D)(51), _198(52)>
  # off_10 = PHI <off_102(D)(51), _207(52)>
  # add_acc_99 = PHI <0(51), add_acc_101(52)>
which is not correct, the default definitions and zero are now from the edge
from end of function and the other values from the edge from the new BB_RTL
successor of ENTRY.  With this patch we get
  # expr_12 = PHI <expr_199(D)(52), part_93(51)>
  # ptr_13 = PHI <ptr_86(D)(52), ptr_13(51)>
  # len_14 = PHI <len_103(D)(52), _198(51)>
  # off_10 = PHI <off_102(D)(52), _207(51)>
  # add_acc_99 = PHI <0(52), add_acc_101(51)>
instead.

2025-06-13  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/120629
	* cfgexpand.cc (construct_init_block): If first_block isn't BB_RTL,
	has any PHI nodes and false_edge->dest_idx before redirection is
	different from make_single_succ_edge result's dest_idx, swap the
	latter with the former last pred edge and their dest_idx members.

diff --git a/gcc/cfgexpand.cc b/gcc/cfgexpand.cc
index f288a54073462c38b0cb8044206ecb8aecf75272..2e6241d1e39abb9f517ee0a729888041e641ce54 100644 (file)
--- a/gcc/cfgexpand.cc
+++ b/gcc/cfgexpand.cc
@@ -6570,9 +6570,22 @@ construct_init_block (void)
   add_bb_to_loop (init_block, ENTRY_BLOCK_PTR_FOR_FN (cfun)->loop_father);
   if (e)
     {
+      unsigned int dest_idx = e->dest_idx;
       first_block = e->dest;
       redirect_edge_succ (e, init_block);
-      make_single_succ_edge (init_block, first_block, flags);
+      e = make_single_succ_edge (init_block, first_block, flags);
+      if ((first_block->flags & BB_RTL) == 0
+         && phi_nodes (first_block)
+         && e->dest_idx != dest_idx)
+       {
+         /* If there are any PHI nodes on dest, swap the new succ edge
+            with the one moved into false_edge's former position, so that
+            PHI arguments don't need adjustment.  */
+         edge e2 = EDGE_PRED (first_block, dest_idx);
+         std::swap (e->dest_idx, e2->dest_idx);
+         std::swap (EDGE_PRED (first_block, e->dest_idx),
+                    EDGE_PRED (first_block, e2->dest_idx));
+       }
     }
   else
     make_single_succ_edge (init_block, EXIT_BLOCK_PTR_FOR_FN (cfun),