expand: drop stack adjustments after barrier [PR118006]

A gimple block with __builtin_unreachable () can't have code after it,
and gimple optimizers ensure there isn't any, even without
optimization.  But if the block requires stack adjustments,
e.g. because of a call that passes arguments on the stack, expand will
emit that after the barrier, and then rtl checkers rightfully
complain.  Arrange to discard adjustments after a barrier.

Strub expanders seem to be necessary to bring about the exact
conditions that require stack adjustments after the block that ends
with a __builtin_unreachable call.


for  gcc/ChangeLog

	PR middle-end/118006
	* cfgexpand.cc (expand_gimple_basic_block): Do not emit
	pending stack adjustments after a barrier.

for  gcc/testsuite/ChangeLog

	PR middle-end/118006
	* gcc.target/i386/strub-pr118006.c: New.

1 files changed, 11 insertions, 1 deletions

diff --git a/gcc/cfgexpand.cc b/gcc/cfgexpand.cc
index 8b6ed0d..abab385 100644
--- a/gcc/cfgexpand.cc
+++ b/gcc/cfgexpand.cc
@@ -6216,7 +6216,17 @@ expand_gimple_basic_block (basic_block bb, bool disable_tail_calls)
       emit_insn_after_noloc (gen_move_insn (dummy, dummy), last, NULL);
     }
 
-  do_pending_stack_adjust ();
+  /* A __builtin_unreachable () will insert a barrier that should end
+     the basic block.  In gimple, any code after it will have already
+     deleted, even without optimization.  If we emit additional code
+     here, as we would to adjust the stack after a call, it should be
+     eventually deleted, but it confuses internal checkers (PR118006)
+     and optimizers before it does, because we don't expect to find
+     barriers inside basic blocks.  */
+  if (!BARRIER_P (get_last_insn ()))
+    do_pending_stack_adjust ();
+  else
+    discard_pending_stack_adjust ();
 
   /* Find the block tail.  The last insn in the block is the insn
      before a barrier and/or table jump insn.  */