Fix miscompilation of Python on HP-PA/Linux

This is the miscompilation of Python at -O2 on HP-PA/Linux present
on the mainline and 10 branch, caused by the presence of a call to
__builtin_unreachable () in the middle of a heavily branchy code,
which confuses the reorg pass.

gcc/
	PR rtl-optimization/96015
	* reorg.c (skip_consecutive_labels): Minor comment tweaks.
	(relax_delay_slots): When deleting a jump to the next active
	instruction over a barrier, first delete the barrier if the
	jump is the only way to reach the target label.

diff --git a/gcc/reorg.c b/gcc/reorg.c
index fc184202778ab551c626bac05e6046c9f9dba685..69c9a768fa9f7f90192912146d7dc24697068e44 100644 (file)
--- a/gcc/reorg.c
+++ b/gcc/reorg.c
@@ -139,9 +139,9 @@ skip_consecutive_labels (rtx label_or_return)
   /* __builtin_unreachable can create a CODE_LABEL followed by a BARRIER.
 
      Since reaching the CODE_LABEL is undefined behavior, we can return
-     any code label and we're OK at runtime.
+     any code label and we're OK at run time.
 
-     However, if we return a CODE_LABEL which leads to a shrinked wrapped
+     However, if we return a CODE_LABEL which leads to a shrink-wrapped
      epilogue, but the path does not have a prologue, then we will trip
      a sanity check in the dwarf2 cfi code which wants to verify that
      the CFIs are all the same on the traces leading to the epilogue.
@@ -3210,6 +3210,23 @@ relax_delay_slots (rtx_insn *first)
              && ! condjump_in_parallel_p (jump_insn)
              && ! (next && switch_text_sections_between_p (jump_insn, next)))
            {
+             rtx_insn *direct_label = as_a<rtx_insn *> (JUMP_LABEL (insn));
+             rtx_insn *prev = prev_nonnote_insn (direct_label);
+
+             /* If the insn jumps over a BARRIER and is the only way to reach
+                its target, then we need to delete the BARRIER before the jump
+                because, otherwise, the target may end up being considered as
+                unreachable and thus also deleted.  */
+             if (BARRIER_P (prev) && LABEL_NUSES (direct_label) == 1)
+               {
+                 delete_related_insns (prev);
+
+                 /* We have just removed a BARRIER, which means that the block
+                    number of the next insns has effectively been changed (see
+                    find_basic_block in resource.c), so clear it.  */
+                 clear_hashed_info_until_next_barrier (direct_label);
+               }
+
              delete_jump (jump_insn);
              continue;
            }