[PATCH] bb-reorder: Remove a misfiring micro-optimization (PR96475)

Segher Boessenkool segher@kernel.crashing.org
Fri Aug 7 21:51:04 GMT 2020

When the compgotos pass copies the tail of blocks ending in an indirect
jump, there is a micro-optimization to not copy the last one, since the
original block will then just be deleted.  This does not work properly
if cleanup_cfg does not merge all pairs of blocks we expect it to.

v2: This also deletes the other use of single_pred_p, which has the same
problem in principle, I just never have triggered it so far.

Tested on powerpc64-linux {-m32,-m64} like before.  Is this okay for


2020-08-07  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/96475
	* bb-reorder.c (maybe_duplicate_computed_goto): Remove single_pred_p
 gcc/bb-reorder.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/gcc/bb-reorder.c b/gcc/bb-reorder.c
index c635010..76e56b5 100644
--- a/gcc/bb-reorder.c
+++ b/gcc/bb-reorder.c
@@ -2680,9 +2680,6 @@ make_pass_reorder_blocks (gcc::context *ctxt)
 static bool
 maybe_duplicate_computed_goto (basic_block bb, int max_size)
-  if (single_pred_p (bb))
-    return false;
   /* Make sure that the block is small enough.  */
   rtx_insn *insn;
   FOR_BB_INSNS (bb, insn)
@@ -2700,10 +2697,9 @@ maybe_duplicate_computed_goto (basic_block bb, int max_size)
       basic_block pred = e->src;
-      /* Do not duplicate BB into PRED if that is the last predecessor, or if
-	 we cannot merge a copy of BB with PRED.  */
-      if (single_pred_p (bb)
-	  || !single_succ_p (pred)
+      /* Do not duplicate BB into PRED if we cannot merge a copy of BB
+	 with PRED.  */
+      if (!single_succ_p (pred)
 	  || e->flags & EDGE_COMPLEX
 	  || pred->index < NUM_FIXED_BLOCKS
 	  || (JUMP_P (BB_END (pred)) && !simplejump_p (BB_END (pred)))

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