[PATCH] shrink-wrap: Fix thinko (PR68520)

[PATCH] shrink-wrap: Fix thinko (PR68520)

[Segher Boessenkool]

Part of the shrink-wrapping algorithm has this comment:

  /* Now see if we can put the prologue at the start of PRO.  Putting it
     there might require duplicating a block that cannot be duplicated,
     or in some cases we cannot insert the prologue there at all.  If PRO
     wont't do, try again with the immediate dominator of PRO, and so on.

     The blocks that need duplicating are those reachable from PRO but
     not dominated by it.  We keep in BB_WITH a bitmap of the blocks
     reachable from PRO that we already found, and in VEC a stack of
     those we still need to consider (to find successors).  */

Two of the cases that push to that stack do not actually check the
bitmap first.  Either I thought those blocks could not be on the stack
already, or more likely I didn't think and it just didn't crash during
any testing.  But of course those blocks *can* already be on the stack
(if you have a hideous loop structure), and then we end up with the
same block on the stack more than once.  This is harmless, except that
(like in the PR) this can overflow the stack.

This fixes it, by doing the necessary bitmap checks before pushing.

Tested on the testcase in the PR, with an x86_64 cross-compiler;
bootstrap+regcheck in progress (on powerpc64-linux).  Is this okay for
trunk if that succeeds?


Segher


2015-11-24  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/68520
	* shrink-wrap.c (try_shrink_wrapping): Don't push a block to VEC if
	its bit was already set in BB_WITH.

---
 gcc/shrink-wrap.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/gcc/shrink-wrap.c b/gcc/shrink-wrap.c
index 61765ff..3a1df84 100644
--- a/gcc/shrink-wrap.c
+++ b/gcc/shrink-wrap.c
@@ -722,8 +722,8 @@ try_shrink_wrapping (edge *entry_edge, bitmap_head *bb_with,
 	{
 	  pro = get_immediate_dominator (CDI_DOMINATORS, pro);
 
-	  bitmap_set_bit (bb_with, pro->index);
-	  vec.quick_push (pro);
+	  if (bitmap_set_bit (bb_with, pro->index))
+	    vec.quick_push (pro);
 	}
 
       basic_block bb = vec.pop ();
@@ -734,8 +734,8 @@ try_shrink_wrapping (edge *entry_edge, bitmap_head *bb_with,
 
 	    pro = get_immediate_dominator (CDI_DOMINATORS, pro);
 
-	    bitmap_set_bit (bb_with, pro->index);
-	    vec.quick_push (pro);
+	    if (bitmap_set_bit (bb_with, pro->index))
+	      vec.quick_push (pro);
 	  }
 
       FOR_EACH_EDGE (e, ei, bb->succs)
-- 
1.9.3

Re: [PATCH] shrink-wrap: Fix thinko (PR68520)

[Bernd Schmidt]

On 11/24/2015 07:13 PM, Segher Boessenkool wrote:
>
> 	PR rtl-optimization/68520
> 	* shrink-wrap.c (try_shrink_wrapping): Don't push a block to VEC if
> 	its bit was already set in BB_WITH.

Ok.


Bernd