Re: shrink-wrapping duplicates BBs across partitions.

[Christian Bruel <christian dot bruel at st dot com>]

The problem stems from tree-ssa-tail-merge that breaks bb->count, The
CFG looks like

     2
   /  \
  /    6
 5 (0) |
 |     3 <-----
 |    /   \   |
 |   7 (1)  8 -
 | /
  4 (1)

(in parenthesis the bb->count from gcov)

     2
   /  \
  /    6
 /      |
 |      3 <--
 |     / |  |
 5 (0)   8 --
 |
 |
  4 (1)

so 5 and 4 are now in different partitions, producing an assertion
because there is no EDGE_CROSSING between them.

I can see 3 solutions to this

1) merge the BB counts in tree-ssa-tail-merge.c, so 5 is in the same
partition that 4

2) don't tail-merge blocks that belong to different partitions.

3) add a EDGE_CROSSING flag on the edge between 4 and 5.

1) fixes the problem, so 5 and 4 are now in the same partition. The fix
is quite trivial, as with attached.

the other solution 2) is more conservative, and also fixes the problem.

I don't think 3) is necessary.

more ideas ?

thanks,

Christian


On 09/11/2012 06:21 PM, Jakub Jelinek wrote:
> On Tue, Sep 11, 2012 at 05:40:30PM +0200, Steven Bosscher wrote:
>> On Tue, Sep 11, 2012 at 5:31 PM, Christian Bruel <christian.bruel@st.com> wrote:
>>> Actually, the edge is fairly simple. I have
>>>
>>> BB5 (BB_COLD_PARTITION) -> BB10 (BB_HOT_PARTITION) -> EXIT
>>>
>>> and BB10 has no other incoming edges. and we are duplicating it.
>>
>> That is wrong, should never happen. Is there a test case to play with?
>> It'd be good to have a PR for this.
> 
> Isn't that the standard case when !HAVE_return ?  Then you can have only a
> single return through epilogue, and when the epilogue is in the hot
> partition, even if cold code is returning, it needs to jump to the epilogue.
> 
> 	Jakub
> 

Index: tree-ssa-tail-merge.c
===================================================================
--- tree-ssa-tail-merge.c	(revision 191129)
+++ tree-ssa-tail-merge.c	(working copy)
@@ -1478,6 +1478,8 @@
     bb2->frequency = BB_FREQ_MAX;
   bb1->frequency = 0;
 
+  bb2->count += bb1->count;
+
   /* Do updates that use bb1, before deleting bb1.  */
   release_last_vdef (bb1);
   same_succ_flush_bb (bb1);