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Re: if-conversion/HOT-COLD partitioning

On Wed, Oct 24, 2012 at 6:11 PM, Christophe Lyon wrote:
> On 24 October 2012 00:42, Steven Bosscher wrote:
>> On Tue, Oct 23, 2012 at 10:29 PM, Christophe Lyon wrote:
>>> Well, both of these functions appear to check that the 2 blocks to
>>> merge belong to the same partition, so it should be OK.
>> In your first email, you said if-convert was merging two blocks from
>> different partitions. can_merge_block_p() would rejected merging the
>> two blocks, so merge_blocks shouldn't be called on them.
>> IIRC cfghooks.c:merge_blocks() used to have a
>> gcc_assert(can_merge_blocks(a,b)) but it's not there now. But if
>> can_merge_blocks() returns false, merge_blocks should fail. Your bug
>> is that merge_blocks is being called at all on those blocks from
>> different partitions.
>>> But not all calls to merge_blocks are guarded by if
>>> (can_merge_block_p()), this is probably where the problem is?
>> Not sure. Depends on what blocks get merged. It may be that
>> if-conversion shouldn't even be attempting whatever transformation
>> it's attempting. Not enough information.
> What happens is that merge_if_block() is called with test_bb, then_bb
> and else_bb in the cold section, while join_bb is in the hot one.

AFAICT that can only happen if the join_bb has more predecessors than
just then_bb and else_bb. Otherwise, you'd be looking at a complete
diamond region, and test_bb and either else_bb or then_bb should be in
the hot partition as well. But if the join_bb has more predecessors,
then merge_blocks shouldn't be able to merge away the join block.

So either something's wrong with the CFG so that merge_if_blocks sees
a join_bb with fewer than 2 predecessors (the only path to the
merge_blocks call in merge_if_blocks), or the profile data is so
corrupted that the partitioning has somehow gone wrong. So...

> merge_if_block calls merge_blocks unconditionally several times (in my
> case, the wrong one is merge_blocks (combo_bb, join_bb)).

... still not quite enough information.

A more detailed explanation of the paths through the code that lead to
the error would be nice. A test case would be good. A PR would be


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