On Fri, Aug 2, 2013 at 5:05 PM, Jan Hubicka <hubi...@ucw.cz> wrote:
>> +/* Called when block BB has been reassigned to a different partition,
>> + to ensure that the region crossing attributes are updated. */
>> +
>> +static void
>> +fixup_bb_partition (basic_block bb)
>> +{
>> + edge e;
>> + edge_iterator ei;
>> +
>> + /* Now need to make bb's pred edges non-region crossing. */
>> + FOR_EACH_EDGE (e, ei, bb->preds)
>> + {
>> + fixup_partition_crossing (e);
>> + }
>> +
>> + /* Possibly need to make bb's successor edges region crossing,
>> + or remove stale region crossing. */
>> + FOR_EACH_EDGE (e, ei, bb->succs)
>> + {
>> + if ((e->flags & EDGE_FALLTHRU)
>> + && BB_PARTITION (bb) != BB_PARTITION (e->dest)
>> + && e->dest != EXIT_BLOCK_PTR)
>> + force_nonfallthru (e);
>> + else
>> + fixup_partition_crossing (e);
>> + }
>> +}
>
> Is there particular reason why preds can not be fallhtrus
Yes, by definition a crossing edge cannot fall through. There is
always a control transfer from one section to another.
>> +/* Sanity check partition hotness to ensure that basic blocks in
>> + the cold partition don't dominate basic blocks in the hot partition.
>> + If FLAG_ONLY is true, report violations as errors. Otherwise
>> + re-mark the dominated blocks as cold, since this is run after
>> + cfg optimizations that may make hot blocks previously reached
>> + by both hot and cold blocks now only reachable along cold paths. */
>
> With profile, I suppose we can have cold blocks dominating hot blocks when the
> hot blocks is in loop whose trip count is high enough.
That is the common case, actually.
> Indeed for partitioning
> reasons it does not make sense to push those into different section.
The partitioning algrorithm makes sure this doesn't happen. The
hottest path from the entry block to a hot basic block is always part
of the hot partition.
> I also wonder, if we finally get the pass stable, can we enable it by default
> and offline probably cold blocks w/o profile?
That is the general idea behind all this work, obviously ;-)
> Primarily blocks reachable only
> by EH + blocks leading to a crash or throw(). For C++ those should be common
> enough to make a difference...
Yup, and IIRC Theresa posted some numbers that showed this.
Ciao!
Steven
