Re: Fix PR 53743 and other -freorder-blocks-and-partition failures (issue6823047)

[Jeff Law <law at redhat dot com>]

On 05/07/13 23:13, Teresa Johnson wrote:
> ----------------------
> Revised patch that fixes failures encountered when enabling
> -freorder-blocks-and-partition, including the failure reported in PR 53743.
>
> This includes new verification code to ensure no cold blocks dominate hot
> blocks contributed by Steven Bosscher.
Seems like a reasonable verification; presumably if we have a cold block 
dominating a hot block, then the block/edge frequencies are badly 
broken.  Ah, just saw the comments for the other case where this 
happens.  cold entry, but hot loop inside pushing over the barrier. 
Arguably given a cold block in the dominator graph, all its children 
should have their frequences scaled down to avoid that situation.

> Additionally, I added a flag to the rtl_data structure to indicate whether
> any partitioning was actually performed, so that optimizations which were
> conservatively disabled whenever the flag_reorder_blocks_and_partition
> is enabled (e.g. try_crossjump_to_edge, part of connect_traces) can be less
> conservative for functions where no partitions were formed (e.g. they are
> completely hot).
> ----------------------
>
> Ok for trunk?
I can't really comment on the cfglayout and related stuff -- it was 
added at a time when I wasn't doing much with GCC and thus I don't know 
much about it.

However, I like the changes to record if we've done partitioning and 
checking those instead of flag_reorder_blocks_and_partition.  That's 
simple enough that I'd support pulling it out as a separate patch and 
installing immediately if that can be done so without major headaches.

I think we could do something similar with the code to verify the idom 
of a hot block is also hot.  Though looking at the implementation I 
wonder if it could be simplified by walking the dominator tree?  I can't 
look at it real closely tonight though.

Could you pull those two logical hunks of work out into individual 
patches.

jeff