On Thu, Apr 25, 2013 at 5:03 AM, Jeff Law <l...@redhat.com> wrote:
> On 04/24/2013 04:54 PM, Steve Ellcey wrote:
>>
>>
>> I am still having trouble with this and with figuring out how to
>> straighten out my PHI nodes. I have decided to try a slightly different
>> tack and see if I could create a routine that would do a generic basic
>> block copy, handling all the needed bookkeeping internally and fixing
>> all the PHI nodes after the copy. I am trying to create a slow but
>> dependable and easy to use function and would consider completely
>> regenerating all the PHI information if that was the easiest thing to
>> do. Here is what I have so far:
>
> Interesting you should mention this; one of the things I really want to get
> back to is a more generic mechanism to copy block regions.
We have gimple_duplicate_sese_region for this. It may be not perfect though.
Eventually it should be changed to handle SEME regions as well and all
loop copying / versioning code should use it as well (though I don't think
any of the loop copying / versioning code handles multiple exits).
I've slowly started to move us in this direction by removing duplicate
functionality
in the compiler as I come along it ...
Richard.
> Threading is really just path isolation by copying and some
> equivalency/redundancy elimination enabled by the path isolation.
>
> We're missing a lot of threading opportunities because the current method
> for copying blocks is so limited. There's finally some good literature on
> this stuff, both in terms of finding the redundancies that lead to useful
> optimization and in terms of identifying regions of blocks that need to be
> copied. All the nonsense we do needs to be reformulated using better known
> methods.
>
>
>>
>>
>> /* Copy the basic block that is the destination block of orig_edge, then
>> modify/replace the edge in orig_edge->src basic block with a new edge
>> that goes to the new block. Fix up any PHI nodes that may need to be
>> updated. Remove the dominance info since it may have been messed up.
>> */
>>
>> edge
>> duplicate_succ_block (edge orig_edge)
>> {
>> edge new_edge;
>> basic_block orig_block, new_block;
>>
>> initialize_original_copy_tables ();
>> orig_block = orig_edge->dest;
>> fprintf(stderr, "Duplicating block %d\n", orig_block->index);
>> new_block = duplicate_block (orig_block, NULL, NULL);
>> update_destination_phis (orig_block, new_block);
>> new_edge = redirect_edge_and_branch (orig_edge, new_block);
>> remove_phi_args (orig_edge);
>> free_dominance_info (CDI_DOMINATORS);
>> free_original_copy_tables ();
>> return new_edge;
>> }
>>
>> When I use this to copy a block I get a failure from verify_ssa.
>
> Well, with that structure you need to update PHIs at the destination of
> every outgoing edge from new_block. That's one of the reasons you want to
> delete the control statement and dead edges in the copy -- that leaves you
> just updating the single successor of new_block.
>
> You don't mention why verify_ssa fails. I'd hazard a guess you've got a use
> not dominated by its set. It'll be important to know where the use occurs
> and where the dominating set is supposed to be. Presumably you call into
> update_ssa or whatever it's called these days before trying to verify_ssa?
>
>
>
>>
>> The block I am trying to copy (based on my original example) is:
>>
>> <bb 8>:
>> # s_1 = PHI <s_6(D)(2), s_8(7)>
>> # t_3 = PHI <0(2), t_2(7)>
>> # c_4 = PHI <c_7(2), c_9(7)>
>> if (c_4 != 0)
>> goto <bb 3>;
>> else
>> goto <bb 9>;
>>
>> There are two edges leading here (from block 2 and block 7) and I want
>> to change the 2->8 edge to be a 2->8_prime edge where 8_prime is my new
>> basic block. That obviously affects the PHI nodes in both block 8 and
>> the new 8_prime block. I don't think any other PHI's are affected in
>> this case, but obviously, I would like my routine to work on any block I
>> want to copy even if it does affect PHI nodes in successor blocks.
>
> You have to update the PHIs in bb3 & bb9. You want to copy the PHI arg
> associated with 8->3 for the 8'->3 edge, similarly for for PHI args
> associated with the 8->9 edge to the 8'->9 edge. See copy_phi_args.
>
> jeff
>
>
