Re: [PATCH] Properly valueize values we value-number to

[Jeff Law <law at redhat dot com>]

On 02/17/15 07:03, Richard Biener wrote:
> This is something I noticed some time ago and that I remembered when
> you added that looping SSA_NAME_VALUE to simplify_control_stmt_condition.
> Currently DOM doesn't make sure that when setting
> SSA_NAME_VALUE (x) = y that SSA_NAME_VALUE (y) == y, thus you could
> get SSA_NAME_VALUE forming a chain until you reach the final value.
>
> Thus the following patch which fixes all occurances and removes the
> looping from simplify_control_stmt_condition.  Did you have any
> testcase when you added that looping?
pr61607, which probably looks familiar :-)





> Note that this is purely by code inspection and I don't have any
> testcase where a SSA_NAME_VALUE chain causes missed optimizations
> (you'd have to disable a lot of other optimizations before dom1
> to be able to create a reasonably simple one).
>
> Anyway - the tree-ssa-dom.c part bootstrapped and tested ok on
> x86_64-unknown-linux-gnu, the tree-ssa-threadedge.c part bootstrapped
> ok ontop of that and testing is still in progress.
>
> Ok?
>
> Thanks,
> Richard.
>
> 2015-02-17  Richard Biener  <rguenther@suse.de>
>
> 	* tree-ssa-dom.c (record_equivalences_from_phis): Valueize PHI arg.
> 	(record_const_or_copy_1): Valueize y.
> 	(record_equivalences_from_stmt): Valueize rhs.
> 	* tree-ssa-threadedge.c (simplify_control_stmt_condition):
> 	Remove repeated valueization.
Given the regression was fixed elsewhere, let's table this until gcc-6.

I want to rip out large hunks of this code, at least on the threading 
side and replace it with a backwards walk similar to what Sebastian did 
for the FSM optimization.  As a nice side effect, that ought to 
completely disentangle DOM and the threader.

At the same time I want to see what effect we'd have by disabling these 
context sensitive propagations in DOM.  They add a lot of hair and I'm 
just not sure they're worth it.  If we really need them, perhaps we can 
get away with something simpler, outside of DOM.

jeff