Re: [tree-ssa] DCE and conditionals

[Andrew MacLeod <amacleod at redhat dot com>]

On Mon, 2003-05-05 at 16:57, law@redhat.com wrote:
> In message <1052167541.2909.116.camel@frodo.toronto.redhat.com>, Diego Novillo 
> writes:
>  >Ah, yes.  So, keep the conditional expression.  After you've brought the
>  >program out of SSA form, run the linearizer once more.  And, this time,
>  >you untie its hands and let it trash COND_EXPRs that have both arms
>  >empty.  Similarly with other empty blocks that couldn't be removed
>  >before because of PHI nodes.
>  >
>  >
>  >> And in order for that to be effective, we
>  >> should look at phi removal when possible since thats what ends up
>  >> keeping the conditions alive... 
>  >> 
>  >PHIs that have all their arguments the same can be easily checked in the
>  >linearizer.
> Note that PHI optimization/removal is pretty simple, except for one
> little problem.  Specifically when there is a cycle in the PHI nodes.
> 
> I've got skeleton code to do PHI -> copy transformations here, but 
> there wasn't much point in doing anything with it until we support
> handling overlapping lifetimes.  

Well, we're within an inch and a quarter of supporting them. Ive been
testing it by running copy-prop, and and I only have 1 additional
failure with copy-prop now. Of course, there is the matter of
bootstrapping left :-). 

I think we're going to need it...

I also have a couple of hacks in place to work around things I need,
like the issue of never rewriting *p's... WHich I will document in the
hack.

Next day or two. I *think* I bootstrapped without copy prop, but I dont
remember so I have to try again.

Andrew