Re: [tree-ssa] -fgcse and friends useless

[Daniel Berlin <dberlin at dberlin dot org>]

On Feb 6, 2004, at 1:28 PM, Zdenek Dvorak wrote:

> Hello,
>
> > It seems that the tree-optimizers do all the work that rtl level gcse 
> > and
> > friends (gcse-lm, gcse-sm, gcse-las) do.
>
> I was not aware we do load & store motion on tree-ssa (or does PRE
> handle this?)

SSAPRE does load motion on loads that occur > 1 time (be they direct or 
indirect loads).

Doing it on loads that occur only once is too expensive right now.
This is mainly because
1. process_left_occs_and_kills needs to be sped up through algorithmic 
changes so that it doesn't have to do O(n^3) work in the worst case.
2. we need to be able to do rename_2 without generating tons of new 
trees.  Though I guess i can now ggc_free this stuff explicitly, which 
may help.

I never implemented SSUPRE (which is gcse-sm), though i could.

But for simplicity sake, we'd probably be better off implementing 
regular FSM (forward store motion) using dominator based 
sinking/removal, at least for now.
--Dan