Re: [PATCH] Fix PR36078, fallout from early loop unrolling

[Zdenek Dvorak <rakdver at kam dot mff dot cuni dot cz>]

Hi,

> On Tue, 29 Apr 2008, Zdenek Dvorak wrote:
> 
> > > This patch adds a missing SSA update after early complete loop unrolling.
> > > Appearantly this is only needed if unrolling doesn't operate on
> > > loop-closed SSA form, but it might also paper over a problem with not
> > > doing so.
> > 
> > why does this help?  We already call update_ssa in
> > try_unroll_loop_completely.
> 
> Hm, the difference with the (indeed extra) update_ssa is
> ...
> as we run CFG cleanup after updating SSA form in 
> try_unroll_loop_completely, some constants are propagated from PHIs
> and the VOPs change during CFG cleanup requiring an SSA update
> (cfg-cleanup vs. non-cfg-cleanup variant):
> ...
>
> This may be a latent general problem of course, which the patch
> (bootstrapped and tested ok meanwhile) only papers over.

this is a known issue with cfg cleanup (see passes.c:904).  An easy fix
is to call update_ssa after cfg cleanup (or possibly to add the update_ssa
call to cfg cleanup itself).

A nicer fix would be to make cfg cleanup preserve ssa form for virtual
ops.  In theory, this is easy -- the phi node elimination should only
make the memory addresses more specific, thus it may only make some of
the vops disappear, in which case we can just "copy propagate" them.  In
practice, alias analysis somehow manages (or at least, used to manage)
to introduce new vops, thus making this impossible.

Zdenek