Re: [tree-ssa] Avoid jump threading from producing irreducible regions

[Jan Hubicka <hubicka at ucw dot cz>]

> In message <20031121152630.GV12030@kam.mff.cuni.cz>, Jan Hubicka writes:
>  >Hi,
>  >as discussed earlier, I believe we must avoid producing irreducible
>  >loops in early stages of compilation as disabling loop optimizations for
>  >no good reason can be disasterous.
>  >regtested/bootstrapped i386
>  >OK?
>  >
>  >2003-11-21  Jan Hubicka  <jh@suse.cz>
>  >	* tree-ssa.dom.c (tree_ssa_domionator_thread_jumps): Mark back edges.
>  >	(thread_across_edge): Do not thread across loop headers.
> Note that the threader is run in two modes.
> 
> The first (and the one you changed) is allowed to thread through blocks
> which begin with PHI nodes.
> 
> The second runs as part of the dominator optimizer, but is not allowed to
> thread through a block with PHI nodes.  You didn't change the behavior of
> this instance.
> 
> [ Note I fully expect that with Andrew's changes that we'll have a single
>   mode for the dominator based jump threader in the future. ]
> 
> Anyway, I'm guessing that you don't have to worry about the second mode
> (no threading through PHIs) since loops are going to have a PHI at the
> the block which starts the natural loop.  Right?
I guess so.  Except for loops with no loop carried variables that are
out of interest anyway.
OK then? :)

Honza
> 
> jeff
>