[PATCH][PR tree-optimization/67816] Fix jump threading when DOM removes conditionals in jump threading path

[Jeff Law]

On 10/07/2015 02:26 AM, Richard Biener wrote:

>
> Hmm, other passes avoid all this by not removing edges or blocks
> themselves but leaving that to cfgcleanup.  They simply replace the
> condition in GIMPLE_CONDs or the switch value in GIMPLE_SWITCHes and
> let cleanup_control_expr_graph do the hard part of the job.
>
> Now - I don't know how that would interact with jump threads
> covering those paths, that is, what kind of "mess" jump threading
> would produce with the conditions/switches mangled that way.
>
> The other nice thing is that CFG cleanup properly deals with loop
> structure changes.
Threading will handle it correctly as that's essentially how we handled 
it until last week ;-)

The "problem" is the threader thread through those blocks.  And while 
that works correctly, it is quite wasteful in terms of compile time and 
memory consumption due to the block duplication required to preserve 
side effects.  It also churns the blocks & ssa-names used in the 
isolated regions which makes comparing debugging dumps before/after 
threading much harder than it ought to be.

Jeff