[Bug tree-optimization/78496] [7 Regression] Missed opportunities for jump threading

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78496

--- Comment #3 from Jeffrey A. Law <law at redhat dot com> ---
So I've been pondering this a bit more.  Essentially I see two paths forward.  

One is to enhance the backwards threader so that it can do more general
lookups/simplifications.  I've cobbled together some code for this, but it's
certainly not gcc-7 material and even once working it may not necessarily
capture this case.

The second would be to take advantage of some of the DOM restructuring we did a
year or so ago and do a block local jump threading pass where the edges/blocks
to consider are seeded by phi-only-cprop.  Depending on the cost iterating
phi-only-cprop and a local threader might be acceptable -- particularly if we
throttled back threading elsewhere.  

I haven't cobbled together any code for the second approach, but it shouldn't
be hard to prototype a non-iterating implementation.  Most of the
infrastructure we need is already in place.