[tree-ssa] Alias analysis speedups

[law@redhat.com]

In message <3E5685F5.8000300@moene.indiv.nluug.nl>, Toon Moene writes:
 >law@redhat.com wrote:
 >
 >> In message <1045844562.8671.18.camel@shadowfax>, Diego Novillo writes:
 >
 >>  >Mixed blessing in this case.  Better alias info may lead to longer
 >>  >compile times and memory problems.  Hence our PHI pruning heuristics.
 >
 >> Yes, but those are very extreme cases.
 >
 >"Elk nadeel hep se foordeel" :-)
 >
 >What would this mean for an extremely non-aliasing language like Fortran  ?
 >
 >Inquiring minds want to know ...
I don't expect it will be a problem for Fortran.  Unless you create 
something which is roughly equivalent to 20001226-1.c.

Specifically you'd need a boatload (read tends of thousands) of 
array references, without any stores into the array to make PHI
node insertion go crazy.  And remember go crazy in this sense means
that it takes a little while.  It still completes thanks to the
heuristics to switch to fully-pruned PHI insertions.

jeff