[Bug rtl-optimization/110034] The first popped allcono doesn't take precedence over later popped in ira coloring

["vmakarov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110034

--- Comment #4 from Vladimir Makarov <vmakarov at gcc dot gnu.org> ---
Thank you for providing the test case.

To be honest I don't see why assigning to hr3 to r134 is better.
Currently we have the following assignments:

hr9->r134; hr3->r173; hr3->r124

and the related preferences:

  cp11:a18(r134)<->a29(r173)@125:shuffle
  pref3:a29(r173)<-hr3@2000
  pref4:a0(r124)<-hr3@125

This removes cost 2000 (pref3) and cost 125 (pref4) and adds cost 125
(cp11).  The profit is 2000

If we started with r173, we would have the following assignments:

hr3->r173; hr3->r134; <some hard reg but hr3>->r124

This would remove cost 2000 (pref3) and cost 125 (cp11) and add cost
125 (pref).  The profit would be the same 2000.

Choice of heuristics is very time consuming.  I spent a lot of time to
try and benchmark numerous ones.  I clearly remember that introduction
of pseudo threads for colorable busket gave visible performance
improvement.  Currently we assign pseudos from a thread with the
biggest frequency first (r173 and r134) and a pseudo (r134) with the
biggest frequency first from the same thread.  I think it is logical.

Also it is always possible to find a test (not this case) where
heuristics give some undesirable results.  RA is NP-complete task even
in the simplest formulation. We can not get the optimal solution for
reasonable time.

Still I am open to change any heuristic if somebody can show that it
improves performance for some credible benchmark (I prefer SPEC2007)
on major GCC targets.