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

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

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

--- Comment #5 from HaoChen Gui <guihaoc at gcc dot gnu.org> ---
(In reply to Vladimir Makarov from comment #4)
> 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.

Thanks for your explanation. I agree with it. I also checked the assembly and
found there is no potential performance gain when r3 is assigned to r134. It
should be not a bug.