[Bug tree-optimization/35805] [ira] error in start_allocno_priorities, at ira-color.c:1806

[Paolo Bonzini]

> I will test this patch, but we still need to resolve your issues with my
> approach.

The problem is that you're really doubling the cost of computing the
live registers.  I know that previously it was wrong, but at this point
there's no difference with the full-blown pass...  Despite the idea of
DF_LR_RUN_DCE being that it was "free", now it would do the same work as
a pass_fast_rtl_dce modulo some O(#bbs) work.

At this point, if your patch costs say 0.3%, and removing all traces of
DF_LR_RUN_DCE (instead scheduling a dozen more pass_fast_rtl_dce in
passes.c) costs 0.5%, I'd rather see the latter, at least it's easier to
look for opportunities to remove some useless DCE.

If it wasn't for verification, we could just decide that DF_LR_RUN_DCE
is only for passes that can tolerate a little inaccurate info...

Paolo