[Bug rtl-optimization/64317] [5 Regression] Ineffective allocation of PIC base register

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64317

--- Comment #9 from Vladimir Makarov <vmakarov at gcc dot gnu.org> ---
(In reply to Jeffrey A. Law from comment #7)
> Vlad,
> 
> What's the rationale behind the 50% probability cutoff for forming an EBB? 
> For the purposes of inheritance, ISTM you want the biggest EBBs possible to
> give you a maximal window in which to find an insn to inherit from?  Or is
> it the case that EBB formation impacts both spilling and inheritance in
> IRA/LRA?

I remember I tried different cut-off probabilities.  But LRA is changing
quickly, may be it is time to check this again.

I'd agree with you that for inheritance the bigger EBB, the better.  But there
are also optional reloads.  So if we had EBB consisting of BB1 and very low
probability BB2

BB1
    op with pseudo spilled
BB2
    the spilled pseudo use

optional reload and inheritance would transform it into

BB1
    load hr, pseudo spilled
    op with hr
BB2
    hr use

With 50% cut-off we would have

BB1
    op with pseudo spilled
BB2
    the spilled pseudo use

which is better code if probability BB1 >> BB2.