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.