[LLVMdev] PBQP & CalcSpillWeights

[Arnaud de Grandmaison]

Hi All,

I finally had a chance to get back to my pbqp trials, now using the 3.0
release. I still hit the same assert : "Attempting to spill already spilled 
value."

This is triggered because in RegAllocPBQP::mapPBQPToRegAlloc, a vreg node is
either :
 - a physical register (problem.isPRegOption(vreg, alloc)),
 - or a spill (problem.isSpillOption(vreg, alloc))

The problem is that pass CalcSpillWeights can 'hint' that it is a poor
idea to spill this specific register with :

CalcSpillWeights.cpp / VirtRegAuxInfo::CalculateWeightAndHint :
  // Mark li as unspillable if all live ranges are tiny.
  if (li.isZeroLength(LIS.getSlotIndexes())) {
    li.markNotSpillable();
    ...

This hint makes the register non spillable at all for the spiller (that's the 
assert above), not just a bad-idea-to-spill-but-feasible. The pbqp allocator 
does not cope with this distinction and allways attempts to spill it.

I would need some guidance on how to modify the pbqp to handle this case 
properly.

Best regards,
-- 
Arnaud de Grandmaison