I've found a bug in either the PBQP register allocator or in VirtRegRewriter.
I'm observing this assertion in VirtRegRewriter::rewrite() fail:
unsigned VirtReg = MO.getReg();
unsigned PhysReg = VRM->getPhys(VirtReg);
assert(!MRI->isReserved(PhysReg) && "Reserved register assignment");
Indeed there is a case where PhysReg may be a reserved physical register. Specificially, RegAllocPBQP::finalizeAlloc() may select a physical register thusly:
const TargetRegisterClass &RC = *MRI.getRegClass(LI.reg);
PReg = RC.getRawAllocationOrder(MF).front();
The documentation for TargetRegisterClass::getRawAllocationOrder() notes that the collection may include reserved registers. So it seems that the PBQP allocator may insert a reserve physical register into the VirtRegMap.
I'm not sure which component should be fixed. Is it fair to say that no-reserved-registers is an invariant of VirtRegMap? If so, shouldn't that invariant be enforced in VirtRegRewriter::assignVirt2Phys() ? Should PBQP iterate over the allocation order collection to find an un-reserved physical register?
D. E. Shaw Research