VirtRegMap invariant: no reserved physical registers?

Hey all,

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();
    VRM.assignVirt2Phys(LI.reg, PReg);

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?

Thank you,
Nick Johnson
D. E. Shaw Research

The generic register allocators have not enough knowledge to safely assign vregs to reserved registers; or put another way the register allocator should only assign allocatable register and reserved registers are by definition not allocatable. So this sounds like a bug in the PBQP allocator to me.

- Matthias

Feel free to send a patch that adds an assert to assignVirt2Phys().

- Matthias

Thanks Matthias. Will do.