[llvm-dev] VirtRegMap invariant: no reserved physical registers?

[Johnson, Nicholas Paul via llvm-dev]

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