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

[Matthias Braun via llvm-dev]

> On Jun 5, 2017, at 9:26 AM, Johnson, Nicholas Paul via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 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?

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