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

[Johnson, Nicholas Paul via llvm-dev]

Thanks Matthias.  Will do.

>-----Original Message-----
>From: mbraun at apple.com [mailto:mbraun at apple.com]
>Sent: Monday, June 05, 2017 5:17 PM
>To: Johnson, Nicholas Paul
>Cc: llvm-dev at lists.llvm.org
>Subject: Re: [llvm-dev] VirtRegMap invariant: no reserved physical registers?
>
>
>> 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?
>Feel free to send a patch that adds an assert to assignVirt2Phys().
>
>- Matthias