[llvm-dev] analyzePhysReg question

[Quentin Colombet via llvm-dev]

> On Dec 3, 2015, at 5:36 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
>> 
>> On Dec 3, 2015, at 5:11 PM, Smith, Kevin B <kevin.b.smith at intel.com <mailto:kevin.b.smith at intel.com>> wrote:
>> 
>> 
>> 
>>> -----Original Message-----
>>> From: Quentin Colombet [mailto:qcolombet at apple.com <mailto:qcolombet at apple.com>]
>>> Sent: Thursday, December 03, 2015 4:43 PM
>>> To: Smith, Kevin B <kevin.b.smith at intel.com <mailto:kevin.b.smith at intel.com>>
>>> Cc: llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>> Subject: Re: [llvm-dev] analyzePhysReg question
>>> 
>>> 
>>>> On Dec 3, 2015, at 4:35 PM, Smith, Kevin B via llvm-dev <llvm-
>>> dev at lists.llvm.org <mailto:dev at lists.llvm.org>> wrote:
>>>> 
>>>> I am looking at results from analyzePhysReg, and am getting results a little
>>> different than I expected for x86.
>>>> 
>>>> The call to this is coming from this code in
>>> llvm::MachineBasicBlock::computeRegisterLiveness
>>>> 1163          MachineOperandIteratorBase::PhysRegInfo Analysis =
>>>> 1164            ConstMIOperands(I).analyzePhysReg(Reg, TRI);
>>>> 
>>>> The instruction I being analyzed is:
>>>> %BX<def> = MOV16rm %EDI, 2, %ECX, 0, %noreg;
>>> mem:LD2[%arrayidx98](tbaa=!18)
>>>> 
>>>> and the Reg being passed in is 21, which is EBX.  The result I get back for
>>> is:
>>>> 
>>>> Analysis: {Clobbers = true, Defines = true, Reads = false, ReadsOverlap =
>>> false,
>>>> DefinesDead = false, Kills = false}
>>>> 
>>>> It seems based on the comment in the definition of PhysRegInfo.Defines,
>>> that Defines should only be true if Reg or a super-register of Reg is
>>>> defined.  BX is not a super-register of EBX, so it seemed like Defines
>>> should be false here, while Clobbers is correct as true.
>>> 
>>> I believe the documentation is wrong here.
>>> EBX is partly defined, so to me Defines == true is conservatively correct
>>> here.
>>> My guess, but I haven’t checked the code, is that super-register should be
>>> understood as any alias of Reg.
>>> 
>>> Worth checking.
>>> 
>>> Q.
>> 
>> I agree EBX is partly defined, but I think that is what Clobbers is for. 
> 
> I think Clobbers is when there is a RegMask.
> Basically, if that helps,
> Clobber: The register cannot live through.
> Define: (Part of) the register is define by this instruction.

Put differently, you can use a register as a definition of an instruction when it is clobbered but not defined.

> 
>>  The interface for isSuperRegister
>> certainly makes a pretty clear distinction between something being a superRegister and something being an
>> overlapping register.  What do you think I should be checking to understand the assumptions/expectations better?
> 
> The code :).
> 
>> 
>>> 
>>>> 
>>>> I wanted to be sure that I wasn't missing something about the interface
>>> definition/expectation.
>>>> 
>>>> Thanks,
>>>> Kevin Smith
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