[llvm-dev] analyzePhysReg question

[Sanjoy Das via llvm-dev]

I think this is related to PR25033:
https://llvm.org/bugs/show_bug.cgi?id=25033#c9

-- Sanjoy

On Thu, Dec 3, 2015 at 5:39 PM, Quentin Colombet via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> 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> wrote:
>
>
>
> -----Original Message-----
> From: Quentin Colombet [mailto:qcolombet at apple.com]
> Sent: Thursday, December 03, 2015 4:43 PM
> To: Smith, Kevin B <kevin.b.smith at intel.com>
> Cc: 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> 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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-- 
Sanjoy Das
http://playingwithpointers.com