[LLVMdev] Determining the register type of a MachineOperand

[Evan Cheng]

Sounds like you should specify a few class of pseudo registers mapped  
to the same set of registers. This is something we are considering for  
x86. That is, have xmm0_64, xmm0_32, and xmm0 all mapping to xmm0. If  
the first one is used, it means only the lower 64-bits are needed.

An alternative is to encode such information in the opcode or as a  
pseudo immediate operand. Or, in the worst case, you can just iterate  
through all the uses / defs to determine the type

Type information just doesn't belong on MachineOperand's. They belong  
to machine instructions, i.e. how they are defined / used.

Evan

On Sep 26, 2008, at 8:59 AM, Villmow, Micah wrote:

> Yes,
> Another reason this is useful is for register-type specific
> representations of said register.
>
> For example, all my registers are 128bit vector registers, however,  
> if I
> am only dealing with 32 bit vector registers, I can add write/read  
> masks
> that tell the underlying hardware not to work on the whole register,  
> but
> just a subset of the components.
> 32bit  scalar mov: mov r1.x___, r0.x000
> 64bit  scalar mov: mov r1.xy__, r0.xy00
> 96bit  scalar mov: mov r1.xyz_, r0.xyz0
> 128bit scalar mov: mov r1, r0
>
> If I had the register type information at code generation time, then I
> can use the exact same tablegen pattern for all my register types and
> would only need to add the write/read masks in the printer instead of
> multiple patterns.
>
> Multiply this by over 100 opcodes and I save A LOT of work.
>
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of David Greene
> Sent: Friday, September 26, 2008 8:39 AM
> To: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Determining the register type of a  
> MachineOperand
>
> On Wednesday 24 September 2008 15:23, Mon Ping Wang wrote:
>> To my knowledge, I don't think there is an easy way to get the MVT
>> information from a MachineOperand.   Why do you need it for?  In my
>
> See the thread I started on this very topic.  Spilling is one place
> you'd like
> to have this information.
>
>> mind, the MachineInstr and its associated operands represent a
>> physical machine instruction and I typically want to think of those  
>> as
>> machine opcodes and machine register files.  I am typically  
>> interested
>> in the mapping of MVTs to register classes when I'm generating  
>> machine
>> instructions.  Note that a register class may map to multiple MVTs
>> depending on your description.
>
> Right, but the instruction opcode should give a clue about what the  
> bits
>
> represent.  That mapping is what's currently missing.
>
>                                             -Dave
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