[llvm-dev] Question about registers and subregisters

[Quentin Colombet via llvm-dev]

> On Sep 17, 2021, at 12:00 PM, Francesco Bertolaccini <francesco at bertolaccini.dev> wrote:
> 
> Thanks Quentin, it does help!
> I'm referring to the docs at
> https://llvm.org/docs/WritingAnLLVMBackend.html#register-set-and-register-classes
> 
> How do register units play into the registers/register classes/register
> sets described there?
> 
> The situation I mentioned in my original mail would be implemented
> something like this, if I am not mistaken:
> 
> def A : Register<"A">;
> def AB : Register<"AB"> {
>  let SubRegs = [ A ];
> };
> def ABC : Register<"ABC"> {
>  let SubRegs = [ AB ];
> };
> def ABCD : Register<"ABCD"> {
>  let SubRegs = [ ABC ];
> };

Looks right to me.

> 
> Are the underlying register units automagically generated by TableGen?

Yes the register units are automatically generated by TableGen.
You can see this in YourTargetGenRegisterInfo.inc in your build directory.
The MCRegisterDesc describes the register units for each register (5th field) (hard to see though :)).
And the full list of units stored in the variable YourTargetRegUnitRoots.

Honestly, unless your needs are not covered by the existing APIs, I wouldn’t dig into that stuff. The way that stuff is represented is not super important.

> 
> Best regards,
> Francesco
> 
> On 17/09/2021 20:39, Quentin Colombet wrote:
>> Hi Francesco,
>> 
>> Yes, the subreg are transitive but the transitivity may not be
>> represented the way you expect.
>> 
>> Essentially all the registers map to a set of register units. Two
>> registers overlap if their register units overlap.
>> The register units are not necessarily actual register.
>> 
>> E.g., in your first example, ABCD maps to the reg unit A, B, C, and D.
>> In your second example, ADE maps to A and AD maps to A as well. Assuming
>> D and E are not physically accessible by themselves.
>> 
>> You can take query the regunits using the related MCRegisterInfo API,
>> e.g., MCRegUnitIterator.
>> 
>> Hope this helps.
>> 
>> Cheers,
>> -Quentin
>> 
> 

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