[llvm-dev] Removing the register block in MIR

[Matthias Braun via llvm-dev]

> On Oct 20, 2017, at 5:07 PM, Justin Bogner via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> The MIR format currently has a short-hand syntax for declaring vreg
> classes and banks in the function body so you can write something like
> this:
> 
>  name: foo
>  body: |
>    %3:gpr(s64) = ...
> 
> rather than the much more verbose and awkward:
> 
>  name: foo
>  registers:
>    - { id: 3, class: gpr }
>  body: |
>    %3(s64) = ...
> 
> I'd like to make this shorthand the only way to do this. There are a few
> things that need to be handled here:
Yes not using the registers list is the way to go.

> 
> - We should only print the class on defs, not all uses. This is
>  sufficient to be unambiguous, easy to be consistent, and avoids
>  getting in the way of readability.
We could in principle have no def if:
- A vreg is completely unused. I don’t have a good reason right now we should serialize it then, so this is probably fine.
- A vreg with no def but uses marked undef. This is also not particularily useful but legal and I could imagine this happening. Should be easy enough to catch this special case in the printer though; something like print regclass on all uses when there isn’t a single def.

> 
> - We'll still need to track the preferred-register elsewhere, so we'll
>  still need a block for those. They're used far less often than
>  classes though, so I think its fine to just let them have their own
>  block when any of them are set.
Fine with me.

> 
> - Basically every single MIR test will need to be updated, and its
>  awkward to do automatically unless the test is already using the
>  update_mir_test_checks script.
> 
> I plan to implement this in a couple of steps:
> 
> 1. Teach the MIRPrinter to print the regclass and update test checks
That would be great. Ahmed had started working on this a while ago, but I think it got stuck or was interrupted by higher priority things...

> 2. Modify existing tests to never provide the registers block
> 3. Move preferred-registers to their own block and remove the registers block
> 
> A patch for step 1 is attached (with the other 241 test updates omitted
> for brevity). I don't expect this direction to be contentious, but since
> the change will touch so many files I'll wait until Monday or Tuesday to
> start committing in case anyone has any concerns.

I think it’s a great thing to do.

- Matthias