[PATCH] D97111: [RISCV] change rvv frame layout

[Hsiangkai Wang via Phabricator via llvm-commits]

HsiangKai added a comment.

In D97111#2577248 <https://reviews.llvm.org/D97111#2577248>, @StephenFan wrote:

> In D97111#2576958 <https://reviews.llvm.org/D97111#2576958>, @rogfer01 wrote:
>
>> Hi @StephenFan. I wonder if we want to do this only when we index via `sp`.
>>
>> My understanding is that the emergency slot will be close enough when we index via `fp`/`bp` in the previous layout. In the previous layout, when indexing a scalar via `sp` we have to cross the RVV part (which needs an extra register) to reach the scalar registers (where the emergency slot will be located). IIUC, your approach it fixes the `sp` case but complicates the `fp`/`bp` case to access the emergency slot because now we need to jump over RVV.
>>
>> My suggestion means we want to have the RVV vectors as far from the frame pointer as possible (the one being used `sp`/`fp`/`bp`), so scalars are still straightforward to access even in the presence of RVV. This means different layouts when indexing with `sp` and  when indexing with `fp`/`bp`. In summary: your new layout for `sp` but the previous one for `fp`/`bp`. Does this make sense?
>
> Make sense to me. I didn't consider the case that needs `fp` to access the emergency spill slot. In my patch, It seems that make use of the `bp` register is necessary when variable-sized local variables and rvv stack  objects exist at the same time. Because if uses `fp` to access the emergency spill slot needs an extra register. In the original patch,  make use of the `fp` register is necessary when rvv stack objects exist. I prefer the current frame layout, Because it only needs a `bp` register when variable-sized local variables and rvv stack objects exist. Do you agree it?

When it is necessary to have fp, the frame layout will look like as below.

  |---------------------------------| <- frame pointer (fp)
  | scalar callee-saved registers   |
  |---------------------------------|
  | scalar local variables          |
  |---------------------------------|
  | ///// realignment /////         |
  |---------------------------------|
  | scalar outgoing arguments       |
  |---------------------------------|
  | RVV local variables &&          |
  | RVV outgoing arguments          |
  |---------------------------------| <- end of frame (sp)

What do you mean "if uses `fp` to access the emergency spill slot needs an extra register"?

I agree with Roger. We only need to change the frame layout when accessing frame objects only using sp. That is, we should put RVV objects as far as 'base' as possible. The 'base' could be sp, fp or bp.


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