[PATCH] D135687: [AArch64] Fix aligning the stack after calling __chkstk

[Martin Storsjö via Phabricator via llvm-commits]

mstorsjo added a comment.

In D135687#3850753 <https://reviews.llvm.org/D135687#3850753>, @efriedma wrote:

> I was thinking you could do something like this:
>
>   mov x15, #(NumBytes/16) ; Stack allocation size
>   mov x9, sp
>   and x9, x9, #AlignMask ; compute aligned stack
>   sub x9, sp, x9 ; compute number of bytes required to align stack
>   add x15, x15, x9, lsr #4 ; add bytes to chkstk input
>   bl __chkstk
>   sub sp, sp, x15, lsl #4
>
> There's probably some slightly more efficient way to write this, but that's the idea.

Hmm, ok, I see. I guess that'd work, but it feels a bit roundabout IMO.

It looks like MSVC does something like this:

  mov x15, #(NumBytes+Align)/16
  bl __chkstk
  sub sp, sp, x15, lsl #4
  mov x8, sp
  add x9, x8, #(Align-1)
  and x0, x9, #AlignMask

(The `mov x8, sp` feels superfluous here?) It doesn't align the stack pointer itself, but I think we should be able to do the same (while wasting fewer registers) by doing the last `and` into `sp`.

I think I'd prefer something like this:

  mov x15, #(NumBytes+Align)/16
  bl __chkstk
  sub sp, sp, x15, lsl #4
  add x9, sp, #(Align -1)
  and sp, x9, #AlignMask


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