[PATCH] D126638: [CGP] Also freeze ctlz/cttz operand when despeculating

Thu Jun 9 02:22:19 PDT 2022

nikic added a comment.

In D126638#3566645 <https://reviews.llvm.org/D126638#3566645>, @spatel wrote:

> https://alive2.llvm.org/ce/z/iwd4ba
> This is needed for correctness, so LGTM.
>
> It might be good to add tests for other targets. I didn't step through this to see why, but the code for AArch and ARM actually looks better when we re-use the frozen value.
>
>   rbit	w9, w0
>   mov	w8, #32
>   clz	w9, w9
>   cmp	w0, #0
>   csel	w0, w8, w9, eq
>
> vs.
>
>   rbit	w8, w0
>   clz	w0, w8

For what input is that? As far as I can tell, AArch64 never uses despeculation, and ARM uses a libcall legalization where it makes no difference. I'm trying this one:

  define i32 @ctlz_i32(i32 %a) nounwind {
    %r = call i32 @llvm.ctlz.i32(i32 %a, i1 false)
    ret i32 %r
  }

  declare i32 @llvm.ctlz.i32(i32, i1)

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D126638/new/

https://reviews.llvm.org/D126638