Re: [PATCH v7 16/31] rust: ptr: add const_align_up()

["David Rheinsberg" <david@readahead.eu>]

Hi Alice!

On Fri, Mar 20, 2026, at 12:12 PM, Alice Ryhl wrote:
> On Fri, Mar 20, 2026 at 11:27 AM David Rheinsberg <david@readahead.eu> wrote:
>> align_up1:
>>         lea     rax, [rsi - 1]
>>         and     rax, rdi
>>         sub     rsi, rax
>>         lea     rdx, [rsi + rdi]
>>         cmp     rdx, rdi
>>         setae   cl
>>         test    rax, rax
>>         sete    al
>>         cmove   rdx, rdi
>>         or      al, cl
>>         movzx   eax, al
>>         ret
>>
>> align_up2:
>>         lea     rdx, [rdi + rsi]
>>         dec     rdx
>>         xor     eax, eax
>>         cmp     rdx, rdi
>>         setae   al
>>         neg     rsi
>>         and     rdx, rsi
>>         ret
>
> The conditional move instruction still seems worse as it breaks data
> dependencies in the cpu pipeline, right?

Oh yeah, it is definitely worse, but I think on an acceptable level. I would argue the cmove is a non-issue, because it is a reg<->reg move, but... meh, not sure I wanna dig deeper.

David