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

[David Rheinsberg]

Hi Alice!

On Fri, Mar 20, 2026, at 10:47 AM, Alice Ryhl wrote:
> On Fri, Mar 20, 2026 at 10:27 AM David Rheinsberg <david@xxxxxxxxxxxx> wrote:
>> Kinda off-topic: why doesn't `Alignment` store a u8 that represents the exponent, rather than the power? The left-shift when needing the power should be effectively free, shouldn't it? It would avoid all the unsafety in the impl.
>
> For one, it mirrors the design of the unstable stdlib Alignment type.
> For another, it'd make Alignment::of and similar a pain to implement.

Fair enough!

>> Do you need this helper then at all? I assume it is added because `Alignable` cannot be used in const. But it hard-codes `usize` as type, yet does not reflect that in the name. It comes down to which one is more readable, I guess:
>>
>>     const_align_up(value, align)
>> vs
>>     value.checked_next_multiple_of(align.as_usize())
>>
>> Meh, I don't mind too much. Just wanted to point out that the standard library provides this exactly.
>
> The stdlib implementation probably invokes an expensive division
> operation in place of our bitwise-and operation when the argument is
> not a compile-time constant value.

Fortunately, it does not! Yay! `Alignment::as_nonzero()` has enough annotations to produce competitive assembly. If you care, see my example on x86_64 below, which uses:

align_up1: v.checked_next_multiple_of(align.as_nonzero())
align_up2: const_align_up(v, align)

Thanks
David


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