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SubjectRe: [RFC PATCH 1/6] kenrel.h: add ALIGN_OF_LAST_BIT()
On Tue, 20 Mar 2012 12:32:14 +0100
Michal Nazarewicz <mina86@mina86.com> wrote:

> >+#define ALIGN_OF_LAST_BIT(x) ((((x)^((x) - 1))>>1) + 1)
>
> Wouldn't ALIGNMENT() be less confusing? After all, that's what this
> macro is calculating, right? Alignment of given address.

Why not just LAST_BIT(x)? It's not particularly specific to pointer
alignment, even though that's the context in which it apparently came
up. So far as I can tell, this isn't even meaningfully defined on
pointer types as such; you'd have to convert. So the implications for
alignment seem a convenient side-effect, really.

It might be instructive to see some example proposed uses; the question
of why I'd care what alignment something had, rather than whether it
was aligned for a given type, is one that will doubtless keep me awake
nights.

I guess this feels like it answers a question that is usually the wrong
question. Imagine if you will a couple-page block of memory, full of
unsigned shorts. Iterate through the array, calculating
ALIGN_OF_LAST_BIT(&a[i]). Do we really *care* that it's PAGE_SIZE for
some i, and 2 (I assume) for other i, and PAGE_SIZE*2 for either i==0 or
i==PAGE_SIZE? (Apologies if this is a silly question; maybe this is
such a commonly-needed feature that it's obvious.)

-s
--
Listen, get this. Nobody with a good compiler needs to be justified.


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