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Subject: Re: [RFC PATCH 01/20] Create generic alignment API (v8)
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: LKML <linux-kernel@vger.kernel.org>, ltt-dev@lists.casi.polymtl.ca,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
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        Frederic Weisbecker <fweisbec@gmail.com>,
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        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
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Date: Tue, 17 Aug 2010 21:25:26 -0400
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On Tue, 2010-08-17 at 19:16 -0400, Mathieu Desnoyers wrote:

> +/*
> + * Align pointer on natural object alignment. Object size must be power of two.
> + */

Hmm, I wonder if we should add a compiler bug here too.

extern void __bug_obj_not_power_of_two(void);

({
	if ((sizeof(*obj) - 1) & sizeof(*obj))
		__bug_obj_not_power_of_two();
	PTR_ALIGN((obj), __alignof__(*(obj)));
})

> +#define object_align(obj)	PTR_ALIGN((obj), __alignof__(*(obj)))
> +#define object_align_floor(obj)	PTR_ALIGN_FLOOR((obj), __alignof__(*(obj)))
> +
> +/**
> + * offset_align - Calculate the offset needed to align an object on its natural
> + *                alignment towards higher addresses.
> + * @align_drift:  object offset from an "alignment"-aligned address.
> + * @alignment:    natural object alignment. Must be non-zero, power of 2.
> + *
> + * Returns the offset that must be added to align towards higher
> + * addresses.
> + */
> +static inline size_t offset_align(size_t align_drift, size_t alignment)
> +{
> +	return (alignment - align_drift) & (alignment - 1);
> +}
> +
> +/**
> + * offset_align_floor - Calculate the offset needed to align an object
> + *                      on its natural alignment towards lower addresses.
> + * @align_drift:  object offset from an "alignment"-aligned address.
> + * @alignment:    natural object alignment. Must be non-zero, power of 2.
> + *
> + * Returns the offset that must be substracted to align towards lower addresses.
> + */
> +static inline size_t offset_align_floor(size_t align_drift, size_t alignment)
> +{
> +	return (align_drift - alignment) & (alignment - 1);
> +}

I take it that theses functions can have variables passed to it for
alignment, thus a check wont help. Although, we could add a test for the
constant case.

-- Steve

> +