[PATCH 1/2] create generic alignment api (v6)

[Mathieu Desnoyers]

* Alexander Shishkin (virtuoso at slind.org) wrote:
> From: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> 
> Rather than re-doing the "alignment on a type size" trick all over again at
> different levels, import the "ltt_align" from LTTng into kernel.h and make this
> available to everyone. Renaming to:
> 
> - object_align()
> - object_align_floor()
> - offset_align()
> - offset_align_floor()
> 
> Changelog since v5:
> - moved alignment apis to a separate header file so that it is possible
>   to use them from other header files which are, for example, included
>   from kernel.h.
> 
> Changelog since v4:
> - add missing ( ) around parameters within object_align() and
>   object_align_floor().
> - More coding style cleanups to ALIGN() (checkpatch.pl is happy now).
> 
> Changelog since v3:
> - optimize object_align*() so fewer instructions are needed for alignment of
>   addresses known dynamically. Use the (already existing) "ALIGN()", and create
>   the "ALIGN_FLOOR()" macro.
> - While we are there, let's clean up the ALIGN() macros wrt coding style. e.g.
>   missing parenthesis around the first use of the "x" parameter in ALIGN().
> 
> Changelog since v2:
> - Fix object_align*(): should use object size alignment, not pointer alignment.
> 
> Changelog since v1:
> - Align on the object natural alignment
>     (rather than min(arch word alignment, natural alignment))
> 
> The advantage of separating the API in "object alignment" and "offset alignment"
> is that it gives more freedom to play with offset alignment. Very useful to
> implement a tracer ring-buffer alignment. (hint hint)
> 
> Typical users will use "object alignment", but infrastructures like tracers
> which need to perform alignment of statically known base+offsets will typically
> use "offset alignment", because it allows to align with respect to a base rather
> than to pass an absolute address.
> 
> We use "sizeof(object)" rather than "__alignof__()" object because alignof
> returns "recommended" object alignment for the architecture, which can be
> sub-optimal on some architectures. By ensuring alignment on the object size, we
> are sure to make the right choice.

Hrm... thinking about this again, maybe it's better to use __alignof__() for 
object_align() and object_align_floor(), because using "sizeof()" will cause
unexpected effects if a structure is passed as an object (you typically want to
align on the size of the largest object in the structure (or bigger) rather than
the structure size).

Thanks,

Mathieu

> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> Signed-off-by: Alexander Shishkin <virtuoso at slind.org>
> CC: Russell King - ARM Linux <linux at arm.linux.org.uk>
> CC: linux-arm-kernel at lists.infradead.org
> CC: Imre Deak <imre.deak at nokia.com>
> CC: Jamie Lokier <jamie at shareable.org>
> CC: rostedt at goodmis.org
> CC: mingo at elte.hu
> ---
>  include/linux/align.h  |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/kernel.h |    6 +-----
>  2 files changed, 49 insertions(+), 5 deletions(-)
>  create mode 100644 include/linux/align.h
> 
> diff --git a/include/linux/align.h b/include/linux/align.h
> new file mode 100644
> index 0000000..8aa2967
> --- /dev/null
> +++ b/include/linux/align.h
> @@ -0,0 +1,48 @@
> +#ifndef _LINUX_ALIGN_H
> +#define _LINUX_ALIGN_H
> +
> +#include <linux/types.h>
> +
> +#define ALIGN(x, a)		__ALIGN_MASK((x), (typeof(x)) (a) - 1)
> +#define __ALIGN_MASK(x, mask)	(((x) + (mask)) & ~(mask))
> +#define PTR_ALIGN(p, a)		((typeof(p)) ALIGN((unsigned long) (p), (a)))
> +#define ALIGN_FLOOR(x, a)	__ALIGN_FLOOR_MASK((x), (typeof(x)) (a) - 1)
> +#define __ALIGN_FLOOR_MASK(x, mask)	((x) & ~(mask))
> +#define PTR_ALIGN_FLOOR(p, a) \
> +			((typeof(p)) ALIGN_FLOOR((unsigned long) (p), (a)))
> +#define IS_ALIGNED(x, a)	(((x) & ((typeof(x)) (a) - 1)) == 0)
> +
> +/*
> + * Align pointer on natural object alignment. Object size must be power of two.
> + */
> +#define object_align(obj)	PTR_ALIGN((obj), sizeof(*(obj)))
> +#define object_align_floor(obj)	PTR_ALIGN_FLOOR((obj), sizeof(*(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);
> +}
> +
> +#endif
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index f4e3184..81d0d15 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -12,6 +12,7 @@
>  #include <linux/stddef.h>
>  #include <linux/types.h>
>  #include <linux/compiler.h>
> +#include <linux/align.h>
>  #include <linux/bitops.h>
>  #include <linux/log2.h>
>  #include <linux/typecheck.h>
> @@ -38,11 +39,6 @@ extern const char linux_proc_banner[];
>  
>  #define STACK_MAGIC	0xdeadbeef
>  
> -#define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
> -#define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))
> -#define PTR_ALIGN(p, a)		((typeof(p))ALIGN((unsigned long)(p), (a)))
> -#define IS_ALIGNED(x, a)		(((x) & ((typeof(x))(a) - 1)) == 0)
> -
>  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
>  
>  #define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))
> -- 
> 1.7.1.1.g15764
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com