Subject: [RFC PATCH] create generic alignment api (v5)

From: [email-masked] (Mathieu Desnoyers)
Date: Sat, 1 May 2010 15:10:45 -0400

* Alexander Shishkin (virtuoso at slind.org) wrote:
> On Sun, Mar 28, 2010 at 08:09:00 -0400, Mathieu Desnoyers wrote:
> > 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()
> 
> Do you plan to have this integrated to the mainline? I'd like to use one of
> these in my next version of the __xchg patch. So, do you need any help or
> feedback or whatnot with this one?

Given that no one seems to oppose to this patch, I think I'll just re-submit it
as-is for real this time.

Thanks,

Mathieu

> 
> > 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.
> > 
> > Signed-off-by: Mathieu Desnoyers <[email-masked]>
> > CC: Russell King - ARM Linux <[email-masked]>
> > Cc: Alexander Shishkin <[email-masked]>,
> > CC: linux-arm-kernel at lists.infradead.org
> > CC: Imre Deak <[email-masked]>
> > CC: Jamie Lokier <[email-masked]>
> > CC: rostedt at goodmis.org
> > CC: mingo at elte.hu
> 
> Acked-by: Alexander Shishkin <[email-masked]>
> 
> > ---
> >  include/linux/kernel.h |   45 +++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 41 insertions(+), 4 deletions(-)
> > 
> > Index: linux-2.6-lttng/include/linux/kernel.h
> > ===================================================================
> > --- linux-2.6-lttng.orig/include/linux/kernel.h	2010-03-27 20:46:07.000000000 -0400
> > +++ linux-2.6-lttng/include/linux/kernel.h	2010-03-28 20:06:27.000000000 -0400
> > @@ -37,10 +37,47 @@ 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 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);
> > +}
> >  
> >  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
> >  
> > 
> > -- 
> > Mathieu Desnoyers
> > Operating System Efficiency R&D Consultant
> > EfficiOS Inc.
> > http://www.efficios.com

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