* Steven Rostedt (rost...@goodmis.org) wrote: > 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))); > })
[...] Nevermind the "Object size must be power of two." part of the comment. That was true when I first implemented the patch, where I used sizeof(), e.g.: #define object_align(obj) PTR_ALIGN(obj, sizeof(*(obj))) #define object_align_floor(obj) PTR_ALIGN_FLOOR(obj, sizeof(*(obj))) But given that we can expect structures and various objects to be passed as parameters, __alignof__ is more appropriate. This is what this patch now implements: #define object_align(obj) PTR_ALIGN(obj, __alignof__(*(obj))) #define object_align_floor(obj) PTR_ALIGN_FLOOR(obj, __alignof__(*(obj))) so we let gcc determine the appropriate alignment. It will automatically be a power of two. I'll just update the comment. [...] > 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. That's right. I could add something like: MAYBE_BUILD_BUG_ON_NOT_POWER_OF_2(alignment); which will add a dependency on kernel.h (for MAYBE_BUILD_BUG_ON) and log2.h (for is_power_of_2). I'll move the #include <linux/align.h> further down in kernel.h, keeping it outside of #ifdef __KERNEL__. To make sure this build test works when a constant is passed to offset_align or offset_align_floor, I'll have to turn them into macros instead of static inlines. The next round will have a patch that adds "MAYBE_BUILD_BUG_ON_NOT_POWER_OF_2" to kernel.h. Thanks, Mathieu -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com _______________________________________________ ltt-dev mailing list ltt-dev@lists.casi.polymtl.ca http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev