Messages in this thread | | | Date | Tue, 11 Nov 2008 09:50:57 +0000 | From | "Will Newton" <> | Subject | Re: [RFC-PATCH 1/5] unaligned: introduce common header |
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On Mon, Nov 10, 2008 at 6:51 PM, Harvey Harrison <harvey.harrison@gmail.com> wrote: > On Mon, 2008-11-10 at 18:35 +0000, Will Newton wrote: >> On Mon, Nov 10, 2008 at 4:51 PM, Harvey Harrison >> <harvey.harrison@gmail.com> wrote: >> > >> > In this particular case, packed isn't right as you know big_data is >> > aligned (as long as you can guarantee the struct alignment), so you'd >> > probably want: >> > >> > struct foo { >> > u64 big_data; >> > u8 small_data; >> > u32 medium_data __attribute__((__packed__)); >> > } >> > >> > But that's not what we're talking about in the kernel's case. >> >> Perhaps that would be a neater way of expressing what is required in >> my simple example, but it's fairly common to use packed on the whole >> struct which could be because a field that is "packed" by default on >> one architecture might not be on another. You could mark every field >> as packed but few people seem to do that and as far as I am aware >> there is no documented difference between packing all members and the >> whole struct. The gcc documentation for packed is pretty short: > > Actually it's documented that putting attribute(packed) on the struct > is equivalent to putting attribute(packed) on _every_ member of the > struct. > >> The packed attribute specifies that a variable or structure field >> should have the smallest >> possible alignment—one byte for a variable, and one bit for a field, >> unless you specify a >> larger value with the aligned attribute. >> >> I'd love to know if the pointer alignment behaviour is widespread and >> then maybe write a patch for the gcc manual. > > Well, it's kind of the whole point of __packed isn't it? Otherwise the > struct members get naturally (or some arch-dependent value) aligned, > which the compiler can rely on unless you say __packed. > > So in my example above, the compiler _knows_ how it has aligned > big_data and small_data and can use whatever access is most efficient, > but it can't make any assumptions about medium_data, so access through > a pointer _must_ be done unaligned. > > struct foo *bar; > bar->medium_data; // compiler must do this unaligned
Agreed, but the packed struct unaligned access code does this:
struct __una_u16 { u16 x __attribute__((packed)); };
On an architecture with struct alignment > 1 this will not work. Which is why I believe that the memmove code must stay for those architectures. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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