On Mon, 20 Jun 2011, Nicolas Pitre wrote:
> Any usage of __packed is potentially making the code less optimal than
> it could, depending on the actual layout of the structure where this is
> applied, because outside of the IO accessor context, the compiler would
> use less than optimal instructions when accessing the structure members.
>
> If what you have is:
>
> struct foo {
> u8 c;
> u32 d;
> u8 e;
> };
>
> If you need that structure to be packed then so be it and nothing else
> can be done about it.
>
> However if you have:
>
> struct foo {
> u32 c;
> u64 d;
> u32 e;
> };
>
> Here the d member is not naturally aligned. On most architectures,
> including ARM with the ABI currently in use, the compiler would insert a
> 32-bit padding between c and d. If you must prevent that from happening
> then you'll mark this struct with __packed. However that will also mark
> it as having an alignment of 1, meaning that all accesses to this
> structure will be done byte by byte and the resulting values
> reconstructed with shifts and ORs.
Agreed.
> Whar ARnd is talking about is _only_ about the IO accessor on ARM which
> behavior changed with newer GCC versions. Changing the IO accessor
> implementation will fix the byte sized access issue to the hardware, but
> the rest of the code will still suck even if it will work correctly.
>
> By adding the aligned(4) attribute here, you're telling the compiler
> that despite the packing attribute, it may assume that the structure is
> still aligned on a 32-bit boundary (which is normally true except if you
> cast a random pointer to this struct of course) and therefore it can
> still use 32-bit sized accesses, and the u64 member will be correctly
> accessed using a pair of 32-bit accesses instead of 8 byte sized
> accesses.
>
> So this is a matter of being intelligent with those attributes and not
> stamping them freely just because a structure might be mapped to some
> hardware representation. In most cases, the packed attribute is just
> unneeded.
Again, agreed. The current code does not have the packed attribute.
> > As far as I can tell, the other structures in ehci.h have
> > ((aligned(32)) simply in order to save space, since there can be large
> > numbers of these structures allocated.
>
> How can increasing the alignment to 32 bytes save space?
No, no -- the alignment is _decreased_ to 32 bits. Without the
attribute the alignment would have been 64 bits.
> Usually a greater alignment is used to ensure proper mapping to CPU
> cache line boundaries, not to save space.
Irrelevant to the point I was making.
Alan Stern
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