Re: [PATCH] Introduce compat_u64 and compat_s64 types

[Arnd Bergmann <arnd@arndb.de>]

On Saturday 16 June 2007, David Woodhouse wrote:
> Will GCC know that it needs to emit code to handle that (mis)alignment?

I've tested this with gcc-4.0.3, and it does the right thing, which
is to split a 4 byte aligned 64 bit load/store into two 32 bit accesses,
if you pass -mstrict-align.

> Preliminary tests show that it does load the value bytewise if we use
> the 'packed' attribute structure on ppc64, but doesn't if we use
> compat_u64. But then, I don't think it actually _needs_ to handle it n
> ppc64 anyway, so maybe that's not such a good test.

Right. Note that the behaviour of compat_u64 is the same as when you
pass attribute((packed,aligned(4))) to the structure, where it also won't
do byte accesses, and split the 64 bit access only if you pass
-mstrict-align.

	Arnd <><