Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH] Introduce compat_u64 and compat_s64 types | Date | Sat, 16 Jun 2007 12:26:40 +0200 |
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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.
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