Re: memcpy to an unaligned address

[Shaun Jackman <sjackman at gmail dot com>]

On 8/4/05, Carl Whitwell <carl.whitwell@gmail.com> wrote:
> Hi, 
>     thought I'd drop you a mail, would put it on gcc mailing list but
> haven't got time to work out how to send it there at this moment. 

The gcc mailing list is gcc@sources.redhat.com.
   
> All testing here is done on x86 processors using gcc under cygwin. 

Are you using an x86 host and an arm target?

> Testing gcc 3.3.4 showed no problems with memcpy alignment.  Using your
> example, the structure s was aligned but the member variable b was
> unaligned. The assembler code produced for the direct copy s->a = p and the
> memcpy replacement were identical. 

Did it produce an open code memcpy, and was it correct?

> Testing gcc 4.0.1 I found the structure s was unaligned such that the member
> variable b was aligned, which was odd. 
> I discovered that the structure appears to be aligned based upon the natural
> alignment of the last element within that structure. 
> Of course this means that the memcpy is now acting on an aligned member
> rather than an unaligned member, and works perfectly well. 

Interesting. Can you post an assembler snippet of this?

> This means that to cause an unaligned element within the structure I had to
> add another element to the structure and retest. 
> On doing this though I found gcc appeared to correctly replace memcpy with
> an unaligned copy. 
>   
> I completely agree that gcc should be handling all the alignment issues
> here, but I'm not sure what it thinks it's doing moving the structure about
> in 4.0.1 
> It may be worth tracking the addresses of the members in all the tests to
> make sure the tests are comparable across gcc versions. 
>   
> Regards, 
> Carl Whitwell 

Cheers,
Shaun