[fedora-arm] Broken sha512sum in coreutils

[Andrew Haley]

On 01/07/2011 08:13 PM, Andy Green wrote:
> On 01/07/11 18:45, Somebody in the thread at some point said:
>
>>> With this issue, a correct structure in a typedef or a struct with
>>> correct alignment padding turns to crap because the structure pointed to
>>> is not on a u32 boundary for example.
>>
>> I see. But shouldn't the compiler be taking care of that? -malign?
>
> For most code the compiler or malloc is ultimately always making the
> pointer and takes care about its alignment so you never see such symptoms.
>
> However take for example structures that are coming into a buffer as
> part of a byte stream, it is uncontrolled how many bytes have come in,
> but you know there is a struct in the buffer n bytes from the start of
> your buffer.  Like -->
>
> struct mystruct *p = (struct mystruct *)&buffer[n];
>
> On x86 this code is always OK, although it may run faster or slower
> depending on alignment.

Yes, but in C code this is always a bug, and should always be fixed.
Even on x86 it may not always generate code that does what you want:
gcc is perfectly entitled to assume that an unaligned pointer is
never used.  And even now, x86 has a big performance hit for word
accesses that straddle a cache line boundary.

Andrew.