Michael S (already5chosen@yahoo.com) on 1/12/11 wrote:
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>Wilco (Wilco.Dijkstra@ntlworld.com) on 1/12/11 wrote:
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>[If the rumors are right and Microsoft indeed finishing porting of their Nt-based
>line to ARM], I see close to zero chance that they will set SCTLR.A=1.

Indeed. Note booting NT on ARM has been a compiler checkin requirement for a long time...

>>So an ARM compiler will use LDM or LDRD
>>only if the access is guaranteed (by the ABI) to be least 4-byte aligned. For badly
>>written software which doesn't understand alignment at all, I added a mode in the
>>ARM compiler which assumes all accesses are unaligned.
>>
>>Wilco
>
>Why do you call it badly written?
>In my book the software does not have to be portable to be considered well-written.

Writing software without considering portability is just madness, it doesn't take much extra effort, and I'd argue that anyone who doesn't understand portability shouldn't really be writing software in the first place. Tomorrow your manager may tell you that your N million line application has been sold - oh and I promised you will deliver the port next month...

>That is, as long as it runs fast and correct within well-defined platform constrains.
>On x86 doing unaligned access is in most cases the best from performance perspective
>so, as long as portability is not part of the spec, the software that in performance-critical
>parts does something else could be considered badly written.

Code with lots of misaligned accesses typically doesn't run fast - not even on x86 as there is always a penalty. There certainly are a few key examples where they improve performance, but apart from those, unaligned accesses don't help much, even there wasn't any penalty.

However you can use unaligned accesses safely and portably. So in cases where it does improve performance or is required to process unaligned data it's perfectly fine. I consider it badly written if the unaligned access isn't explicitly marked - in those cases it is often not only a correctness bug but also a performance hog.

Wilco