[tom_]

And one of the anythings permitted would be to behave in a documented manner characteristic of the target environment. The program is after all almost certainly being built to run on an actual machine; if you know what that actual machine does, it would sometimes be useful to be able to take advantage of that. We might not be able to demand this on the basis that the standard requires it, but as a quality of implementation issue I think it a reasonable request.

This is such an obvious thing to do that I'm surprised the C standard doesn't include wording along those lines to accommodate it. But I suppose even if it did, people would just ignore it.

[robinsonb5]

The problem is that what the machine does isn't necessarily consistent. If you're using old-as-the-green-hills integer instructions then yes, the CPU supports unaligned access. If you want to benefit from the speedup afforded by the latest vector instructions, now it suddenly it doesn't.

Also, to be fair, GCC does appear to back off the optimisations when dealing with, for example, a struct with the packed attribute.

[saagarjha]

That depends on which vector instructions you use.