atomic accesses

[Segher Boessenkool]

>>>> The Linux kernel, and probably some user-space applications and 
>>>> libraries
>>>> as well, depend on GCC guaranteeing (a variant of) the following:
>>>> 	"any access to a naturally aligned scalar object in memory
>>>> 	that is not a bit-field will be performed by a single machine
>>>> 	instruction whenever possible"
>>>> and it seems the current compiler actually does work like this.
>>> Seems a pity to have the bit-field exception here, why is it there?
>> Bit-fields will generally require a read-modify-write instruction,
>> and I don't think we actually guarantee to generate one right now.
>
> Well if they do require more than one instruction, the rule has
> no effect ("whenever possible"). If they can be done in one
> instruction  (as on the x86), then why not require this, why
> make a special case?

Because current GCC doesn't work like this AFAIK.  I'm aiming for
a documentation-only change here, we can always extend it later.


Segher