atomic accesses

[Robert Dewar]

Segher Boessenkool wrote:
>>>>> 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.

Fair enough, we don't want to document something we don't do!

Does this rule extend to the use of floating-point instructions
to guarantee atomic access to 64-bit long_long_integer, as
written it does!
> 
> 
> Segher