Re: Modified proposals for Ada-Comment

[Simon Wright <simon@pushface.org>]

Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> writes:

> was defined as Atomic. However, the generated code showed that when a
> bitfield, instead of the record as a whole, was referenced, GNAT used
> ldrb/strb (byte level access instructions) instead of ldr/str (32 bit
> access instructions) when the bitfield was within the first 8 bits
> of the record.

Likewise when the bitfield was in the MS 8 bits. I haven't tried with
bitfields of length greater than 1.

> C.6(15) states:
>
> 	For an atomic object (including an atomic component) all reads and
> 	updates of the object as a whole are indivisible.
>
> There are conflicting opinions about the above rule in comp.lang.ada.
> The words "as a whole" in C.6(15) were used to justify the position
> that access to this single bitfield is not required to be in units of
> the record size which on the surface seemed reasonable.

I think the problem is with the phrase "as a whole". If it means that
partial access need not be indivisible it probably needs to spell it out
better, because that is completely at odds with what I would expect of
"atomic".

> However, other opinions are that C.6(15) does apply when accessing
> this single bitfield.

I think that the "other opinions" are that "as a whole" is a mistake, so
that C.6(15) should say

   For an atomic object (including an atomic component) all reads and
   updates of the object are indivisible.


Aside from these niggles, a useful pair of proposals.