[Info-vax] LLVM, volatile and async VMS I/O and system calls

[Simon Clubley]

On 2021-09-26, chris <chris-nospam at tridac.net> wrote:
> On 09/25/21 19:46, Simon Clubley wrote:
>>
>> Looking at the generated code has proved interesting at times. :-)
>>
>> The following Ada Issue is a direct result of me looking at someone's
>> problem on comp.lang.ada a number of years ago which was caused by
>> the code the Ada compiler had generated:
>>
>> http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai12s/ai12-0128-1.txt?rev=1.15&raw=N
>>
>> The following AI is also directly related to this:
>>
>> http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai12s/ai12-0127-1.txt?rev=1.27&raw=N
>>
>> [I am _not_ a member of the ARG or anything like that. I am just a
>> normal programmer who includes Ada in the list of languages I know.]
>>
>
> Interesting examples, but just how much of that was due to system
> complexity ?.
>

Not directly in this case IIRC. Ada is about modelling the problem
using a very strict type system and a very strong type system that
allows a problem to be modelled in detail.

However, in this case IIRC, the optimiser simply did something
unexpected when a bitfield within a record instead of the 32-bit
record itself (as would be the case in C) was updated even through
the 32-bit record was marked as Atomic.

As you can see, the suggested changes allow Ada to still be Ada but
also make sure the correct sized memory read and write occurs.

To be honest however, I'm surprised the issue wasn't discovered prior
to the problem report showing up in comp.lang.ada.

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
Walking destinations on a map are further away than they appear.