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

[Simon Clubley]

On 2021-09-22, Jan-Erik Söderholm <jan-erik.soderholm at telia.com> wrote:
> Den 2021-09-23 kl. 02:07, skrev Arne Vajhøj:
>> 
>> But instead of spreading out volatile keyword wouldn't
>> __MB() do the same? (on VMS - it is a VMS C specific thing
>> I believe)
>> 
>
> Sorry. Seems to be something about a "memory barrier". I don't know
> what that is in this context and if it works as an volatile.

Assuming __MB() is a hardware memory barrier operation, the answer is no.

Memory barrier instructions are designed to get bits of hardware back
into sync with each other so that when you read something it is valid.

Volatile OTOH is a purely software construct and is used to tell the
compiler to insert bits of code into the generated code to _always_
first re-read the memory location even if the compiler thinks from
looking at the source code that the value could not have changed.

There's no point getting the hardware back into sync, if the generated
code is missing the bit to then unconditionally read that value again
before doing something with the variable.

Even if memory barriers could be made to so the same thing somehow
by having the compiler look for them, they would have to be inserted
within the executable code. Volatile however is a variable definition
attribute so only ever appears in the source code when the volatile
variables are initially defined.

Simon.

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