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

[Dave Froble]

On 9/23/2021 10:10 AM, chris wrote:

> So what is the issue here ?. Keywords like volatile would not normally
> ever be used at app level, being reserved for low level kernel and
> driver code where it touches real hardware registers. or perhaps
> memory locations reserved for a specific purpose. The 4th edition of
> Harbison & Steele C reference manual has 2 or 3 pages devoted to the
> volatile keyword and might be worth looking at; Section 4.4.5. The
> whole point of a black box kernel is to isolate the internal workings
> from an application. Thinking layers, of course.
>
> If systems and code are designed and written properly, then it should
> compile to usable code irrespective of compiler, including
> optimisation level, so long as the compiler is standards compliant.
> Anything else is s system design issue...

If your code was executed as you intended, then there should not be any 
issue, as you mention.

But what if your code is NOT executed as you intended.  An optimizer 
just might figure that it doesn't need to execute some instructions, 
that they are redundant.  They do that.  However, if they are not 
redundant, then results may not be as expected.

So, one might consider "volatile" (or whatever else is used) as an edict 
to "don't optimize".

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