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

[Bob Gezelter]

On Wednesday, September 22, 2021 at 10:00:54 AM UTC-4, abrsvc wrote:
> On Wednesday, September 22, 2021 at 9:12:14 AM UTC-4, xyzz... at gmail.com wrote: 
> > On Wednesday, September 22, 2021 at 9:06:29 AM UTC-4, Arne Vajhøj wrote: 
> > > On 9/22/2021 8:28 AM, Simon Clubley wrote: 
> > > > Jan-Erik's questions about ASTs in COBOL have reminded me about something 
> > > > I asked a while back. 
> > > > 
> > > > VMS I/O and system calls are much more asynchronous than on other operating 
> > > > systems and data can appear in buffers and variables in general can be 
> > > > changed outside of the normal sequence points (such as at a function call 
> > > > boundary). 
> > > > 
> > > > With the move to LLVM, and its different optimiser, have any examples 
> > > > appeared in VMS code for x86-64 where volatile attributes are now required 
> > > > on variable definitions where you would have got away with not using them 
> > > > before (even if technically, they should have been marked as volatile anyway) ? 
> > > > 
> > > > Just curious if there's any places in code running on VMS x86-64 that will 
> > > > need to cleaned up to do things in the correct way that you would have 
> > > > got away with doing less correctly previously. 
> > > To state the obvious. 
> > > 
> > > Correct C aka C code with defined behavior by C standard will work 
> > > on any standard compliant C compiler. 
> > > 
> > > C code with implementation specific or undefined behavior is 
> > > throwing a dice. 
> > > 
> > > Maybe John Reagan has some ideas about what may break, but I cannot see 
> > > VSI systematic document how Itanium to x86-64 migration will impact 
> > > C code with implementation specific or undefined behavior. 
> > > 
> > > Arne 
> > Simon, you asked this exact same question before. You can search back for the guesses. 
> > 
> > Moving architectures can always expose bugs regardless of platform, OS, or compiler. 
> > 
> > GEM has different optimizations on Alpha vs Itanium. Did you have to add any volatiles in that transition? 
> > 
> > Linux people, how many people have code that works with -O0 and -O1 but not with -O3 or-Ofast? 
> > 
> > The current cross-compilers are no-optimize so there is no real world experience for missing volatiles.
> I would also ask why does seemingly every question have a negative bent toward OpenVMS? 
> Why is what LLVM does "correct' where potentially what OpenVMS does buggy? 
> Isn't it possible that a correct sequence on OpenVMS can reveal a bug in LLVM where an implementation is not correct in all cases?

absrv,

Absolutely. Many an optimizer has incorrectly optimized that which should not be optimized.

- Bob Gezelter, http://www.rlgsc.com