[Simh] clang (was Re: XCode and LTO)

[Mark Pizzolato - Info Comm]

On Friday, April 27, 2012, at 1:15 PM, Craig A. Berry wrote:
> On Apr 27, 2012, at 2:52 PM, Mark Pizzolato - Info Comm wrote:
> 
> > On Friday, April 27, 2012, at 10:44 AM, Sergey Oboguev wrote:
> >> Declaring "loopval" as volatile (and perhaps global, rather than
> >> on-stack, to reduce chances of compiler disregarding volatile
> >> declaration) might do the trick.
> >
> > The goal really isn't to cause a memory reference, but really to avoid the
> inlining on this specific function.  We're trying to do enough 'spinning' to slow
> things down predictably here.
> 
> Doesn't it amount to the same thing?  If the function gets inlined, then a
> subsequent optimization step may realize loopval never changes and
> eliminate even the inlined function calls.  Preventing inlining prevents that.
> But making loopval volatile is like saying it's a device register or something
> that can be changed from outside of the program, right?  So that also
> prevents it from optimizing out references to loopval (even if the function it's
> passed to gets inlined) since the compiler doesn't know when it might
> change.  This is speculative as I haven't tried to analyze any assembler listings
> (and they don't always reflect exactly what the optimizer did), but I suspect
> that's the gist of it.

I guess, in the end it either way is OK (as long as the variable is global as Sergey suggested).  I'm checking in that version.
 
> In other words, I'm not sure if it matters much which way we go, especially
> since I *think* this code is just involved with spinning up the virtual ROM and
> doesn't do much after the simulator is up and running.

It's only purpose is to allow the ROM diagnostics to pass which will then let the
ROM believe that the hardware is in good enough shape to allow the system to boot.
If a powerup diag fails, the ROM won't allow a boot.

- Mark