[Info-vax] yet another sys$qiow question

[Stephen Hoffman]

On 2015-08-21 20:32:00 +0000, Bob Gezelter said:

> I do not recall what the C standard says on the subject offhand, but in 
> general, one must presume when generating code that variables in a 
> global scope are subject to change when invoking an out-of-line 
> subroutine (This was the reasoning behind my previous comments that an 
> out-of-line invocation ends a basic block).

AFAICT, the salient difference here is that the variables involved in 
this case can change after the call has returned; when the variable is 
otherwise apparently and fully back in scope.  This implies that the 
compiler either has to assume that any variable that gets passed by 
reference and that goes out of scope might then change at any time 
(probably playing havoc with performance, at least for some code), or 
it has to figure out which variables can change (involving entirely 
non-trivial heuristics, involving the detection of the IOSB, LKSB, 
EVSB, etc), or the variables involved have to be marked volatile or 
some platform-specific equivalent.  Or we have to assume (hope?) that 
the compiler doesn't have enough registers and won't try to cache the 
IOSB, LKSB, EVSB, etc.  Fun times...


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