[m-rev.] for review: fix misleading io__report_stats

[Fergus Henderson]

On 30-Apr-2001, Zoltan Somogyi <zs at cs.mu.OZ.AU> wrote:
> On 26-Apr-2001, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> > > runtime/mercury_wrapper.[ch]:
> > > 	Add a global variable that records the size of the zones allocated
> > > 	via Boehm gc since the last call to io__report_stats.
> > > 
> > > runtime/mercury_memory_zones.c:
> > > 	Increment this global variable as appropriate.
> > > 
> > > library/benchmarking.m:
> > > 	Subtract the global variable from the value reported for heap cell
> > > 	allocations, then zero it to prevent double counting.
> > 
> > This fix doesn't work, I'm pretty sure.  E.g. it will do the wrong thing for
> > 
> > 	main -->
> > 		io__report_stats,
> > 		io__report_stats.
> > 
> > and for
> > 
> > 	main -->
> > 		gc__garbage_collect,
> > 		io__report_stats.
> 
> Why? What wrong thing do you think it will do?

Calling io__report_stats twice in a row shoud give the same
answer for "heap allocated since last GC", but because you
zero the global variable in report_stats, it won't.

Calling io__report_stats after gc__garbage_collect should report zero for
"heap allocated since last GC", but because you subtract the global
variable, I think it will report something negative (probably wrapped
around mod 2^32 or 2^64, since it is using unsigned arithmetic, I think).

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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