[m-rev.] diff: reset contains_trace_goal if necessary

[Ian MacLarty]

On Mon, Nov 06, 2006 at 06:19:57PM +1100, Zoltan Somogyi wrote:
> compiler/simplify.m:
> 	Finish something I left undone when I added trace goals. When a trace
> 	goal is deleted from the HLDS, the goals that used to contain it still
> 	have a feature that says they contain a trace goal, which inhibits
> 	their optimization (since in some respects they are treated as if they
> 	were impure). This diff resets that feature for those goals.
> 

Is the behaviour of not optimising away code that contains trace goals wise?
This behaviour might mean that the act of adding a trace goal to try and
diagnose a bug may cause the bug to disappear.  Consider a bug involving
a false purity promise where the promised pure code is optimised away
by the compiler.  Now suppose I want to test if the promised pure code
is being run in my application.  I add a trace goal and run my program.
Now all of a sudden my program does the correct thing, because the
code that was falsly promised pure is now run.

This behaviour also means trace goals are not useful for determining how
many times a goal was run in optimised code, something that might be
useful for performance profiling.

I think you should remove the feature that prevents code containing
trace goals from being optimised away.  If I add a trace goal then I
only want to see the effects of the trace goal if the conjunct it
appears is run under normal circumstances and I definitely don't want
the behaviour of my program to change, besides the extra output
generated by the trace goals.

Ian.
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