[m-dev.] [reuse] fixes of some naughty bugs

[Fergus Henderson]

On 12-Mar-2001, Peter Schachte <schachte at cs.mu.OZ.AU> wrote:
> Suppose we defined p/1 as:
> 
> 	p(Term) :- 
> 		Term = f(X, Y),
> 		X = Y,
> 		destructively update first argument of Term to be Y.
> 
> If X and Y are big, this will free up a lot of space at the next GC.  But
> Mercury compilers are allowed to reorder conjunctions, so there's no way
> for a programmer to write
> 
> 	p(X), X = f(Y,_), q(Y)
> 
> and be confident the X = f(Y,_) won't be moved before the call to p/1,
> thereby screwing up the GC possibility.

The programmer can always use --no-reorder-conj or its equivalent.
Or they can use if-then-else to force sequential evaluation in just
this particular spot.  (Yes, a more syntactically convenient way of
forcing sequential conjunction would be nice.)

But normally the programmer should just trust the compiler to do
a reasonably good job.  If reordering X = f(Y,_) to before the call to p/1
is going to slow the program down, then the compiler shouldn't do it.

Sure, the compiler *might* do it.  But the compiler might also decide
to allocate every floating point value on a misaligned address that
causes a trap to the OS for every floating point access, or to generate
code for "hello world" that links in a 2 gigabyte Java library that
gets dynamically loaded from a web site in outer Mongolia via a 300-baud
modem.  At some point you have to rely on quality of implementation.

-- 
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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