Proper tail calls would help sometimes, but not always.
For instance, using a fictional LazyList class,
int foo() {
LazyList xs = LazyList.replicate(10000000, new Object());
int x = LazyList.length(xs); // this call fully evaluates the tails
of xs
return 2 + x;
}
In this case, LazyList.length is not a tail call, but it is the
problematic call, since xs will be referenced throughout the call.
Note also that the JIT will compile LazyList.length, since its loop
will execute often enough, but it will not compile foo(), which is the
method retaining the unneeded reference to xs.
- Malcolm
On Oct 30, 1:29 am, Ingo Wechsung <[EMAIL PROTECTED]> wrote:
> On 29 Okt., 19:37, Malcolm Sharpe <[EMAIL PROTECTED]> wrote:
>
>
>
> > Hi,
>
> > We've encountered an interesting Java memory usage problem that occurs
> > when using lazily-constructed data structures. The problem is that a
> > single data structure, such as a list, may not fit into memory all at
> > once, so it's important to not hold a reference to the parts that are
> > no longer in use. Unfortunately, Sun's Java GC follows local variables
> > even when they can be statically known to be no longer in use, such as
> > after they are passed to a function call and never again referenced.
> > This only seems to be the case with the bytecode interpreter, but an
> > out-of-memory error can occur well before the affected code is JIT-
> > compiled.
>
> > This issue is most common in lazy languages implemented on the JVM,
> > such as our language, CAL, but it affects any JVM program using lazy
> > data structures.
>
> Right.
> This is also a problem brought about by the inability of the JVM to do
> tail calls. Proper tail calls would null out the local variables of
> the caller automatically, instantly and cheaply.
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