More range woes: composed ranges are unsafe to return from functions

[H. S. Teoh]

On Tue, Oct 16, 2012 at 08:44:56PM +0200, monarch_dodra wrote:
> On Tuesday, 16 October 2012 at 17:17:36 UTC, Jonathan M Davis
> wrote:
> >On Tuesday, October 16, 2012 06:40:54 H. S. Teoh wrote:
> >>Notice in the second output range, that the third element is [20,
> >>203] instead of [100, 203], and thereafter a bunch of elements are
> >>skipped, and things just start going haywire after that.
> >>
> >>The only difference between the two is that the first is computed
> >>within main(), and the second is returned from a function call. Does
> >>this mean that it's unsafe to return composed ranges from functions?
> >
> >No. I don't know what's going on, but it sounds like there's a bug
> >somewhere.
> >
> >>I'm trying to think what might be going wrong. Could it be that the
> >>composed ranges are stack-allocated temporaries that go out of scope
> >>upon returning from the function, so the returned range is actually
> >>accessing invalid memory? (That is a really scary thought.)
> >
> >If you have a range over static array, then yes, you will have
> >serious issues if you return a range over it, because the data is
> >going away, but that's not going to happen with a struct or class.
> >The class would be safely on the heap, and the struct will get
> >appropriately copied. But one area that could get hairy if dmd is
> >buggy is if you're using delegates which access the stack frame. It's
> >supposed to work just fine, but it requires that a closure be
> >allocated so that the state of the stack frame is saved (and is
> >therefore valid after the function call has completed). So, things
> >could definitely go funny if there are any bugs in there.

Hmm. There *is* a delegate being passed to map(). Would that cause
problems? Theoretically it shouldn't, but as you said, if dmd isn't
handling it correctly that could cause problems.


> >I have no idea what's going wrong for you here (I'd have to spend
> >time studying exactly what your code is doing), but there's either a
> >bug in your code or a compiler bug which is causing you problems,
> >because aside from slices of static arrays, it's perfectly safe to
> >return stack-allocated stuff functions - including several layers of
> >ranges.

I'm at a loss as to where the bug in my code could be, since I have two
identical copies of the same code, one works, and the other doesn't, and
the only difference between them is that one is returning a range from a
separate function.


[...]
> No idea either, but I'll volunteer to investigate ;)
> 
> I agree with Jonathan though, sounds like a bug somewhere. There is no
> reason for your code to fail.
> 
> I'll post back if I find anything.

Thanks! This problem has been bugging me (har har) all through last
night and this morning.


T

-- 
"Real programmers can write assembly code in any language. :-)" -- Larry Wall