Re: ivopts vs. garbage collection

[Jeff Law <law at redhat dot com>]

On 01/06/2016 08:17 AM, Ian Lance Taylor wrote:
> The bug report https://golang.org/issue/13662 describes a case in
> which ivopts appears to be breaking garbage collection for the Go
> compiler.  There is an array allocated in memory, and there is a loop
> over that array.  The ivopts optimization is taking the only pointer
> to the array and subtracting 8 from it before entering the loop.
> There are function calls in between this subtraction and the actual
> use of the array.  During those function calls, a garbage collection
> occurs.  Since the only pointer to the array no longer points to the
> actual memory being used, the array is unexpectedly freed.
>
> That all seems clear enough, although I'm not sure how best to fix it.
> What I'm wondering in particular is whether Java does anything to
> avoid this kind of problem.  I don't see anything obvious.  Thanks for
> any pointers.
The only solution here is for ivopts to keep a pointer to the array, not 
a pointer to some location near, but outside of the array.

Java doesn't do anything special to the best of my knowledge, it just 
relies on the fact that this kind of situation is very rare.

This is related, but not the same as the issue we have with Ada's 
virtual origins for array accesses where the front-end would set up a 
similar situation, which resulted in a "pointer" that points outside the 
object.  When we actually dereference the pointer, it's done so with a 
base+index access which brings the effective address back into the 
object.  That kind of scheme wrecks havoc with segmented targets where 
the segment selection may be from the base register rather than the full 
effective address.

jeff