Re: [Fwd: Uninitialized stack gaps and conservative garbage collection]

[Bryce McKinlay <mckinlay at redhat dot com>]

Ranjit Mathew wrote:

> Interesting stuff found on the GCC list. Doesn't this
> affect GCJ as well where we use the conservative
> Boehm GC?
>  
>
> -------- Original Message --------
> Subject: Uninitialized stack gaps and conservative garbage collection
> Date: Tue, 24 May 2005 17:40:46 -0400
> From: Camm Maguire <camm@enhanced.com>
> Newsgroups: gmane.comp.gcc.devel,gmane.lisp.gcl.devel
>
> Greetings!  GCL is a lisp system that compiles to native object code
> via the intermediary of gcc.  It uses a conservative garbage
> collection algorithm, which means that it walks the C stack to find
> likely objects in use by automatic variables, and holds on to these.
> This works quite well in practice.
>
> For very large systems, the likelihood of holding onto objects which
> should be collected increases.  In looking into this, it has come to
> my attention that regardless of how carefully the C programmer
> initializes variables on the stack, gcc will quite commonly allocate
> extra space inaccessbile via any C variable.  These 'stack gaps' can
> wind up permanently preventing a large portion of user memory from
> ever being collected with this algorithm.


Sure, with a conservative GC there is always the possibility of 
uninitialized "junk" on the stack being mis-identified as a valid 
pointer. There's even the possibility of live non-pointer data, such as 
an int, being mis-identified as a pointer. Hopefully this is rare in 
GCJ, however, as the GC only considers pointers to the start of an 
object to be valid - interior pointers are disregarded, significantly 
reducing the chance of a mis-identification. On a 64-bit system, due to 
the larger address space, the chance of this happening reduces even further.

Bryce