[Fwd: Uninitialized stack gaps and conservative garbage collection]

Wed May 25 14:18:00 GMT 2005

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: 2005年5月24日 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