[Gcl-devel] Re: Uninitialized stack gaps and conservative garbage collection

[Raymond Toy]

Camm Maguire wrote:
> 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.
>
> Ideally, I'd like to be able to have each C function carefully
> initialize a contiguous stack to minimize or eliminate this problem.
> Even better would be a gcc switch which would simply zero out each
> stack frame at the beginning of each function, though this could be
> expensive in terms of performance.  Advice is most appreciated.  I'd

Not directly related, perhaps, but CMUCL has a similar problem.  There's 
a control stack that is allocated in the same way with areas on the 
stack not initialized in any way, which can cause garbage to be retained.

On the sparc port, this area can be zeroed out with appropriate 
optimization settings.  I ran some tests using Eric Marsden's cl-bench. 
   If the stack is always cleared, the cost of some benchmarks go up, 
but some go down, because the cost of GC is decreased.  (The benchmarks 
include GC time.)

Anyway, just some random information.

Ray