Re: Precise GC

[Jim Blandy <jimb@red-bean.com>]

> There was some talk a while back about why precise GC was too hard
> in a C environment.  I thought I might show people how I did it in a
> little scheme I wrote a while back. It's pretty simple and pretty
> easy to use, but I'll let people more expert comment on it. You
> _don't_ have to use a special macro to assign objects around.
> 
> In a header file I put these definitions...
> #define s_beginGcArea() int s_gcStackPos = s_gcContext.cStack.pos
> #define s_prepGc(v) pushStackAutoResize(&s_gcContext.cStack, &(v))
> #define s_defGc(type, name, init) type name = (type)(s_prepGc(name), init)
> #define s_endGcArea() popStackTo(&s_gcContext.cStack, s_gcStackPos)

This is basically what Emacs does.  I found that we frequently forgot
to do the equivalent of s_defGc when it was necessary.  In your
example, it's easy to see that rtn->length is protected, since rtn is
protected, but when the function is doing something more complicated
(walking and editing a tree), it got harder.

Don't get me wrong --- it feels nice and simple to me, too.  But it is
a fact that I made a lot of mistakes.  Stallman did too.