recursive evals now share the same stack.

since in a minimal chibi heap the stack accounts for a large
amount of the space, this makes a big difference - you can
now load (chibi match) in a 2MB heap on a 64-bit system and
it won't grow the heap.

TODO

@@ -9,11 +9,8 @@
    - State "DONE"       [2009-04-09 Thu 14:45]
 ** TODO native x86 backend
 ** TODO fasl/image files
-** TODO shared stack on EVAL
-   Arguably a bug, at the moment we create a new stack on every EVAL
-   (which includes every macro definition, and in particular every
-   call to let-syntax that a macro may expand into - I'm looking at
-   you, (chibi loop)).
+** DONE shared stack on EVAL
+   - State "DONE"       [2009-12-26 Sat 08:22]
 
 * compiler optimizations
 ** DONE constant folding

eval.c

@@ -2653,13 +2653,18 @@ sexp sexp_compile (sexp ctx, sexp x) {
 }
 
 sexp sexp_eval (sexp ctx, sexp obj, sexp env) {
+  sexp_sint_t top;
   sexp ctx2;
   sexp_gc_var1(res);
   sexp_gc_preserve1(ctx, res);
-  ctx2 = sexp_make_eval_context(ctx, NULL, (env ? env : sexp_context_env(ctx)));
+  top = sexp_context_top(ctx);
+  ctx2 = sexp_make_eval_context(ctx,
+                                sexp_context_stack(ctx),
+                                (env ? env : sexp_context_env(ctx)));
   res = sexp_compile(ctx2, obj);
   if (! sexp_exceptionp(res))
     res = sexp_apply(ctx2, res, SEXP_NULL);
+  sexp_context_top(ctx) = top;
   sexp_gc_release1(ctx);
   return res;
 }