restore module cache

The module cache was added in 97ce26b1 (April 16, 2011),
but it was accidentally disabled in e9721058 (May 5, 2011).

This time, I figured out a way to test whether the cache is
working (other than to benchmark examples, which is how I
discovered that it wasn't working).

collects/tests/racket/namespac.rktl

@@ -202,6 +202,31 @@
       (kill-thread th)))
   (test #t namespace? (make-base-empty-namespace)))
 
+;; ----------------------------------------
+;; Check module caching by monitoring `current-eval'.
+;; If the module cache works, then it turns out that
+;; the `module-compiled-imports' of modules to evaluate
+;; will be `eq?' the second time around to the first time
+;; around. This is a fragile and imprecise check, but it's
+;; the best idea we have to checking that the module cache
+;; works.
+
+(let ([codes (make-hash)])
+  (define (go-once)
+    (parameterize ([current-namespace (make-base-namespace)]
+                   [current-eval
+                    (let ([orig (current-eval)])
+                      (lambda (x)
+                        (when (syntax? x)
+                          (when (compiled-module-expression? (syntax-e x))
+                            (hash-set! codes (module-compiled-imports (syntax-e x)) #t)))
+                        (orig x)))])
+      (dynamic-require 'racket/string #f)))
+  (go-once)
+  (let ([pre (hash-count codes)])
+    (go-once)
+    (test pre hash-count codes)))
+
 ;; ----------------------------------------
 
 (report-errs)

src/racket/src/module.c

@@ -5334,7 +5334,7 @@ static Scheme_Object *do_module_execute(Scheme_Object *data, Scheme_Env *genv, i
 
 Scheme_Object *scheme_module_execute(Scheme_Object *data, Scheme_Env *genv)
 {
-  return do_module_execute(data, genv, 0);
+  return do_module_execute(data, genv, 1);
 }
 
 static Scheme_Object *rebuild_et_vec(Scheme_Object *naya, Scheme_Object *vec, Resolve_Prefix *rp)