derivations: 'build-expression->derivation' caches its module derivations.

This reduces the number of lookups in the 'add-data-to-store' cache from
7505 to 3329 (hit rate from 68% to 27%) when running:

  GUIX_PROFILING=add-data-to-store-cache guix build libreoffice -nd

The execution time of "guix build libreoffice -nd" goes from 2.12s to 1.87s.

* guix/derivations.scm (%module-cache): New variable.
(imported+compiled-modules)[key]: New variable.
Lookup KEY in %MODULE-CACHE and populate %MODULE-CACHE upon cache miss.

1 files changed, 14 insertions, 2 deletions

diff --git a/guix/derivations.scm b/guix/derivations.scm
index 8309f845d9..140c22b620 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -1207,13 +1207,25 @@ they can refer to each other."
                                   #:guile-for-build guile
                                   #:local-build? #t)))
 
+(define %module-cache
+  ;; Map a list of modules to its 'imported+compiled-modules' result.
+  (make-weak-value-hash-table))
+
 (define* (imported+compiled-modules store modules #:key
                                     (system (%current-system))
                                     (guile (%guile-for-build)))
   "Return a pair containing the derivation to import MODULES and that where
 MODULES are compiled."
-  (cons (%imported-modules store modules #:system system #:guile guile)
-        (%compiled-modules store modules #:system system #:guile guile)))
+  (define key
+    (list modules (derivation-file-name guile) system))
+
+  (or (hash-ref %module-cache key)
+      (let ((result (cons (%imported-modules store modules
+                                             #:system system #:guile guile)
+                          (%compiled-modules store modules
+                                             #:system system #:guile guile))))
+        (hash-set! %module-cache key result)
+        result)))
 
 (define* (build-expression->derivation store name exp ;deprecated
                                        #:key