* Cache the result of file evaluation (i.e, memoize evalFile()). This

  prevents files from being evaluated and stored as values multiple
  times.  For instance, evaluation of the ‘system’ attribute in NixOS
  causes ‘nixpkgs/pkgs/lib/lists.nix’ to be evaluated 2019 times.

  Caching gives a modest speedup and a decent memory footprint
  reduction (e.g., from 1.44s to 1.28s, and from 81 MiB to 59 MiB with
  GC_INITIAL_HEAP_SIZE=100000 on my system).

2 files changed, 22 insertions, 11 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 5aee3f05eaa6..d0bdaf23822a 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -431,17 +431,19 @@ Value * EvalState::maybeThunk(Env & env, Expr * expr)
 
 void EvalState::evalFile(const Path & path, Value & v)
 {
-    startNest(nest, lvlTalkative, format("evaluating file `%1%'") % path);
-
-    Expr * e = parseExprFromFile(path);
-    
-    try {
-        /* !!! Maybe we should cache the evaluation result. */
-        eval(e, v);
-    } catch (Error & e) {
-        addErrorPrefix(e, "while evaluating the file `%1%':\n", path);
-        throw;
-    }
+    FileEvalCache::iterator i = fileEvalCache.find(path);
+    if (i == fileEvalCache.end()) {
+        startNest(nest, lvlTalkative, format("evaluating file `%1%'") % path);
+        Expr * e = parseExprFromFile(path);
+        try {
+            eval(e, v);
+        } catch (Error & e) {
+            addErrorPrefix(e, "while evaluating the file `%1%':\n", path);
+            throw;
+        }
+        fileEvalCache[path] = v;
+    } else
+        v = i->second;
 }
 
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 413234f2bf19..694d4407b847 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -211,8 +211,17 @@ private:
 
     bool allowUnsafeEquality;
 
+    /* A cache from path names to parse trees. */
     std::map<Path, Expr *> parseTrees;
 
+    /* A cache from path names to values. */
+#if HAVE_BOEHMGC
+    typedef std::map<Path, Value, std::less<Path>, gc_allocator<std::pair<const Path, Value> > > FileEvalCache;
+#else
+    typedef std::map<Path, Value> FileEvalCache;
+#endif
+    FileEvalCache fileEvalCache;
+
     typedef list<std::pair<string, Path> > SearchPath;
     SearchPath searchPath;
     SearchPath::iterator searchPathInsertionPoint;