From 2d663b502da16d6dc480bff7f58297d176d04246 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Sat, 6 Aug 2011 19:45:43 +0000
Subject: [PATCH] =?UTF-8?q?*=20Cache=20the=20result=20of=20file=20evaluati?=
 =?UTF-8?q?on=20(i.e,=20memoize=20evalFile()).=20=20This=20=20=20prevents?=
 =?UTF-8?q?=20files=20from=20being=20evaluated=20and=20stored=20as=20value?=
 =?UTF-8?q?s=20multiple=20=20=20times.=20=20For=20instance,=20evaluation?=
 =?UTF-8?q?=20of=20the=20=E2=80=98system=E2=80=99=20attribute=20in=20NixOS?=
 =?UTF-8?q?=20=20=20causes=20=E2=80=98nixpkgs/pkgs/lib/lists.nix=E2=80=99?=
 =?UTF-8?q?=20to=20be=20evaluated=202019=20times.?=
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  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).
---
 src/libexpr/eval.cc | 24 +++++++++++++-----------
 src/libexpr/eval.hh |  9 +++++++++
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 5aee3f05e..d0bdaf238 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 413234f2b..694d4407b 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;