builtins.cache: Cache regular expressions

The evaluator was spending about 1% of its time compiling a small
number of regexes over and over again.

2 files changed, 9 insertions, 3 deletions

diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index cabc92d15..34a212aa4 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -114,6 +114,9 @@ private:
     /* Cache used by checkSourcePath(). */
     std::unordered_map<Path, Path> resolvedPaths;
 
+    /* Cache used by prim_match(). */
+    std::unordered_map<std::string, std::regex> regexCache;
+
 public:
 
     EvalState(const Strings & _searchPath, ref<Store> store);
@@ -314,6 +317,7 @@ private:
     friend struct ExprOpConcatLists;
     friend struct ExprSelect;
     friend void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v);
+    friend void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v);
 };
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 29302c9b6..4cd28698c 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1811,19 +1811,21 @@ static void prim_hashString(EvalState & state, const Pos & pos, Value * * args,
 
 /* Match a regular expression against a string and return either
    ‘null’ or a list containing substring matches. */
-static void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
+void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     auto re = state.forceStringNoCtx(*args[0], pos);
 
     try {
 
-        std::regex regex(re, std::regex::extended);
+        auto regex = state.regexCache.find(re);
+        if (regex == state.regexCache.end())
+            regex = state.regexCache.emplace(re, std::regex(re, std::regex::extended)).first;
 
         PathSet context;
         const std::string str = state.forceString(*args[1], context, pos);
 
         std::smatch match;
-        if (!std::regex_match(str, match, regex)) {
+        if (!std::regex_match(str, match, regex->second)) {
             mkNull(v);
             return;
         }