Add some basic profiling support to the evaluator

Setting the environment variable NIX_COUNT_CALLS to 1 enables some
basic profiling in the evaluator.  It will count calls to functions
and primops as well as evaluations of attributes.

For example, to see where evaluation of a NixOS configuration spends
its time:

$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 ./src/nix-instantiate/nix-instantiate '<nixos>' -A system --readonly-mode
...
calls to 39 primops:
    239532 head
    233962 tail
    191252 hasAttr
...
calls to 1595 functions:
    224157 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:19'
    221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:14'
    221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:10'
...
evaluations of 7088 attributes:
    167377 undefined position
    132459 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:119:41'
     47322 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:13:21'
...

1 files changed, 14 insertions, 2 deletions

diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 5103ae8ce..540f1e200 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -244,9 +244,21 @@ private:
     unsigned long nrAttrsets;
     unsigned long nrOpUpdates;
     unsigned long nrOpUpdateValuesCopied;
-    
-    friend class RecursionCounter;
+
+    bool countCalls;
+
+    typedef std::map<Symbol, unsigned int> PrimOpCalls;
+    PrimOpCalls primOpCalls;
+
+    typedef std::map<Pos, unsigned int> FunctionCalls;
+    FunctionCalls functionCalls;
+
+    typedef std::map<Pos, unsigned int> AttrSelects;
+    AttrSelects attrSelects;
+
     friend class ExprOpUpdate;
+    friend class ExprSelect;
+    friend void prim_getAttr(EvalState & state, Value * * args, Value & v);
 };