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, 9 insertions, 0 deletions

diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 4c1a0bb2d5fb..bc6c3287c79d 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -27,6 +27,15 @@ struct Pos
     Pos() : line(0), column(0) { };
     Pos(const string & file, unsigned int line, unsigned int column)
         : file(file), line(line), column(column) { };
+    bool operator < (const Pos & p2) const
+    {
+        int d = file.compare(p2.file);
+        if (d < 0) return true;
+        if (d > 0) return false;
+        if (line < p2.line) return true;
+        if (line > p2.line) return false;
+        return column < p2.column;
+    }
 };
 
 extern Pos noPos;