[m-rev.] for review: compile time evaluation of string.count_codepoints

[Julien Fischer]

For review by anyone.

-----------

Compile time evaluation of string.count_codepoints.

compiler/const_prop.m:
     Do compile time evaluation of string.count_codepoints where possible.

     Fix an old error in a comment: functions with two arguments are
     binary, not unary.

Julien.

diff --git a/compiler/const_prop.m b/compiler/const_prop.m
index 83c407b..568ee9e 100644
--- a/compiler/const_prop.m
+++ b/compiler/const_prop.m
@@ -158,10 +158,14 @@ evaluate_det_call(ModuleName, ProcName, ModeNum, CrossCompiling, Args,
              ModuleName = "float",
              evaluate_det_call_float_2(ProcName, ModeNum, CrossCompiling,
                  X, Y, OutputArg, OutputArgVal)
+        ;
+            ModuleName = "string",
+            evaluate_det_call_string_2(ProcName, ModeNum, CrossCompiling,
+                X, Y, OutputArg, OutputArgVal)
          )
      ;
          Args = [X, Y, Z],
-        % Unary functions.
+        % Binary functions.
          (
              ModuleName = "int",
              evaluate_det_call_int_3(ProcName, ModeNum, CrossCompiling,
@@ -263,7 +267,20 @@ evaluate_det_call_float_2(ProcName, ModeNum, _CrossCompiling, X, Y,
          OutputArg = Y,
          OutputArgVal = -XVal
      ).
+ 
+:- pred evaluate_det_call_string_2(string::in, int::in, bool::in,
+    arg_hlds_info::in, arg_hlds_info::in, arg_hlds_info::out,
+    cons_id::out) is semidet.

+evaluate_det_call_string_2(ProcName, ModeNum, _CrossCompiling, X, Y,
+        OutputArg, OutputArgVal) :-
+    ProcName = "count_codepoints",
+    ModeNum = 0,
+    X ^ arg_inst = bound(_, _, [bound_functor(string_const(XVal), [])]),
+    OutputArg = Y,
+    CodePointCountX = string.count_codepoints(XVal),
+    OutputArgVal = int_const(CodePointCountX).
+
  :- pred evaluate_det_call_int_3(string::in, int::in, bool::in,
      arg_hlds_info::in, arg_hlds_info::in, arg_hlds_info::in,
      arg_hlds_info::out, cons_id::out) is semidet.