Add control to enable function evaluation at compile time.

Pure functions with constant operands can be evaluated at compile
time rather than at run time. This patch provides a global control
to enable this optimisation. Until constant function support is
complete, by default it is disabled.

compiler.h

@@ -104,6 +104,15 @@ extern bool debug_elaborate;
 extern bool debug_synth2;
 extern bool debug_optimizer;
 
+/* Control evaluation of functions at compile time:
+ *   0 = only for functions in constant expressions
+ *   1 = only for automatic functions
+ *   2 = for all functions
+ * Level 2 should only be used if the user can guarantee that a
+ * function's local variables are never accessed from outside the
+ * function. */
+extern unsigned opt_const_func;
+
 /* Possibly temporary flag to control virtualization of pin arrays */
 extern bool disable_virtual_pins;
 

eval_tree.cc

@@ -1985,10 +1985,16 @@ NetExpr* NetEUFunc::eval_tree()
       if (!func()->is_const_func())
             return 0;
 
+	// If we neither want nor need to evaluate the function at
+	// compile time, give up now.
+      if (!opt_const_func && !need_const_)
+            return 0;
+
         // Variables inside static functions can be accessed from outside
         // the function, so we can't be sure they are constant unless the
-        // function was called in a constant context.
-      if (!func()->is_auto() && !need_const_)
+        // function was called in a constant context or the user has told
+	// us this is safe.
+      if (!func()->is_auto() && !need_const_ && (opt_const_func < 2))
             return 0;
 
         // Run through the input parameters to check they are constants.
@@ -2000,7 +2006,6 @@ NetExpr* NetEUFunc::eval_tree()
             return 0;
       }
 
-      if (need_const_) {
       NetFuncDef*def = func_->func_def();
       ivl_assert(*this, def);
 
@@ -2011,6 +2016,3 @@ NetExpr* NetEUFunc::eval_tree()
       NetExpr*res = def->evaluate_function(*this, args);
       return res;
 }
-
-      return 0;
-}

main.cc

@@ -169,6 +169,11 @@ bool debug_elaborate = false;
 bool debug_synth2 = false;
 bool debug_optimizer = false;
 
+/*
+ * Optimization control flags.
+ */
+unsigned opt_const_func = 0;
+
 /*
  * Miscellaneous flags.
  */