avoid adding risc instructions twice

that causes loops in the chain
infinite loops in the code that are hard to debug
closes #11

lib/mom/instruction/jump.rb

@@ -12,8 +12,7 @@ module Mom
       @label = label
     end
     def to_risc(compiler)
-      label = @label.to_risc(compiler)
-      compiler.add_code Risc::Branch.new(self , label)
+      compiler.add_code Risc::Branch.new(self , @label.risc_label)
     end
   end
 

lib/mom/instruction/label.rb

@@ -8,6 +8,19 @@ module Mom
   #
   # A Label has a name which is mainly used for debugging.
   #
+  # A Mom::Label converts one2one to a Risc::Label. So in a way it could not be more
+  # simple.
+  # Alas, since almost by definition several roads lead to this label, all those
+  # several converted instructions must also point to the identical label on the
+  # risc level.
+  #
+  # This is achieved by caching the created Risc::Label in an instance variable.
+  # All branches that lead to this label can thus safely call the to_risc and
+  # whoever calls first triggers the labels creation, but all get the same label.
+  #
+  # Off course some specific place still has to be responsible for actually
+  # adding the label to the instruction list (usually an if/while)
+
   class Label < Instruction
     attr_reader :name
     def initialize(name)
@@ -18,20 +31,20 @@ module Mom
       "Label: #{name}"
     end
 
-    # A Mom::Label converts one2one to a Risc::Label. So in a way it could not be more
-    # simple.
-    # Alas, since almost by definition several roads lead to this label, all those
-    # several converted instructions must also point to the identical label on the
-    # risc level.
-    #
-    # This is achieved by caching the created Risc::Label in an instance variable.
-    # All branches that lead to this label can thus safely call the to_risc and
-    # whoever calls first triggers the labels creation, but all get the same label.
-    #
-    # Off course some specific place still has to be responsible for actually
-    # adding the label to the instruction list (usually an if/while)
-    def to_risc(compiler)
+    # generate the risc label lazily
+    def risc_label
       @risc_label ||= Risc.label(self,name)
     end
+
+    # add the risc_label to the compiler (instruction flow)
+    # should only be called once
+    def to_risc(compiler)
+      if( @added )
+        raise "added already #{@added}"
+      else
+        @added = true
+        compiler.add_code( risc_label )
+      end
+    end
   end
 end

lib/mom/instruction/not_same_check.rb

@@ -26,7 +26,7 @@ module Mom
       l_reg = left.to_register(compiler, self)
       r_reg = right.to_register(compiler, self)
       compiler.add_code Risc.op( self , :- , l_reg , r_reg)
-      compiler.add_code Risc::IsZero.new( self, false_jump.to_risc(compiler))
+      compiler.add_code Risc::IsZero.new( self, false_jump.risc_label)
     end
   end
 end

lib/mom/instruction/truth_check.rb

@@ -19,7 +19,7 @@ module Mom
     end
 
     def to_risc(compiler)
-      false_label = @false_jump.to_risc(compiler)
+      false_label = @false_jump.risc_label
       builder = compiler.builder("TruthCheck")
       condition_reg = @condition.to_register(compiler,self)
       builder.build do