diff --git a/lib/rscons.rb b/lib/rscons.rb
index 9a09e3c..e9cbbf6 100644
--- a/lib/rscons.rb
+++ b/lib/rscons.rb
@@ -23,6 +23,7 @@ module Rscons
   # Names of the default builders which will be added to all newly created
   # {Environment} objects.
   DEFAULT_BUILDERS = [
+    :Barrier,
     :Command,
     :Copy,
     :Directory,
@@ -66,6 +67,13 @@ module Rscons
       target.is_a?(Symbol)
     end
 
+    # Generate a random phony target name.
+    #
+    # @return [Symbol] Phony target name.
+    def gen_phony_target
+      ("t" + sprintf("%08x", rand(1_000_000..4_000_000_000))).to_sym
+    end
+
     # Return the system shell and arguments for executing a shell command.
     #
     # @return [Array<String>] The shell and flag.
@@ -136,6 +144,7 @@ require_relative "rscons/builders/mixins/object_deps"
 require_relative "rscons/builders/mixins/program"
 
 # default builders
+require_relative "rscons/builders/barrier"
 require_relative "rscons/builders/command"
 require_relative "rscons/builders/copy"
 require_relative "rscons/builders/directory"
diff --git a/lib/rscons/builder_set.rb b/lib/rscons/builder_set.rb
index 8373d78..f0bae36 100644
--- a/lib/rscons/builder_set.rb
+++ b/lib/rscons/builder_set.rb
@@ -37,7 +37,7 @@ module Rscons
     #   The number of remaining build steps.
     def build_steps_remaining
       self.reduce(0) do |result, (target, builders)|
-        result + builders.size
+        result + builders.count {|b| !b.is_a?(Rscons::Builders::Barrier)}
       end
     end
 
diff --git a/lib/rscons/builders/barrier.rb b/lib/rscons/builders/barrier.rb
new file mode 100644
index 0000000..2328b9f
--- /dev/null
+++ b/lib/rscons/builders/barrier.rb
@@ -0,0 +1,15 @@
+module Rscons
+  module Builders
+    # The Barrier builder does not perform any action. It exists as a builder
+    # on which to place dependencies to ensure that each of its sources are
+    # built before any build targets which depend on the barrier build target.
+    class Barrier < Builder
+
+      # Run the builder.
+      def run(options)
+        true
+      end
+
+    end
+  end
+end
diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index f27cd35..ebaaba6 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -397,7 +397,11 @@ module Rscons
         expand(ud)
       end
       @user_deps[target] ||= []
-      @user_deps[target] = (@user_deps[target] + user_deps).uniq
+      (@user_deps[target] + user_deps).each do |ud|
+        unless Rscons.phony_target?(ud) || @user_deps[target].include?(ud)
+          @user_deps[target] << ud
+        end
+      end
       build_after(target, user_deps)
     end
 
@@ -623,7 +627,9 @@ module Rscons
     #
     # @return [void]
     def run_builder(builder)
-      builder.build_step ||= get_next_build_step
+      unless builder.is_a?(Rscons::Builders::Barrier)
+        builder.build_step ||= get_next_build_step
+      end
       case result = builder.run({})
       when Array
         result.each do |waititem|
@@ -649,8 +655,10 @@ module Rscons
           Cache.instance.register_build(side_effect, nil, [], self, side_effect: true)
           @side_effects.delete(side_effect)
         end
-        @build_hooks[:post].each do |build_hook_block|
-          build_hook_block.call(builder)
+        unless builder.is_a?(Rscons::Builders::Barrier)
+          @build_hooks[:post].each do |build_hook_block|
+            build_hook_block.call(builder)
+          end
         end
         process_remove_wait(builder)
       else
@@ -727,8 +735,10 @@ module Rscons
       if @builder_sets.size > 0
         if builder = @builder_sets[0].get_next_builder_to_run(targets_still_building)
           builder.vars = @varset.merge(builder.vars)
-          @build_hooks[:pre].each do |build_hook_block|
-            build_hook_block.call(builder)
+          unless builder.is_a?(Rscons::Builders::Barrier)
+            @build_hooks[:pre].each do |build_hook_block|
+              build_hook_block.call(builder)
+            end
           end
           return run_builder(builder)
         end