diff --git a/llvm/Makefile.rules b/llvm/Makefile.rules
index 0d4a0c8aedf2..5bddff8e8a22 100644
--- a/llvm/Makefile.rules
+++ b/llvm/Makefile.rules
@@ -612,6 +612,13 @@ $(RecursiveTargets)::
 	done
 endif
 
+#-----------------------------------------------------------
+# Handle the OPTIONAL_PARALLEL_DIRS options for optional parallel construction
+#-----------------------------------------------------------
+ifdef OPTIONAL_PARALLEL_DIRS
+  PARALLEL_DIRS += $(foreach T,$(OPTIONAL_PARALLEL_DIRS),$(shell test -d $(PROJ_SRC_DIR)/$(T) && echo "$(T)"))
+endif
+
 #-----------------------------------------------------------
 # Handle the PARALLEL_DIRS options for parallel construction
 #-----------------------------------------------------------
diff --git a/llvm/tools/Makefile b/llvm/tools/Makefile
index 2726c1f3e43e..519717ad98de 100644
--- a/llvm/tools/Makefile
+++ b/llvm/tools/Makefile
@@ -8,6 +8,10 @@
 ##===----------------------------------------------------------------------===##
 
 LEVEL := ..
+
+# Build clang if present.
+OPTIONAL_PARALLEL_DIRS := clang
+
 # NOTE: The tools are organized into five groups of four consisting of one
 # large and three small executables. This is done to minimize memory load
 # in parallel builds.  Please retain this ordering.