Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -4965,6 +4965,9 @@
     // assembler on assembly source files.
     CmdArgs.push_back("-dwarf-debug-producer");
     CmdArgs.push_back(Args.MakeArgString(getClangFullVersion()));
+
+    // And pass along -I options
+    Args.AddAllArgs(CmdArgs, options::OPT_I);
   }
 
   // Optionally embed the -cc1as level arguments into the debug info, for build
@@ -7296,6 +7299,7 @@
   if (NeedsKPIC)
     addAssemblerKPIC(Args, CmdArgs);
 
+  Args.AddAllArgs(CmdArgs, options::OPT_I);
   Args.AddAllArgValues(CmdArgs, options::OPT_Wa_COMMA,
                        options::OPT_Xassembler);
 
Index: test/Driver/as-options.s
===================================================================
--- /dev/null
+++ test/Driver/as-options.s
@@ -0,0 +1,12 @@
+// PR21000: Test that -I is passed to assembler. 
+// RUN: %clang -### -c -no-integrated-as %s -Ifoo_dir 2>&1 | FileCheck --check-prefix=BARE_INCLUDE1 %s
+// BARE_INCLUDE1: "-I" "foo_dir"
+
+// RUN: %clang -### -c -no-integrated-as %s -I foo_dir 2>&1 | FileCheck --check-prefix=BARE_INCLUDE2 %s
+// BARE_INCLUDE2: "-I" "foo_dir"
+
+// RUN: %clang -### -c -integrated-as %s -Ifoo_dir 2>&1 | FileCheck --check-prefix=BARE_INT_INCLUDE1 %s
+// BARE_INT_INCLUDE1: "-I" "foo_dir"
+
+// RUN: %clang -### -c -integrated-as %s -I foo_dir 2>&1 | FileCheck --check-prefix=BARE_INT_INCLUDE2 %s
+// BARE_INT_INCLUDE2: "-I" "foo_dir"