Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -5320,6 +5320,9 @@
     // 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
@@ -7797,6 +7800,7 @@
   if (NeedsKPIC)
     addAssemblerKPIC(Args, CmdArgs);
 
+  Args.AddAllArgs(CmdArgs, options::OPT_I);
   Args.AddAllArgValues(CmdArgs, options::OPT_Wa_COMMA, options::OPT_Xassembler);
 
   CmdArgs.push_back("-o");
Index: test/Driver/as-options.s
===================================================================
--- /dev/null
+++ test/Driver/as-options.s
@@ -0,0 +1,37 @@
+// PR21000: Test that -I is passed to both external and integrated assemblers.
+
+// RUN: %clang -target x86_64-linux-gnu -c -no-integrated-as %s \
+// RUN:   -Ifoo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// RUN: %clang -target x86_64-linux-gnu -c -no-integrated-as %s \
+// RUN:   -I foo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// RUN: %clang -target x86_64-linux-gnu -c -integrated-as %s \
+// RUN:   -Ifoo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// RUN: %clang -target x86_64-linux-gnu -c -integrated-as %s \
+// RUN:   -I foo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// Other GNU targets
+
+// RUN: %clang -target aarch64-linux-gnu -c -no-integrated-as %s \
+// RUN:   -Ifoo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// RUN: %clang -target aarch64-linux-gnu -c -integrated-as %s \
+// RUN:   -Ifoo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// RUN: %clang -target armv7-linux-gnueabihf -c -no-integrated-as %s \
+// RUN:   -Ifoo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// RUN: %clang -target armv7-linux-gnueabihf -c -integrated-as %s \
+// RUN:   -Ifoo_dir -### 2>&1 \
+// RUN:   | FileCheck %s
+
+// CHECK: "-I" "foo_dir"