diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
index 3dbe47f4cf54..3b7272e5acc5 100644
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -94,5 +94,7 @@ def warn_drv_conflicting_deployment_targets : Warning<
 def warn_drv_treating_input_as_cxx : Warning<
   "treating '%0' input as '%1' when in C++ mode, this behavior is deprecated">,
   InGroup<Deprecated>;
+def warn_drv_objc_gc_unsupported : Warning<
+  "Objective-C garbage collection is not supported on this platform, ignoring '%0'">;
 
 }
diff --git a/clang/include/clang/Driver/ToolChain.h b/clang/include/clang/Driver/ToolChain.h
index 9a82973dca27..b58205cc4d92 100644
--- a/clang/include/clang/Driver/ToolChain.h
+++ b/clang/include/clang/Driver/ToolChain.h
@@ -120,6 +120,9 @@ public:
   /// particular PIC mode.
   virtual const char *GetForcedPicModel() const = 0;
 
+  /// Does this tool chain support Objective-C garbage collection.
+  virtual bool SupportsObjCGC() const { return false; }
+
   /// UseDwarfDebugFlags - Embed the compile options to clang into the Dwarf
   /// compile unit information.
   virtual bool UseDwarfDebugFlags() const { return false; }
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 4bb3246eddcd..80dea504a7f2 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -663,6 +663,11 @@ const char *Darwin::GetForcedPicModel() const {
   return 0;
 }
 
+bool Darwin::SupportsObjCGC() const {
+  // Garbage collection is supported everywhere except on iPhone OS.
+  return !isTargetIPhoneOS();
+}
+
 /// Generic_GCC - A tool chain using the 'gcc' command to perform
 /// all subcommands; this relies on gcc translating the majority of
 /// command line options.
diff --git a/clang/lib/Driver/ToolChains.h b/clang/lib/Driver/ToolChains.h
index 0d5b2a3f4747..9e61b8972f7d 100644
--- a/clang/lib/Driver/ToolChains.h
+++ b/clang/lib/Driver/ToolChains.h
@@ -176,6 +176,8 @@ public:
   virtual const char *GetDefaultRelocationModel() const;
   virtual const char *GetForcedPicModel() const;
 
+  virtual bool SupportsObjCGC() const;
+
   virtual bool UseDwarfDebugFlags() const;
 
   virtual bool UseSjLjExceptions() const;
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index be79e79fd05b..6a74ff47438f 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -1107,10 +1107,23 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                     options::OPT_fno_lax_vector_conversions))
     CmdArgs.push_back("-fno-lax-vector-conversions");
 
+  // Handle -fobjc-gc and -fobjc-gc-only. They are exclusive, and -fobjc-gc-only
+  // takes precedence.
+  const Arg *GCArg = Args.getLastArg(options::OPT_fobjc_gc_only);
+  if (!GCArg)
+    GCArg = Args.getLastArg(options::OPT_fobjc_gc);
+  if (GCArg) {
+    if (getToolChain().SupportsObjCGC()) {
+      GCArg->render(Args, CmdArgs);
+    } else {
+      // FIXME: We should move this to a hard error.
+      D.Diag(clang::diag::warn_drv_objc_gc_unsupported)
+        << GCArg->getAsString(Args);
+    }
+  }
+
   Args.AddLastArg(CmdArgs, options::OPT_fno_caret_diagnostics);
   Args.AddLastArg(CmdArgs, options::OPT_fno_show_column);
-  Args.AddLastArg(CmdArgs, options::OPT_fobjc_gc_only);
-  Args.AddLastArg(CmdArgs, options::OPT_fobjc_gc);
   Args.AddLastArg(CmdArgs, options::OPT_fobjc_sender_dependent_dispatch);
   Args.AddLastArg(CmdArgs, options::OPT_fdiagnostics_print_source_range_info);
   Args.AddLastArg(CmdArgs, options::OPT_ftime_report);
diff --git a/clang/test/Driver/darwin-objc-gc.m b/clang/test/Driver/darwin-objc-gc.m
new file mode 100644
index 000000000000..aecb9a6b146f
--- /dev/null
+++ b/clang/test/Driver/darwin-objc-gc.m
@@ -0,0 +1,19 @@
+// Check that we warn, but accept, -fobjc-gc for iPhone OS.
+
+// RUN: %clang -ccc-host-triple i386-apple-darwin9 -miphoneos-version-min=3.0 -fobjc-gc -flto -S -o %t %s 2> %t.err
+// RUN: FileCheck --check-prefix=IPHONE_OBJC_GC_LL %s < %t 
+// RUN: FileCheck --check-prefix=IPHONE_OBJC_GC_STDERR %s < %t.err
+
+// IPHONE_OBJC_GC_LL: define void @f0
+// IPHONE_OBJC_GC_LL-NOT: objc_assign_ivar
+// IPHONE_OBJC_GC_LL: }
+
+// IPHONE_OBJC_GC_STDERR: warning: Objective-C garbage collection is not supported on this platform, ignoring '-fobjc-gc'
+
+@interface A {
+@public
+ id x;
+}
+@end
+
+void f0(A *a, id x) { a->x = x; }