diff --git a/clang/lib/Driver/ToolChains/Arch/PPC.cpp b/clang/lib/Driver/ToolChains/Arch/PPC.cpp
--- a/clang/lib/Driver/ToolChains/Arch/PPC.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/PPC.cpp
@@ -39,7 +39,12 @@
   if (Arg *A = Args.getLastArg(clang::driver::options::OPT_mcpu_EQ)) {
     StringRef CPUName = A->getValue();
 
-    if (CPUName == "generic")
+    // Clang/LLVM does not actually support code generation
+    // for the 405 CPU. However, there are uses of this CPU ID
+    // in projects that previously used GCC and rely on Clang
+    // accepting it. Clang has always ignored it and passed the
+    // generic CPU ID to the back end.
+    if (CPUName == "generic" || CPUName == "405")
       return getPPCGenericTargetCPU(T);
 
     if (CPUName == "native") {
diff --git a/clang/test/Driver/ppc-cpus.c b/clang/test/Driver/ppc-cpus.c
--- a/clang/test/Driver/ppc-cpus.c
+++ b/clang/test/Driver/ppc-cpus.c
@@ -31,3 +31,9 @@
 
 // RUN: %clang -### -c --target=powerpc64 %s -mcpu=generic -mtune=pwr9 2>&1 | FileCheck %s --check-prefix=TUNE
 // TUNE: "-target-cpu" "ppc64" "-tune-cpu" "pwr9"
+
+/// Test mcpu options that are equivalent to "generic"
+// RUN: %clang -### -c -target powerpc64 %s -mcpu=generic 2>&1 | FileCheck %s --check-prefix=GENERIC
+// RUN: %clang -### -c -target powerpc64 %s -mcpu=405     2>&1 | FileCheck %s --check-prefix=GENERIC
+//
+// GENERIC: "-target-cpu" "ppc64"