diff --git a/clang/lib/Basic/Targets/PPC.cpp b/clang/lib/Basic/Targets/PPC.cpp
--- a/clang/lib/Basic/Targets/PPC.cpp
+++ b/clang/lib/Basic/Targets/PPC.cpp
@@ -257,6 +257,9 @@
     Builder.defineMacro("__powerpc64__");
     Builder.defineMacro("__ppc64__");
     Builder.defineMacro("__PPC64__");
+  } else if (getTriple().isOSAIX()) {
+    // The XL compilers on AIX define _ARCH_PPC64 for both 32 and 64-bit modes.
+    Builder.defineMacro("_ARCH_PPC64");
   }
   if (getTriple().isOSAIX()) {
     Builder.defineMacro("__THW_PPC__");
diff --git a/clang/test/Preprocessor/init-ppc.c b/clang/test/Preprocessor/init-ppc.c
--- a/clang/test/Preprocessor/init-ppc.c
+++ b/clang/test/Preprocessor/init-ppc.c
@@ -391,6 +391,7 @@
 // PPC-AIX-NOT:#define __64BIT__ 1
 // PPC-AIX:#define _AIX 1
 // PPC-AIX:#define _ARCH_PPC 1
+// PPC-AIX:#define _ARCH_PPC64 1
 // PPC-AIX:#define _BIG_ENDIAN 1
 // PPC-AIX:#define _IBMR2 1
 // PPC-AIX:#define _LONG_LONG 1