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
@@ -172,6 +172,10 @@
       Builder.defineMacro("__LONG_DOUBLE_IBM128__");
   }
 
+  if (getTriple().isOSAIX() && LongDoubleWidth == 64) {
+    Builder.defineMacro("__LONGDOUBLE64");
+  }
+
   // Define this for elfv2 (64-bit only) or 64-bit darwin.
   if (ABI == "elfv2" ||
       (getTriple().getOS() == llvm::Triple::Darwin && PointerWidth == 64))
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
@@ -512,6 +512,7 @@
 // PPC-AIX:#define __LDBL_MIN_10_EXP__ (-307)
 // PPC-AIX:#define __LDBL_MIN_EXP__ (-1021)
 // PPC-AIX:#define __LDBL_MIN__ 2.2250738585072014e-308L
+// PPC-AIX:#define __LONGDOUBLE64 1
 // PPC-AIX:#define __LONG_LONG_MAX__ 9223372036854775807LL
 // PPC-AIX:#define __LONG_MAX__ 2147483647L
 // PPC-AIX-NOT:#define __LP64__ 1