diff --git a/clang/lib/Headers/float.h b/clang/lib/Headers/float.h
--- a/clang/lib/Headers/float.h
+++ b/clang/lib/Headers/float.h
@@ -14,11 +14,11 @@
  * additional definitions provided for Windows.
  * For more details see http://msdn.microsoft.com/en-us/library/y0ybw9fy.aspx
  *
- * Also fall back on Darwin to allow additional definitions and
+ * Also fall back on Darwin and AIX to allow additional definitions and
  * implementation-defined values.
  */
-#if (defined(__APPLE__) || (defined(__MINGW32__) || defined(_MSC_VER))) && \
-    __STDC_HOSTED__ && __has_include_next(<float.h>)
+#if (defined(__APPLE__) || (defined(__MINGW32__) || defined(_MSC_VER))) ||     \
+    defined(_AIX) && __STDC_HOSTED__ && __has_include_next(<float.h>)
 
 /* Prior to Apple's 10.7 SDK, float.h SDK header used to apply an extra level
  * of #include_next<float.h> to keep Metrowerks compilers happy. Avoid this
@@ -37,7 +37,8 @@
 #  undef FLT_MANT_DIG
 #  undef DBL_MANT_DIG
 #  undef LDBL_MANT_DIG
-#  if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) || __cplusplus >= 201103L
+#if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201103L || defined(_AIX) && defined(_ALL_SOURCE)
 #    undef DECIMAL_DIG
 #  endif
 #  undef FLT_DIG
@@ -64,7 +65,8 @@
 #  undef FLT_MIN
 #  undef DBL_MIN
 #  undef LDBL_MIN
-#  if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) || __cplusplus >= 201703L
+#if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201703L || defined(_AIX) && defined(_ALL_SOURCE)
 #    undef FLT_TRUE_MIN
 #    undef DBL_TRUE_MIN
 #    undef LDBL_TRUE_MIN
@@ -87,7 +89,8 @@
 #define DBL_MANT_DIG __DBL_MANT_DIG__
 #define LDBL_MANT_DIG __LDBL_MANT_DIG__
 
-#if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) || __cplusplus >= 201103L
+#if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201103L || defined(_AIX) && defined(_ALL_SOURCE)
 #  define DECIMAL_DIG __DECIMAL_DIG__
 #endif
 
@@ -123,7 +126,8 @@
 #define DBL_MIN __DBL_MIN__
 #define LDBL_MIN __LDBL_MIN__
 
-#if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) || __cplusplus >= 201703L
+#if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201703L || defined(_AIX) && defined(_ALL_SOURCE)
 #  define FLT_TRUE_MIN __FLT_DENORM_MIN__
 #  define DBL_TRUE_MIN __DBL_DENORM_MIN__
 #  define LDBL_TRUE_MIN __LDBL_DENORM_MIN__
diff --git a/clang/test/Headers/float.c b/clang/test/Headers/float.c
--- a/clang/test/Headers/float.c
+++ b/clang/test/Headers/float.c
@@ -4,6 +4,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++11 -ffreestanding %s
 // RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++14 -ffreestanding %s
 // RUN: %clang_cc1 -fsyntax-only -verify -xc++ -std=c++17 -ffreestanding %s
+// RUN: %clang_cc1 -triple powerpc-ibm-aix -fsyntax-only -verify -std=c99 -ffreestanding -D_ALL_SOURCE %s
 // expected-no-diagnostics
 
 /* Basic floating point conformance checks against:
@@ -45,7 +46,8 @@
 #endif
 
 
-#if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) || __cplusplus >= 201703L
+#if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201703L || defined(_AIX) && defined(_ALL_SOURCE)
     #ifndef FLT_DECIMAL_DIG
         #error "Mandatory macro FLT_DECIMAL_DIG is missing."
     #elif   FLT_DECIMAL_DIG < 6
@@ -101,7 +103,8 @@
 #endif
 
 
-#if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) || __cplusplus >= 201103L
+#if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201103L || defined(_AIX) && defined(_ALL_SOURCE)
     #ifndef DECIMAL_DIG
         #error "Mandatory macro DECIMAL_DIG is missing."
     #elif   DECIMAL_DIG < 10
@@ -215,13 +218,15 @@
 _Static_assert(DBL_MANT_DIG == __DBL_MANT_DIG__, "");
 _Static_assert(LDBL_MANT_DIG == __LDBL_MANT_DIG__, "");
 
-#if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) || __cplusplus >= 201703L
+#if __STDC_VERSION__ >= 201112L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201703L || defined(_AIX) && defined(_ALL_SOURCE)
 _Static_assert(FLT_DECIMAL_DIG == __FLT_DECIMAL_DIG__, "");
 _Static_assert(DBL_DECIMAL_DIG == __DBL_DECIMAL_DIG__, "");
 _Static_assert(LDBL_DECIMAL_DIG == __LDBL_DECIMAL_DIG__, "");
 #endif
 
-#if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) || __cplusplus >= 201103L
+#if __STDC_VERSION__ >= 199901L || !defined(__STRICT_ANSI__) ||                \
+    __cplusplus >= 201103L || defined(_AIX) && defined(_ALL_SOURCE)
 _Static_assert(DECIMAL_DIG == __DECIMAL_DIG__, "");
 #endif