diff --git a/configure b/configure
index 7a6bfc2..d6ecb33 100755
--- a/configure
+++ b/configure
@@ -14650,6 +14650,46 @@ $as_echo "$pgac_cv_have_ppc_mutex_hint" >&6; }
 $as_echo "#define HAVE_PPC_LWARX_MUTEX_HINT 1" >>confdefs.h
 
     fi
+    # Check if compiler accepts "i"(x) when __builtin_constant_p(x).
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether __builtin_constant_p(x) implies \"i\"(x) acceptance" >&5
+$as_echo_n "checking whether __builtin_constant_p(x) implies \"i\"(x) acceptance... " >&6; }
+if ${pgac_cv_have_i_constraint__builtin_constant_p+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+static inline int
+     addi(int ra, int si)
+     {
+         int res = 0;
+         if (__builtin_constant_p(si))
+             __asm__ __volatile__(
+                 " addi %0,%1,%2\n" : "=r"(res) : "r"(ra), "i"(si));
+         return res;
+     }
+     int test_adds(int x) { return addi(3, x) + addi(x, 5); }
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_have_i_constraint__builtin_constant_p=yes
+else
+  pgac_cv_have_i_constraint__builtin_constant_p=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_have_i_constraint__builtin_constant_p" >&5
+$as_echo "$pgac_cv_have_i_constraint__builtin_constant_p" >&6; }
+    if test x"$pgac_cv_have_i_constraint__builtin_constant_p" = xyes ; then
+
+$as_echo "#define HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P 1" >>confdefs.h
+
+    fi
   ;;
 esac
 
diff --git a/configure.in b/configure.in
index dde3eec..510bd76 100644
--- a/configure.in
+++ b/configure.in
@@ -1541,6 +1541,26 @@ case $host_cpu in
     if test x"$pgac_cv_have_ppc_mutex_hint" = xyes ; then
 	AC_DEFINE(HAVE_PPC_LWARX_MUTEX_HINT, 1, [Define to 1 if the assembler supports PPC's LWARX mutex hint bit.])
     fi
+    # Check if compiler accepts "i"(x) when __builtin_constant_p(x).
+    AC_CACHE_CHECK([whether __builtin_constant_p(x) implies "i"(x) acceptance],
+                   [pgac_cv_have_i_constraint__builtin_constant_p],
+    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+    [static inline int
+     addi(int ra, int si)
+     {
+         int res = 0;
+         if (__builtin_constant_p(si))
+             __asm__ __volatile__(
+                 " addi %0,%1,%2\n" : "=r"(res) : "r"(ra), "i"(si));
+         return res;
+     }
+     int test_adds(int x) { return addi(3, x) + addi(x, 5); }], [])],
+    [pgac_cv_have_i_constraint__builtin_constant_p=yes],
+    [pgac_cv_have_i_constraint__builtin_constant_p=no])])
+    if test x"$pgac_cv_have_i_constraint__builtin_constant_p" = xyes ; then
+      AC_DEFINE(HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P, 1,
+                [Define to 1 if __builtin_constant_p(x) implies "i"(x) acceptance.])
+    fi
   ;;
 esac
 
diff --git a/src/include/pg_config.h.in b/src/include/pg_config.h.in
index 512213a..9be84fc 100644
--- a/src/include/pg_config.h.in
+++ b/src/include/pg_config.h.in
@@ -332,6 +332,9 @@
 /* Define to 1 if you have isinf(). */
 #undef HAVE_ISINF
 
+/* Define to 1 if __builtin_constant_p(x) implies "i"(x) acceptance. */
+#undef HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P
+
 /* Define to 1 if you have the <langinfo.h> header file. */
 #undef HAVE_LANGINFO_H
 
diff --git a/src/include/port/atomics/arch-ppc.h b/src/include/port/atomics/arch-ppc.h
index 344b394..35d602e 100644
--- a/src/include/port/atomics/arch-ppc.h
+++ b/src/include/port/atomics/arch-ppc.h
@@ -25,5 +25,103 @@
 #define pg_write_barrier_impl()		__asm__ __volatile__ ("lwsync" : : : "memory")
 #endif
 
+#define PG_HAVE_ATOMIC_U32_SUPPORT
+typedef struct pg_atomic_uint32
+{
+	volatile uint32 value;
+} pg_atomic_uint32;
+
+/* 64bit atomics are only supported in 64bit mode */
+#ifdef __64BIT__
+#define PG_HAVE_ATOMIC_U64_SUPPORT
+typedef struct pg_atomic_uint64
+{
+	volatile uint64 value pg_attribute_aligned(8);
+} pg_atomic_uint64;
+
+#endif /* __64BIT__ */
+
+#define PG_HAVE_ATOMIC_FETCH_ADD_U32
+static inline uint32
+pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
+{
+	uint32 _t;
+	uint32 res;
+
+	/*
+	 * xlc has a no-longer-documented __fetch_and_add() intrinsic.  In xlc
+	 * 12.01.0000.0000, it emits a leading "sync" and trailing "isync".  In
+	 * xlc 13.01.0003.0004, it emits neither.  Hence, using the intrinsic
+	 * would add redundant syncs on xlc 12.
+	 */
+#ifdef HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P
+	if (__builtin_constant_p(add_) &&
+		add_ <= PG_INT16_MAX && add_ >= PG_INT16_MIN)
+		__asm__ __volatile__(
+			"	sync				\n"
+			"	lwarx   %1,0,%4		\n"
+			"	addi    %0,%1,%3	\n"
+			"	stwcx.  %0,0,%4		\n"
+			"	bne     $-12		\n"		/* branch to lwarx */
+			"	isync				\n"
+:			"=&r"(_t), "=&r"(res), "+m"(ptr->value)
+:			"i"(add_), "r"(&ptr->value)
+:			"memory", "cc");
+	else
+#endif
+		__asm__ __volatile__(
+			"	sync				\n"
+			"	lwarx   %1,0,%4		\n"
+			"	add     %0,%1,%3	\n"
+			"	stwcx.  %0,0,%4		\n"
+			"	bne     $-12		\n"		/* branch to lwarx */
+			"	isync				\n"
+:			"=&r"(_t), "=&r"(res), "+m"(ptr->value)
+:			"r"(add_), "r"(&ptr->value)
+:			"memory", "cc");
+
+	return res;
+}
+
+#ifdef PG_HAVE_ATOMIC_U64_SUPPORT
+#define PG_HAVE_ATOMIC_FETCH_ADD_U64
+static inline uint64
+pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
+{
+	uint64 _t;
+	uint64 res;
+
+	/* Like u32, but s/lwarx/ldarx/; s/stwcx/stdcx/ */
+#ifdef HAVE_I_CONSTRAINT__BUILTIN_CONSTANT_P
+	if (__builtin_constant_p(add_) &&
+		add_ <= PG_INT16_MAX && add_ >= PG_INT16_MIN)
+		__asm__ __volatile__(
+			"	sync				\n"
+			"	ldarx   %1,0,%4		\n"
+			"	addi    %0,%1,%3	\n"
+			"	stdcx.  %0,0,%4		\n"
+			"	bne     $-12		\n"		/* branch to ldarx */
+			"	isync				\n"
+:			"=&r"(_t), "=&r"(res), "+m"(ptr->value)
+:			"i"(add_), "r"(&ptr->value)
+:			"memory", "cc");
+	else
+#endif
+		__asm__ __volatile__(
+			"	sync				\n"
+			"	ldarx   %1,0,%4		\n"
+			"	add     %0,%1,%3	\n"
+			"	stdcx.  %0,0,%4		\n"
+			"	bne     $-12		\n"		/* branch to ldarx */
+			"	isync				\n"
+:			"=&r"(_t), "=&r"(res), "+m"(ptr->value)
+:			"r"(add_), "r"(&ptr->value)
+:			"memory", "cc");
+
+	return res;
+}
+
+#endif /* PG_HAVE_ATOMIC_U64_SUPPORT */
+
 /* per architecture manual doubleword accesses have single copy atomicity */
 #define PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY
diff --git a/src/include/port/atomics/generic-xlc.h b/src/include/port/atomics/generic-xlc.h
index 8b5c732..8330b45 100644
--- a/src/include/port/atomics/generic-xlc.h
+++ b/src/include/port/atomics/generic-xlc.h
@@ -18,23 +18,6 @@
 
 #if defined(HAVE_ATOMICS)
 
-#define PG_HAVE_ATOMIC_U32_SUPPORT
-typedef struct pg_atomic_uint32
-{
-	volatile uint32 value;
-} pg_atomic_uint32;
-
-
-/* 64bit atomics are only supported in 64bit mode */
-#ifdef __64BIT__
-#define PG_HAVE_ATOMIC_U64_SUPPORT
-typedef struct pg_atomic_uint64
-{
-	volatile uint64 value pg_attribute_aligned(8);
-} pg_atomic_uint64;
-
-#endif /* __64BIT__ */
-
 #define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U32
 static inline bool
 pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
@@ -69,33 +52,6 @@ pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
 	return ret;
 }
 
-#define PG_HAVE_ATOMIC_FETCH_ADD_U32
-static inline uint32
-pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
-{
-	uint32 _t;
-	uint32 res;
-
-	/*
-	 * xlc has a no-longer-documented __fetch_and_add() intrinsic.  In xlc
-	 * 12.01.0000.0000, it emits a leading "sync" and trailing "isync".  In
-	 * xlc 13.01.0003.0004, it emits neither.  Hence, using the intrinsic
-	 * would add redundant syncs on xlc 12.
-	 */
-	__asm__ __volatile__(
-		"	sync				\n"
-		"	lwarx   %1,0,%4		\n"
-		"	add     %0,%1,%3	\n"
-		"	stwcx.  %0,0,%4		\n"
-		"	bne     $-12		\n"		/* branch to lwarx */
-		"	isync				\n"
-:		"=&r"(_t), "=&r"(res), "+m"(ptr->value)
-:		"r"(add_), "r"(&ptr->value)
-:		"memory", "cc");
-
-	return res;
-}
-
 #ifdef PG_HAVE_ATOMIC_U64_SUPPORT
 
 #define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U64
@@ -115,28 +71,6 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 	return ret;
 }
 
-#define PG_HAVE_ATOMIC_FETCH_ADD_U64
-static inline uint64
-pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
-{
-	uint64 _t;
-	uint64 res;
-
-	/* Like u32, but s/lwarx/ldarx/; s/stwcx/stdcx/ */
-	__asm__ __volatile__(
-		"	sync				\n"
-		"	ldarx   %1,0,%4		\n"
-		"	add     %0,%1,%3	\n"
-		"	stdcx.  %0,0,%4		\n"
-		"	bne     $-12		\n"		/* branch to ldarx */
-		"	isync				\n"
-:		"=&r"(_t), "=&r"(res), "+m"(ptr->value)
-:		"r"(add_), "r"(&ptr->value)
-:		"memory", "cc");
-
-	return res;
-}
-
 #endif /* PG_HAVE_ATOMIC_U64_SUPPORT */
 
 #endif /* defined(HAVE_ATOMICS) */