Index: compiler-rt/lib/builtins/atomic.c
===================================================================
--- compiler-rt/lib/builtins/atomic.c
+++ compiler-rt/lib/builtins/atomic.c
@@ -122,36 +122,51 @@
   return locks + (hash & SPINLOCK_MASK);
 }
 
-/// Macros for determining whether a size is lock free.  Clang can not yet
-/// codegen __atomic_is_lock_free(16), so for now we assume 16-byte values are
-/// not lock free.
-#define IS_LOCK_FREE_1 __c11_atomic_is_lock_free(1)
-#define IS_LOCK_FREE_2 __c11_atomic_is_lock_free(2)
-#define IS_LOCK_FREE_4 __c11_atomic_is_lock_free(4)
-#define IS_LOCK_FREE_8 __c11_atomic_is_lock_free(8)
-#define IS_LOCK_FREE_16 0
+/// Macros for determining whether a size is lock free.
+/// __c11_atomic_lock_free(16) is sometimes a dynamic property (e.g. not all
+/// x86_64 CPUs have cmpxchg16) but compiler-rt does not implement the required
+/// libcall yet so we use an approximation valid on all CPUs where it is a
+/// static property.
+#define IS_LOCK_FREE_1(ptr) (__c11_atomic_is_lock_free(1) && (uintptr_t)ptr % 1 == 0)
+#define IS_LOCK_FREE_2(ptr) (__c11_atomic_is_lock_free(2) && (uintptr_t)ptr % 2 == 0)
+#define IS_LOCK_FREE_4(ptr) (__c11_atomic_is_lock_free(4) && (uintptr_t)ptr % 4 == 0)
+#define IS_LOCK_FREE_8(ptr) (__c11_atomic_is_lock_free(8) && (uintptr_t)ptr % 8 == 0)
+#define IS_LOCK_FREE_16(ptr) (__atomic_always_lock_free(16, 0) && (uintptr_t)ptr % 16 == 0)
+
+// Most 32-bit platforms do not have __int128_t, but fortunately they also don't
+// support 16-byte atomics so in that case the horribly wrong use of uint64_t
+// will never actually happen.
+#if defined(__SIZEOF_INT128__)
+#define LOCK_FREE_16_TYPE __uint128_t
+#else
+_Static_assert(!IS_LOCK_FREE_16(0), "lock free 16-byte atomic attempted but nothing to implement it with");
+#define LOCK_FREE_16_TYPE uint64_t
+#endif
 
 /// Macro that calls the compiler-generated lock-free versions of functions
 /// when they exist.
-#define LOCK_FREE_CASES() \
+#define LOCK_FREE_CASES(ptr) \
   do {\
   switch (size) {\
+    case 1:\
+      if (IS_LOCK_FREE_1(ptr)) {\
+        LOCK_FREE_ACTION(uint8_t);\
+      }\
     case 2:\
-      if (IS_LOCK_FREE_2) {\
+      if (IS_LOCK_FREE_2(ptr)) {\
         LOCK_FREE_ACTION(uint16_t);\
       }\
     case 4:\
-      if (IS_LOCK_FREE_4) {\
+      if (IS_LOCK_FREE_4(ptr)) {\
         LOCK_FREE_ACTION(uint32_t);\
       }\
     case 8:\
-      if (IS_LOCK_FREE_8) {\
+    if (IS_LOCK_FREE_8(ptr)) {\
         LOCK_FREE_ACTION(uint64_t);\
       }\
     case 16:\
-      if (IS_LOCK_FREE_16) {\
-        /* FIXME: __uint128_t isn't available on 32 bit platforms.
-        LOCK_FREE_ACTION(__uint128_t);*/\
+      if (IS_LOCK_FREE_16(ptr)) {\
+        LOCK_FREE_ACTION(LOCK_FREE_16_TYPE);\
       }\
   }\
   } while (0)
@@ -163,7 +178,7 @@
 #define LOCK_FREE_ACTION(type) \
     *((type*)dest) = __c11_atomic_load((_Atomic(type)*)src, model);\
     return;
-  LOCK_FREE_CASES();
+  LOCK_FREE_CASES(src);
 #undef LOCK_FREE_ACTION
   Lock *l = lock_for_pointer(src);
   lock(l);
@@ -177,7 +192,7 @@
 #define LOCK_FREE_ACTION(type) \
     __c11_atomic_store((_Atomic(type)*)dest, *(type*)dest, model);\
     return;
-  LOCK_FREE_CASES();
+  LOCK_FREE_CASES(dest);
 #undef LOCK_FREE_ACTION
   Lock *l = lock_for_pointer(dest);
   lock(l);
@@ -195,7 +210,7 @@
 #define LOCK_FREE_ACTION(type) \
   return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, (type*)expected,\
       *(type*)desired, success, failure)
-  LOCK_FREE_CASES();
+  LOCK_FREE_CASES(ptr);
 #undef LOCK_FREE_ACTION
   Lock *l = lock_for_pointer(ptr);
   lock(l);
@@ -216,7 +231,7 @@
     *(type*)old = __c11_atomic_exchange((_Atomic(type)*)ptr, *(type*)val,\
         model);\
     return;
-  LOCK_FREE_CASES();
+  LOCK_FREE_CASES(ptr);
 #undef LOCK_FREE_ACTION
   Lock *l = lock_for_pointer(ptr);
   lock(l);
@@ -246,7 +261,7 @@
 
 #define OPTIMISED_CASE(n, lockfree, type)\
 type __atomic_load_##n(type *src, int model) {\
-  if (lockfree)\
+  if (lockfree(src))\
     return __c11_atomic_load((_Atomic(type)*)src, model);\
   Lock *l = lock_for_pointer(src);\
   lock(l);\
@@ -259,7 +274,7 @@
 
 #define OPTIMISED_CASE(n, lockfree, type)\
 void  __atomic_store_##n(type *dest, type val, int model) {\
-  if (lockfree) {\
+  if (lockfree(dest)) {\
     __c11_atomic_store((_Atomic(type)*)dest, val, model);\
     return;\
   }\
@@ -274,7 +289,7 @@
 
 #define OPTIMISED_CASE(n, lockfree, type)\
 type __atomic_exchange_##n(type *dest, type val, int model) {\
-  if (lockfree)\
+  if (lockfree(dest))\
     return __c11_atomic_exchange((_Atomic(type)*)dest, val, model);\
   Lock *l = lock_for_pointer(dest);\
   lock(l);\
@@ -289,7 +304,7 @@
 #define OPTIMISED_CASE(n, lockfree, type)\
 int __atomic_compare_exchange_##n(type *ptr, type *expected, type desired,\
     int success, int failure) {\
-  if (lockfree)\
+  if (lockfree(ptr))\
     return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, expected, desired,\
         success, failure);\
   Lock *l = lock_for_pointer(ptr);\
@@ -311,7 +326,7 @@
 ////////////////////////////////////////////////////////////////////////////////
 #define ATOMIC_RMW(n, lockfree, type, opname, op) \
 type __atomic_fetch_##opname##_##n(type *ptr, type val, int model) {\
-  if (lockfree) \
+  if (lockfree(ptr))\
     return __c11_atomic_fetch_##opname((_Atomic(type)*)ptr, val, model);\
   Lock *l = lock_for_pointer(ptr);\
   lock(l);\