[libclc] [libclc] Fix memory fence scope mapping for OpenCL (PR #170542)

[Victor Mustya via cfe-commits]

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@@ -13,16 +13,15 @@
 #include <clc/mem_fence/clc_mem_semantic.h>
 #include <clc/opencl/synchronization/cl_mem_fence_flags.h>
 
-_CLC_INLINE int __opencl_get_memory_scope(cl_mem_fence_flags flag) {
-  int memory_scope = 0;
+static _CLC_INLINE int __opencl_get_memory_scope(cl_mem_fence_flags flag) {
   if (flag & CLK_GLOBAL_MEM_FENCE)
-    memory_scope |= __MEMORY_SCOPE_DEVICE;
+    return __MEMORY_SCOPE_DEVICE;
   if (flag & CLK_LOCAL_MEM_FENCE)
-    memory_scope |= __MEMORY_SCOPE_WRKGRP;
-  return memory_scope;
+    return __MEMORY_SCOPE_WRKGRP;
+  return __MEMORY_SCOPE_SINGLE;
----------------
vmustya wrote:

According to the [OpenCL C spec](https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_C.html#synchronization-functions), the synchronization functions allow the `0` to be passed as a valid argument value. When `0` is passed, the single-thread fence looks reasonable to me, because the implementation shouldn't issue any cross-thread memory synchronization in that case. The `__builtin_unreachable()` won't work correctly.

https://github.com/llvm/llvm-project/pull/170542