macros.h: Alphabetize the compiler-specific macros

cpp/src/arrow/util/macros.h

@@ -58,40 +58,42 @@
 // https://blog.regehr.org/archives/1096
 #define ARROW_UNUSED(x) (void)(x)
 #define ARROW_ARG_UNUSED(x)
-#if defined(__GNUC__) // GCC and clang
-#define ARROW_NORETURN __attribute__((noreturn))
-#define ARROW_NOINLINE __attribute__((noinline))
-#define ARROW_FORCE_INLINE __attribute__((always_inline))
-#define ARROW_PREDICT_FALSE(x) (__builtin_expect(!!(x), 0))
-#define ARROW_PREDICT_TRUE(x) (__builtin_expect(!!(x), 1))
-#define ARROW_PREFETCH(addr) __builtin_prefetch(addr)
-#define ARROW_RESTRICT __restrict
-#if defined(__clang__) // clang-specific
+#if defined(__GNUC__)
+// Both GCC and clang define __GNUC__
+#if defined(__clang__)
+// Only clang defines __clang__
 #define ARROW_COMPILER_ASSUME(expr) __builtin_assume(expr)
 #else
 #define ARROW_COMPILER_ASSUME(expr) \
  if (!(expr)) { \
  __builtin_unreachable(); \
  }
 #endif
+#define ARROW_FORCE_INLINE __attribute__((always_inline))
+#define ARROW_NOINLINE __attribute__((noinline))
+#define ARROW_NORETURN __attribute__((noreturn))
+#define ARROW_PREDICT_FALSE(x) (__builtin_expect(!!(x), 0))
+#define ARROW_PREDICT_TRUE(x) (__builtin_expect(!!(x), 1))
+#define ARROW_PREFETCH(addr) __builtin_prefetch(addr)
+#define ARROW_RESTRICT __restrict
 #elif defined(_MSC_VER) // MSVC
-#define ARROW_NORETURN __declspec(noreturn)
-#define ARROW_NOINLINE __declspec(noinline)
+#define ARROW_COMPILER_ASSUME(expr) __assume(expr)
 #define ARROW_FORCE_INLINE __declspec(forceinline)
+#define ARROW_NOINLINE __declspec(noinline)
+#define ARROW_NORETURN __declspec(noreturn)
 #define ARROW_PREDICT_FALSE(x) (x)
 #define ARROW_PREDICT_TRUE(x) (x)
 #define ARROW_PREFETCH(addr)
 #define ARROW_RESTRICT __restrict
-#define ARROW_COMPILER_ASSUME(expr) __assume(expr)
 #else
-#define ARROW_NORETURN
-#define ARROW_NOINLINE
+#define ARROW_COMPILER_ASSUME(expr)
 #define ARROW_FORCE_INLINE
+#define ARROW_NOINLINE
+#define ARROW_NORETURN
 #define ARROW_PREDICT_FALSE(x) (x)
 #define ARROW_PREDICT_TRUE(x) (x)
 #define ARROW_PREFETCH(addr)
 #define ARROW_RESTRICT
-#define ARROW_COMPILER_ASSUME(expr)
 #endif
 
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