[PATCH] Use Clang's __has_* macros in Compiler.h to test for features

[Reid Kleckner]

This actually needs more testing.  But feel free to comment on the
direction.


On Tue, Jul 2, 2013 at 2:21 PM, Reid Kleckner <rnk at google.com> wrote:

> When targetting Windows, clang does not define __GNUC__, and as a result
> we don't use our attributes with it.  This leads to warnings about
> unused functions that are already annotated with LLVM_ATTRIBUTE_UNUSED.
>
> While I'm here, conditionally define and use __GNUC_PREREQ for gcc
> version checks.  Spelling the check out with three comparisons is
> verbose and error prone.
>
> http://llvm-reviews.chandlerc.com/D1080
>
> Files:
>   include/llvm/Support/Compiler.h
>
> Index: include/llvm/Support/Compiler.h
> ===================================================================
> --- include/llvm/Support/Compiler.h
> +++ include/llvm/Support/Compiler.h
> @@ -21,6 +21,21 @@
>  # define __has_feature(x) 0
>  #endif
>
> +#ifndef __has_attribute
> +# define __has_attribute(x) 0
> +#endif
> +
> +/// \macro __GNUC_PREREQ
> +/// \brief Defines __GNUC_PREREQ if glibc's features.h isn't available.
> +#ifndef __GNUC_PREREQ
> +# if defined(__GNUC__) && defined(__GNUC_MINOR__)
> +#  define __GNUC_PREREQ(maj, min) \
> +    ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
> +# else
> +#  define __GNUC_PREREQ(maj, min) 0
> +# endif
> +#endif
> +
>  /// \brief Does the compiler support r-value references?
>  /// This implies that <utility> provides the one-argument std::move;  it
>  /// does not imply the existence of any other C++ library features.
> @@ -146,13 +161,13 @@
>  /// into a shared library, then the class should be private to the
> library and
>  /// not accessible from outside it.  Can also be used to mark variables
> and
>  /// functions, making them private to any shared library they are linked
> into.
> -#if (__GNUC__ >= 4) && !defined(__MINGW32__) && !defined(__CYGWIN__)
> +#if __GNUC_PREREQ(4, 0) && !defined(__MINGW32__) && !defined(__CYGWIN__)
>  #define LLVM_LIBRARY_VISIBILITY __attribute__ ((visibility("hidden")))
>  #else
>  #define LLVM_LIBRARY_VISIBILITY
>  #endif
>
> -#if (__GNUC__ >= 4 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1))
> +#if __has_attribute(used) || __GNUC_PREREQ(3, 1)
>  #define LLVM_ATTRIBUTE_USED __attribute__((__used__))
>  #else
>  #define LLVM_ATTRIBUTE_USED
> @@ -166,31 +181,33 @@
>  // more portable solution:
>  //   (void)unused_var_name;
>  // Prefer cast-to-void wherever it is sufficient.
> -#if (__GNUC__ >= 4 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 1))
> +#if __has_attribute(unused) || __GNUC_PREREQ(3, 1)
>  #define LLVM_ATTRIBUTE_UNUSED __attribute__((__unused__))
>  #else
>  #define LLVM_ATTRIBUTE_UNUSED
>  #endif
>
> -#if (__GNUC__ >= 4) && !defined(__MINGW32__) && !defined(__CYGWIN__)
> +#if __GNUC_PREREQ(4, 0) && !defined(__MINGW32__) && !defined(__CYGWIN__)
>  #define LLVM_ATTRIBUTE_WEAK __attribute__((__weak__))
>  #else
>  #define LLVM_ATTRIBUTE_WEAK
>  #endif
>
> -#ifdef __GNUC__ // aka 'CONST' but following LLVM Conventions.
> +#if __has_attribute(const) || defined(__GNUC__)
> +// aka 'CONST' but following LLVM Conventions.
>  #define LLVM_READNONE __attribute__((__const__))
>  #else
>  #define LLVM_READNONE
>  #endif
>
> -#ifdef __GNUC__  // aka 'PURE' but following LLVM Conventions.
> +#if __has_attribute(pure) || defined(__GNUC__)
> +// aka 'PURE' but following LLVM Conventions.
>  #define LLVM_READONLY __attribute__((__pure__))
>  #else
>  #define LLVM_READONLY
>  #endif
>
> -#if (__GNUC__ >= 4)
> +#if __has_builtin(__builtin_expect) || __GNUC_PREREQ(4, 0)
>  #define LLVM_LIKELY(EXPR) __builtin_expect((bool)(EXPR), true)
>  #define LLVM_UNLIKELY(EXPR) __builtin_expect((bool)(EXPR), false)
>  #else
> @@ -213,7 +230,7 @@
>
>  /// LLVM_ATTRIBUTE_NOINLINE - On compilers where we have a directive to
> do so,
>  /// mark a method "not for inlining".
> -#if (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4))
> +#if __GNUC_PREREQ(3, 4)
>  #define LLVM_ATTRIBUTE_NOINLINE __attribute__((noinline))
>  #elif defined(_MSC_VER)
>  #define LLVM_ATTRIBUTE_NOINLINE __declspec(noinline)
> @@ -225,7 +242,7 @@
>  /// so, mark a method "always inline" because it is performance
> sensitive. GCC
>  /// 3.4 supported this but is buggy in various cases and produces
> unimplemented
>  /// errors, just use it in GCC 4.0 and later.
> -#if __GNUC__ > 3
> +#if __GNUC_PREREQ(3, 0)
>  #define LLVM_ATTRIBUTE_ALWAYS_INLINE inline __attribute__((always_inline))
>  #elif defined(_MSC_VER)
>  #define LLVM_ATTRIBUTE_ALWAYS_INLINE __forceinline
> @@ -267,27 +284,23 @@
>  /// LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
>  /// to an expression which states that it is undefined behavior for the
>  /// compiler to reach this point.  Otherwise is not defined.
> -#if defined(__clang__) || (__GNUC__ > 4) \
> - || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
> +#if __has_builtin(__builtin_unreachable) || __GNUC_PREREQ(4, 5)
>  # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
>  #elif defined(_MSC_VER)
>  # define LLVM_BUILTIN_UNREACHABLE __assume(false)
>  #endif
>
>  /// LLVM_BUILTIN_TRAP - On compilers which support it, expands to an
> expression
>  /// which causes the program to exit abnormally.
> -#if defined(__clang__) || (__GNUC__ > 4) \
> - || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)
> +#if __has_builtin(__builtin_trap) || __GNUC_PREREQ(4, 3)
>  # define LLVM_BUILTIN_TRAP __builtin_trap()
>  #else
>  # define LLVM_BUILTIN_TRAP *(volatile int*)0x11 = 0
>  #endif
>
>  /// \macro LLVM_ASSUME_ALIGNED
>  /// \brief Returns a pointer with an assumed alignment.
> -#if !defined(__clang__) && ((__GNUC__ > 4) \
> - || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
> -// FIXME: Enable on clang when it supports it.
> +#if __has_builtin(__builtin_assume_aligned) && __GNUC_PREREQ(4, 7)
>  # define LLVM_ASSUME_ALIGNED(p, a) __builtin_assume_aligned(p, a)
>  #elif defined(LLVM_BUILTIN_UNREACHABLE)
>  # define LLVM_ASSUME_ALIGNED(p, a) \
>
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