mlpack/fastlib/base/compiler.h

// Copyright 2007 Georgia Institute of Technology. All rights reserved.
/**
 * @file compiler.h
 *
 * Defines compiler-specific directives and optimizations.  Especially
 * useful are branch-prediction macros "likely" and "unlikely", which
 * can dramatically affect your code's running time.
 */

#ifndef BASE_COMPILER_H
#define BASE_COMPILER_H

/**
 * Begin listing C function prototypes.
 *
 * Needed for proper linking in C++.
 */
#ifdef __cplusplus
#define EXTERN_C_BEGIN extern "C" {
#else
#define EXTERN_C_BEGIN
#endif

/**
 * Finish listing C function prototypes.
 *
 * Needed for proper linking in C++.
 */
#ifdef __cplusplus
#define EXTERN_C_END };
#else
#define EXTERN_C_END
#endif

/* Check for availability of compiler optimizations and directives. */
#if !defined(__GNUC__) && !defined(__INTEL_COMPILER)
#warning Unknown compiler; optimizations and directives disabled.
#ifndef NO_COMPILER_DEFS
#define NO_COMPILER_DEFS
#endif
#endif



/**
 * Optimize compilation for an expression having a given value.
 *
 * @param expr the expression to evaluate
 * @param value the expected result of expr
 */
#ifndef NO_COMPILER_DEFS
#define expect(expr, value) (__builtin_expect((expr), (value)))
#else
#define expect(expr, value) (expr)
#endif

/** Optimize compilation for a condition being true. */
#define likely(cond) expect(!!(cond), 1)

/** Optimize compilation for a condition being false. */
#define unlikely(cond) expect(!!(cond), 0)

/** Returns 1 if the compiler can prove the expression is constant. */
#ifndef NO_COMPILER_DEFS
#define IS_CONST_EXPR(expr) (__builtin_constant_p(expr))
#else
#define IS_CONST_EXPR(expr) 0
#endif



/**
 * Define a function that never returns, e.g. aborts the program.
 *
 * Supresses potential warning messages about not returning a value in
 * non-void functions.
 *
 * Example:
 * @code
 *   COMPILER_NO_RETURN
 *   void my_abort();
 * @endcode
 */
#ifndef NO_COMPILER_DEFS
#define COMPILER_NO_RETURN __attribute__((noreturn))
#else
#define COMPILER_NO_RETURN
#endif

/**
 * Define a function with printf-style arguments.
 *
 * Enables compile-time warning messages for mismatched printf format
 * strings and argument lists.
 *
 * Example:
 * @code
 *   COMPILER_PRINTF(2, 4)
 *   void my_printf(int i, const char *format, int j, ...);
 * @endcode
 *
 * Note that C++ member funtions reserve argument 1 for "this", so the
 * first visible argument is 2.
 */
#ifndef NO_COMPILER_DEFS
#define COMPILER_PRINTF(format_arg, dotdotdot_arg) \
    __attribute__((format(printf, format_arg, dotdotdot_arg)))
#else
#define COMPILER_PRINTF(format_arg, dotdotdot_arg)
#endif

/**
 * Define a function with no (visible) side-effects.
 *
 * Allows the compiler to optimize out repeated calls or rearrange
 * calls, and supresses warning messages when multiple functions are
 * called in an expression.  (By definition, C/C++ may evaluate
 * expressions in any order, causing problems for functions with
 * side-effects).
 *
 * WARNING: due to complier bugs, never use for functions of pointers.
 *
 * Example:
 * @code
 *   COMPILER_FUNCTIONAL
 *   double my_hypot(double x, double y);
 * @endcode
 */
#ifndef NO_COMPILER_DEFS
#define COMPILER_FUNCTIONAL __attribute__((const))
#else
#define COMPILER_FUNCTIONAL
#endif

/**
 * Disables the in-lining of a function.
 *
 * Used to reduce compiled binary size, but rarely worthwhile.
 */
#ifndef NO_COMPILER_DEFS
#define COMPILER_NO_INLINE __attribute__((noinline))
#else
#define COMPILER_NO_INLINE
#endif



/**
 * Computes the stride (alignment) of a type.
 *
 * Input must be a type, not a variable as is possible with sizeof.
 */
#ifdef __cplusplus
#define strideof(T) (compiler_strideof<T>::STRIDE)
template <typename T>
struct compiler_strideof {
  struct S {T x; char c;};
  /* The compiler gives this stride in a struct. */
  static const int NATURAL_STRIDE =
      (sizeof(S) > sizeof(T)) ? (sizeof(S) - sizeof(T)) : sizeof(T);
  /* This power-of-two stride is likely to be faster. */
  static const int PREFERRED_STRIDE =
      sizeof(T) >= 8 ? 8 : sizeof(T) >= 4 ? 4 : 0;
  /* Use the larger of the two. */
  enum { STRIDE = NATURAL_STRIDE > PREFERRED_STRIDE
      ? NATURAL_STRIDE : PREFERRED_STRIDE };
};
#else
#define strideof(T) (sizeof(struct {T x; char c;}) - sizeof(T))
#endif

/** Aligns a size_t to a multiple of a type's stride, rounding up. */
#define stride_align(num, T) \
    (((size_t)(num) + strideof(T) - 1) / strideof(T) * strideof(T))

/** The maximum stride of any object on the platform. */
#define MAX_STRIDE 16           /* TODO: confirm correct */

/** Aligns a size_t to a multiple of the maximum stride. */
#define stride_align_max(num) \
    (((size_t)(num) + MAX_STRIDE - 1) & ~(size_t)(MAX_STRIDE - 1))

/* Fill possibly lacking definition for member memory offsets. */
#ifndef offsetof
#define offsetof(S, member) ((size_t)(&((S const *)0)->member))
#endif



/** Performs C++ casts in either C or C++. */
#ifdef __cplusplus
#define COMPILER_CAST(cast, T, val) (cast< T >(val))
#else
#define COMPILER_CAST(cast, T, val) ((T)(val))
#endif

/**
 * Performs the equivalent of static_cast in either C or C++.
 *
 * This is useful if you compile with -Wold-style-cast.
 */
#define STATIC_CAST(T, val) COMPILER_CAST(static_cast, T, val)

/**
 * Performs the equivalent of reinterpret_cast in either C or C++.
 *
 * This is useful if you compile with -Wold-style-cast.
 */
#define REINTERPRET_CAST(T, val) COMPILER_CAST(reinterpret_cast, T, val)



/** Use a constant reference in C++, or constant pointer in C. */
#ifdef __cplusplus
#define CONST_REF const &
#else
#define CONST_REF const *
#endif

/** Use a reference in C++, or pointer in C. */
#ifdef __cplusplus
#define REF &
#else
#define REF *
#endif



#endif /* BASE_COMPILER_H */