macros.h

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// Copyright 2014 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef V8_BASE_MACROS_H_
#define V8_BASE_MACROS_H_
 
#include <cstring>
 
#include "include/v8stdint.h"
#include "src/base/build_config.h"
#include "src/base/compiler-specific.h"
#include "src/base/logging.h"
 
 
// The expression OFFSET_OF(type, field) computes the byte-offset
// of the specified field relative to the containing type. This
// corresponds to 'offsetof' (in stddef.h), except that it doesn't
// use 0 or NULL, which causes a problem with the compiler warnings
// we have enabled (which is also why 'offsetof' doesn't seem to work).
// Here we simply use the non-zero value 4, which seems to work.
#define OFFSET_OF(type, field)                                          \
  (reinterpret_cast<intptr_t>(&(reinterpret_cast<type*>(4)->field)) - 4)
 
 
// ARRAYSIZE_UNSAFE performs essentially the same calculation as arraysize,
// but can be used on anonymous types or types defined inside
// functions.  It's less safe than arraysize as it accepts some
// (although not all) pointers.  Therefore, you should use arraysize
// whenever possible.
//
// The expression ARRAYSIZE_UNSAFE(a) is a compile-time constant of type
// size_t.
//
// ARRAYSIZE_UNSAFE catches a few type errors.  If you see a compiler error
//
//   "warning: division by zero in ..."
//
// when using ARRAYSIZE_UNSAFE, you are (wrongfully) giving it a pointer.
// You should only use ARRAYSIZE_UNSAFE on statically allocated arrays.
//
// The following comments are on the implementation details, and can
// be ignored by the users.
//
// ARRAYSIZE_UNSAFE(arr) works by inspecting sizeof(arr) (the # of bytes in
// the array) and sizeof(*(arr)) (the # of bytes in one array
// element).  If the former is divisible by the latter, perhaps arr is
// indeed an array, in which case the division result is the # of
// elements in the array.  Otherwise, arr cannot possibly be an array,
// and we generate a compiler error to prevent the code from
// compiling.
//
// Since the size of bool is implementation-defined, we need to cast
// !(sizeof(a) & sizeof(*(a))) to size_t in order to ensure the final
// result has type size_t.
//
// This macro is not perfect as it wrongfully accepts certain
// pointers, namely where the pointer size is divisible by the pointee
// size.  Since all our code has to go through a 32-bit compiler,
// where a pointer is 4 bytes, this means all pointers to a type whose
// size is 3 or greater than 4 will be (righteously) rejected.
#define ARRAYSIZE_UNSAFE(a)     \
  ((sizeof(a) / sizeof(*(a))) / \
   static_cast<size_t>(!(sizeof(a) % sizeof(*(a)))))  // NOLINT
 
 
#if V8_OS_NACL
 
// TODO(bmeurer): For some reason, the NaCl toolchain cannot handle the correct
// definition of arraysize() below, so we have to use the unsafe version for
// now.
#define arraysize ARRAYSIZE_UNSAFE
 
#else  // V8_OS_NACL
 
// The arraysize(arr) macro returns the # of elements in an array arr.
// The expression is a compile-time constant, and therefore can be
// used in defining new arrays, for example.  If you use arraysize on
// a pointer by mistake, you will get a compile-time error.
//
// One caveat is that arraysize() doesn't accept any array of an
// anonymous type or a type defined inside a function.  In these rare
// cases, you have to use the unsafe ARRAYSIZE_UNSAFE() macro below.  This is
// due to a limitation in C++'s template system.  The limitation might
// eventually be removed, but it hasn't happened yet.
#define arraysize(array) (sizeof(ArraySizeHelper(array)))
 
 
// This template function declaration is used in defining arraysize.
// Note that the function doesn't need an implementation, as we only
// use its type.
template <typename T, size_t N>
char (&ArraySizeHelper(T (&array)[N]))[N];
 
 
#if !V8_CC_MSVC
// That gcc wants both of these prototypes seems mysterious. VC, for
// its part, can't decide which to use (another mystery). Matching of
// template overloads: the final frontier.
template <typename T, size_t N>
char (&ArraySizeHelper(const T (&array)[N]))[N];
#endif
 
#endif  // V8_OS_NACL
 
 
// The COMPILE_ASSERT macro can be used to verify that a compile time
// expression is true. For example, you could use it to verify the
// size of a static array:
//
//   COMPILE_ASSERT(ARRAYSIZE_UNSAFE(content_type_names) == CONTENT_NUM_TYPES,
//                  content_type_names_incorrect_size);
//
// or to make sure a struct is smaller than a certain size:
//
//   COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large);
//
// The second argument to the macro is the name of the variable. If
// the expression is false, most compilers will issue a warning/error
// containing the name of the variable.
#if V8_HAS_CXX11_STATIC_ASSERT
 
// Under C++11, just use static_assert.
#define COMPILE_ASSERT(expr, msg) static_assert(expr, #msg)
 
#else
 
template <bool>
struct CompileAssert {};
 
#define COMPILE_ASSERT(expr, msg)                \
  typedef CompileAssert<static_cast<bool>(expr)> \
      msg[static_cast<bool>(expr) ? 1 : -1] ALLOW_UNUSED
 
// Implementation details of COMPILE_ASSERT:
//
// - COMPILE_ASSERT works by defining an array type that has -1
//   elements (and thus is invalid) when the expression is false.
//
// - The simpler definition
//
//     #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1]
//
//   does not work, as gcc supports variable-length arrays whose sizes
//   are determined at run-time (this is gcc's extension and not part
//   of the C++ standard).  As a result, gcc fails to reject the
//   following code with the simple definition:
//
//     int foo;
//     COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is
//                               // not a compile-time constant.
//
// - By using the type CompileAssert<(bool(expr))>, we ensures that
//   expr is a compile-time constant.  (Template arguments must be
//   determined at compile-time.)
//
// - The outer parentheses in CompileAssert<(bool(expr))> are necessary
//   to work around a bug in gcc 3.4.4 and 4.0.1.  If we had written
//
//     CompileAssert<bool(expr)>
//
//   instead, these compilers will refuse to compile
//
//     COMPILE_ASSERT(5 > 0, some_message);
//
//   (They seem to think the ">" in "5 > 0" marks the end of the
//   template argument list.)
//
// - The array size is (bool(expr) ? 1 : -1), instead of simply
//
//     ((expr) ? 1 : -1).
//
//   This is to avoid running into a bug in MS VC 7.1, which
//   causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1.
 
#endif
 
 
// bit_cast<Dest,Source> is a template function that implements the
// equivalent of "*reinterpret_cast<Dest*>(&source)".  We need this in
// very low-level functions like the protobuf library and fast math
// support.
//
//   float f = 3.14159265358979;
//   int i = bit_cast<int32>(f);
//   // i = 0x40490fdb
//
// The classical address-casting method is:
//
//   // WRONG
//   float f = 3.14159265358979;            // WRONG
//   int i = * reinterpret_cast<int*>(&f);  // WRONG
//
// The address-casting method actually produces undefined behavior
// according to ISO C++ specification section 3.10 -15 -.  Roughly, this
// section says: if an object in memory has one type, and a program
// accesses it with a different type, then the result is undefined
// behavior for most values of "different type".
//
// This is true for any cast syntax, either *(int*)&f or
// *reinterpret_cast<int*>(&f).  And it is particularly true for
// conversions between integral lvalues and floating-point lvalues.
//
// The purpose of 3.10 -15- is to allow optimizing compilers to assume
// that expressions with different types refer to different memory.  gcc
// 4.0.1 has an optimizer that takes advantage of this.  So a
// non-conforming program quietly produces wildly incorrect output.
//
// The problem is not the use of reinterpret_cast.  The problem is type
// punning: holding an object in memory of one type and reading its bits
// back using a different type.
//
// The C++ standard is more subtle and complex than this, but that
// is the basic idea.
//
// Anyways ...
//
// bit_cast<> calls memcpy() which is blessed by the standard,
// especially by the example in section 3.9 .  Also, of course,
// bit_cast<> wraps up the nasty logic in one place.
//
// Fortunately memcpy() is very fast.  In optimized mode, with a
// constant size, gcc 2.95.3, gcc 4.0.1, and msvc 7.1 produce inline
// code with the minimal amount of data movement.  On a 32-bit system,
// memcpy(d,s,4) compiles to one load and one store, and memcpy(d,s,8)
// compiles to two loads and two stores.
//
// I tested this code with gcc 2.95.3, gcc 4.0.1, icc 8.1, and msvc 7.1.
//
// WARNING: if Dest or Source is a non-POD type, the result of the memcpy
// is likely to surprise you.
template <class Dest, class Source>
V8_INLINE Dest bit_cast(Source const& source) {
  COMPILE_ASSERT(sizeof(Dest) == sizeof(Source), VerifySizesAreEqual);
 
  Dest dest;
  memcpy(&dest, &source, sizeof(dest));
  return dest;
}
 
 
// A macro to disallow the evil copy constructor and operator= functions
// This should be used in the private: declarations for a class
#define DISALLOW_COPY_AND_ASSIGN(TypeName)  \
  TypeName(const TypeName&) V8_DELETE;      \
  void operator=(const TypeName&) V8_DELETE
 
 
// A macro to disallow all the implicit constructors, namely the
// default constructor, copy constructor and operator= functions.
//
// This should be used in the private: declarations for a class
// that wants to prevent anyone from instantiating it. This is
// especially useful for classes containing only static methods.
#define DISALLOW_IMPLICIT_CONSTRUCTORS(TypeName)  \
  TypeName() V8_DELETE;                           \
  DISALLOW_COPY_AND_ASSIGN(TypeName)
 
 
// Newly written code should use V8_INLINE and V8_NOINLINE directly.
#define INLINE(declarator)    V8_INLINE declarator
#define NO_INLINE(declarator) V8_NOINLINE declarator
 
 
// Newly written code should use WARN_UNUSED_RESULT.
#define MUST_USE_RESULT WARN_UNUSED_RESULT
 
 
// Define V8_USE_ADDRESS_SANITIZER macros.
#if defined(__has_feature)
#if __has_feature(address_sanitizer)
#define V8_USE_ADDRESS_SANITIZER 1
#endif
#endif
 
// Define DISABLE_ASAN macros.
#ifdef V8_USE_ADDRESS_SANITIZER
#define DISABLE_ASAN __attribute__((no_sanitize_address))
#else
#define DISABLE_ASAN
#endif
 
 
#if V8_CC_GNU
#define V8_IMMEDIATE_CRASH() __builtin_trap()
#else
#define V8_IMMEDIATE_CRASH() ((void(*)())0)()
#endif
 
 
// Use C++11 static_assert if possible, which gives error
// messages that are easier to understand on first sight.
#if V8_HAS_CXX11_STATIC_ASSERT
#define STATIC_ASSERT(test) static_assert(test, #test)
#else
// This is inspired by the static assertion facility in boost.  This
// is pretty magical.  If it causes you trouble on a platform you may
// find a fix in the boost code.
template <bool> class StaticAssertion;
template <> class StaticAssertion<true> { };
// This macro joins two tokens.  If one of the tokens is a macro the
// helper call causes it to be resolved before joining.
#define SEMI_STATIC_JOIN(a, b) SEMI_STATIC_JOIN_HELPER(a, b)
#define SEMI_STATIC_JOIN_HELPER(a, b) a##b
// Causes an error during compilation of the condition is not
// statically known to be true.  It is formulated as a typedef so that
// it can be used wherever a typedef can be used.  Beware that this
// actually causes each use to introduce a new defined type with a
// name depending on the source line.
template <int> class StaticAssertionHelper { };
#define STATIC_ASSERT(test)                                                    \
  typedef                                                                     \
    StaticAssertionHelper<sizeof(StaticAssertion<static_cast<bool>((test))>)> \
    SEMI_STATIC_JOIN(__StaticAssertTypedef__, __LINE__) ALLOW_UNUSED
 
#endif
 
 
// The USE(x) template is used to silence C++ compiler warnings
// issued for (yet) unused variables (typically parameters).
template <typename T>
inline void USE(T) { }
 
 
#define IS_POWER_OF_TWO(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
 
 
// Define our own macros for writing 64-bit constants.  This is less fragile
// than defining __STDC_CONSTANT_MACROS before including <stdint.h>, and it
// works on compilers that don't have it (like MSVC).
#if V8_CC_MSVC
# define V8_UINT64_C(x)   (x ## UI64)
# define V8_INT64_C(x)    (x ## I64)
# if V8_HOST_ARCH_64_BIT
#  define V8_INTPTR_C(x)  (x ## I64)
#  define V8_PTR_PREFIX   "ll"
# else
#  define V8_INTPTR_C(x)  (x)
#  define V8_PTR_PREFIX   ""
# endif  // V8_HOST_ARCH_64_BIT
#elif V8_CC_MINGW64
# define V8_UINT64_C(x)   (x ## ULL)
# define V8_INT64_C(x)    (x ## LL)
# define V8_INTPTR_C(x)   (x ## LL)
# define V8_PTR_PREFIX    "I64"
#elif V8_HOST_ARCH_64_BIT
# if V8_OS_MACOSX
#  define V8_UINT64_C(x)   (x ## ULL)
#  define V8_INT64_C(x)    (x ## LL)
# else
#  define V8_UINT64_C(x)   (x ## UL)
#  define V8_INT64_C(x)    (x ## L)
# endif
# define V8_INTPTR_C(x)   (x ## L)
# define V8_PTR_PREFIX    "l"
#else
# define V8_UINT64_C(x)   (x ## ULL)
# define V8_INT64_C(x)    (x ## LL)
# define V8_INTPTR_C(x)   (x)
# define V8_PTR_PREFIX    ""
#endif
 
#define V8PRIxPTR V8_PTR_PREFIX "x"
#define V8PRIdPTR V8_PTR_PREFIX "d"
#define V8PRIuPTR V8_PTR_PREFIX "u"
 
// Fix for Mac OS X defining uintptr_t as "unsigned long":
#if V8_OS_MACOSX
#undef V8PRIxPTR
#define V8PRIxPTR "lx"
#endif
 
// The following macro works on both 32 and 64-bit platforms.
// Usage: instead of writing 0x1234567890123456
//      write V8_2PART_UINT64_C(0x12345678,90123456);
#define V8_2PART_UINT64_C(a, b) (((static_cast<uint64_t>(a) << 32) + 0x##b##u))
 
 
// Compute the 0-relative offset of some absolute value x of type T.
// This allows conversion of Addresses and integral types into
// 0-relative int offsets.
template <typename T>
inline intptr_t OffsetFrom(T x) {
  return x - static_cast<T>(0);
}
 
 
// Compute the absolute value of type T for some 0-relative offset x.
// This allows conversion of 0-relative int offsets into Addresses and
// integral types.
template <typename T>
inline T AddressFrom(intptr_t x) {
  return static_cast<T>(static_cast<T>(0) + x);
}
 
 
// Return the largest multiple of m which is <= x.
template <typename T>
inline T RoundDown(T x, intptr_t m) {
  DCHECK(IS_POWER_OF_TWO(m));
  return AddressFrom<T>(OffsetFrom(x) & -m);
}
 
 
// Return the smallest multiple of m which is >= x.
template <typename T>
inline T RoundUp(T x, intptr_t m) {
  return RoundDown<T>(static_cast<T>(x + m - 1), m);
}
 
#endif   // V8_BASE_MACROS_H_