gecko-dev/mfbt/Util.h

331 lines
7.9 KiB
C++

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/*
* Miscellaneous uncategorized functionality. Please add new functionality to
* new headers, or to other appropriate existing headers, not here.
*/
#ifndef mozilla_Util_h_
#define mozilla_Util_h_
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Types.h"
#ifdef __cplusplus
namespace mozilla {
/**
* DebugOnly contains a value of type T, but only in debug builds. In release
* builds, it does not contain a value. This helper is intended to be used with
* MOZ_ASSERT()-style macros, allowing one to write:
*
* DebugOnly<bool> check = func();
* MOZ_ASSERT(check);
*
* more concisely than declaring |check| conditional on #ifdef DEBUG, but also
* without allocating storage space for |check| in release builds.
*
* DebugOnly instances can only be coerced to T in debug builds. In release
* builds they don't have a value, so type coercion is not well defined.
*/
template<typename T>
struct DebugOnly
{
#ifdef DEBUG
T value;
DebugOnly() { }
DebugOnly(const T& other) : value(other) { }
DebugOnly(const DebugOnly& other) : value(other.value) { }
DebugOnly& operator=(const T& rhs) {
value = rhs;
return *this;
}
void operator++(int) {
value++;
}
void operator--(int) {
value--;
}
T *operator&() { return &value; }
operator T&() { return value; }
operator const T&() const { return value; }
T& operator->() { return value; }
#else
DebugOnly() { }
DebugOnly(const T&) { }
DebugOnly(const DebugOnly&) { }
DebugOnly& operator=(const T&) { return *this; }
void operator++(int) { }
void operator--(int) { }
#endif
/*
* DebugOnly must always have a destructor or else it will
* generate "unused variable" warnings, exactly what it's intended
* to avoid!
*/
~DebugOnly() {}
};
/*
* This class, and the corresponding macro MOZ_ALIGNOF, figure out how many
* bytes of alignment a given type needs.
*/
template<class T>
class AlignmentFinder
{
struct Aligner
{
char c;
T t;
};
public:
static const size_t alignment = sizeof(Aligner) - sizeof(T);
};
#define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment
/*
* Declare the MOZ_ALIGNED_DECL macro for declaring aligned types.
*
* For instance,
*
* MOZ_ALIGNED_DECL(char arr[2], 8);
*
* will declare a two-character array |arr| aligned to 8 bytes.
*/
#if defined(__GNUC__)
# define MOZ_ALIGNED_DECL(_type, _align) \
_type __attribute__((aligned(_align)))
#elif defined(_MSC_VER)
# define MOZ_ALIGNED_DECL(_type, _align) \
__declspec(align(_align)) _type
#else
# warning "We don't know how to align variables on this compiler."
# define MOZ_ALIGNED_DECL(_type, _align) _type
#endif
/*
* AlignedElem<N> is a structure whose alignment is guaranteed to be at least N
* bytes.
*
* We support 1, 2, 4, 8, and 16-bit alignment.
*/
template<size_t align>
struct AlignedElem;
/*
* We have to specialize this template because GCC doesn't like __attribute__((aligned(foo))) where
* foo is a template parameter.
*/
template<>
struct AlignedElem<1>
{
MOZ_ALIGNED_DECL(uint8_t elem, 1);
};
template<>
struct AlignedElem<2>
{
MOZ_ALIGNED_DECL(uint8_t elem, 2);
};
template<>
struct AlignedElem<4>
{
MOZ_ALIGNED_DECL(uint8_t elem, 4);
};
template<>
struct AlignedElem<8>
{
MOZ_ALIGNED_DECL(uint8_t elem, 8);
};
template<>
struct AlignedElem<16>
{
MOZ_ALIGNED_DECL(uint8_t elem, 16);
};
/*
* This utility pales in comparison to Boost's aligned_storage. The utility
* simply assumes that uint64_t is enough alignment for anyone. This may need
* to be extended one day...
*
* As an important side effect, pulling the storage into this template is
* enough obfuscation to confuse gcc's strict-aliasing analysis into not giving
* false negatives when we cast from the char buffer to whatever type we've
* constructed using the bytes.
*/
template<size_t nbytes>
struct AlignedStorage
{
union U {
char bytes[nbytes];
uint64_t _;
} u;
const void* addr() const { return u.bytes; }
void* addr() { return u.bytes; }
};
template<class T>
struct AlignedStorage2
{
union U {
char bytes[sizeof(T)];
uint64_t _;
} u;
const T* addr() const { return reinterpret_cast<const T*>(u.bytes); }
T* addr() { return static_cast<T*>(static_cast<void*>(u.bytes)); }
};
/*
* Small utility for lazily constructing objects without using dynamic storage.
* When a Maybe<T> is constructed, it is |empty()|, i.e., no value of T has
* been constructed and no T destructor will be called when the Maybe<T> is
* destroyed. Upon calling |construct|, a T object will be constructed with the
* given arguments and that object will be destroyed when the owning Maybe<T>
* is destroyed.
*
* N.B. GCC seems to miss some optimizations with Maybe and may generate extra
* branches/loads/stores. Use with caution on hot paths.
*/
template<class T>
class Maybe
{
AlignedStorage2<T> storage;
bool constructed;
T& asT() { return *storage.addr(); }
public:
Maybe() { constructed = false; }
~Maybe() { if (constructed) asT().~T(); }
bool empty() const { return !constructed; }
void construct() {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T();
constructed = true;
}
template<class T1>
void construct(const T1& t1) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1);
constructed = true;
}
template<class T1, class T2>
void construct(const T1& t1, const T2& t2) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2);
constructed = true;
}
template<class T1, class T2, class T3>
void construct(const T1& t1, const T2& t2, const T3& t3) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3);
constructed = true;
}
template<class T1, class T2, class T3, class T4>
void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4) {
MOZ_ASSERT(!constructed);
::new (storage.addr()) T(t1, t2, t3, t4);
constructed = true;
}
T* addr() {
MOZ_ASSERT(constructed);
return &asT();
}
T& ref() {
MOZ_ASSERT(constructed);
return asT();
}
const T& ref() const {
MOZ_ASSERT(constructed);
return const_cast<Maybe*>(this)->asT();
}
void destroy() {
ref().~T();
constructed = false;
}
void destroyIfConstructed() {
if (!empty())
destroy();
}
private:
Maybe(const Maybe& other) MOZ_DELETE;
const Maybe& operator=(const Maybe& other) MOZ_DELETE;
};
/*
* Safely subtract two pointers when it is known that end >= begin. This avoids
* the common compiler bug that if (size_t(end) - size_t(begin)) has the MSB
* set, the unsigned subtraction followed by right shift will produce -1, or
* size_t(-1), instead of the real difference.
*/
template<class T>
MOZ_ALWAYS_INLINE size_t
PointerRangeSize(T* begin, T* end)
{
MOZ_ASSERT(end >= begin);
return (size_t(end) - size_t(begin)) / sizeof(T);
}
/*
* Compute the length of an array with constant length. (Use of this method
* with a non-array pointer will not compile.)
*
* Beware of the implicit trailing '\0' when using this with string constants.
*/
template<typename T, size_t N>
size_t
ArrayLength(T (&arr)[N])
{
return N;
}
/*
* Compute the address one past the last element of a constant-length array.
*
* Beware of the implicit trailing '\0' when using this with string constants.
*/
template<typename T, size_t N>
T*
ArrayEnd(T (&arr)[N])
{
return arr + ArrayLength(arr);
}
} /* namespace mozilla */
#endif /* __cplusplus */
#endif /* mozilla_Util_h_ */