gecko-dev/mfbt/Util.h

253 строки
7.1 KiB
C++

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
* vim: set ts=8 sw=4 et tw=99 ft=cpp:
*
* ***** BEGIN LICENSE BLOCK *****
* Version: MPL 1.1/GPL 2.0/LGPL 2.1
*
* The contents of this file are subject to the Mozilla Public License Version
* 1.1 (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at:
* http://www.mozilla.org/MPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*
* The Original Code is Mozilla Code.
*
* The Initial Developer of the Original Code is
* The Mozilla Foundation
* Portions created by the Initial Developer are Copyrigght (C) 2011
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
*
* Alternatively, the contents of this file may be used under the terms of
* either the GNU General Public License Version 2 or later (the "GPL"), or
* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
* in which case the provisions of the GPL or the LGPL are applicable instead
* of those above. If you wish to allow use of your version of this file only
* under the terms of either the GPL or the LGPL, and not to allow others to
* use your version of this file under the terms of the MPL, indicate your
* decision by deleting the provisions above and replace them with the notice
* and other provisions required by the GPL or the LGPL. If you do not delete
* the provisions above, a recipient may use your version of this file under
* the terms of any one of the MPL, the GPL or the LGPL.
*
* ***** END LICENSE BLOCK ***** */
#ifndef mozilla_Util_h_
#define mozilla_Util_h_
#include "mozilla/Types.h"
/*
* XXX: we're cheating here in order to avoid creating object files
* for mfbt /just/ to provide a function like FatalError() to be used
* by MOZ_ASSERT(). (It'll happen eventually, but for just ASSERT()
* it isn't worth the pain.) JS_Assert(), although unfortunately
* named, is part of SpiderMonkey's stable, external API, so this
* isn't quite as bad as it seems.
*
* Once mfbt needs object files, this unholy union with JS_Assert()
* will be broken.
*/
MOZ_BEGIN_EXTERN_C
extern MFBT_API(void)
JS_Assert(const char *s, const char *file, JSIntn ln);
MOZ_END_EXTERN_C
/*
* MOZ_ASSERT() is a "strong" assertion of state, like libc's
* assert(). If a MOZ_ASSERT() fails in a debug build, the process in
* which it fails will stop running in a loud and dramatic way.
*/
#ifdef DEBUG
# define MOZ_ASSERT(expr_) \
((expr_) ? (void)0 : JS_Assert(#expr_, __FILE__, __LINE__))
#else
# define MOZ_ASSERT(expr_) ((void)0)
#endif /* DEBUG */
#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 along with ASSERT()-style macros, allowing one
* to write
*
* DebugOnly<bool> check = Func();
* 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& operator=(const T& rhs) {
value = rhs;
return *this;
}
operator T&() { return value; }
operator const T&() const { return value; }
#else
DebugOnly() {}
DebugOnly(const T&) {}
DebugOnly& operator=(const T&) {}
#endif
/*
* DebugOnly must always have a destructor or else it will
* generate "unused variable" warnings, exactly what it's intended
* to avoid!
*/
~DebugOnly() {}
};
/*
* This utility pales in comparison to Boost's aligned_storage. The utility
* simply assumes that JSUint64 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 _;
} 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 _;
} u;
const T *addr() const { return (const T *)u.bytes; }
T *addr() { return (T *)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(); }
explicit Maybe(const Maybe &other);
const Maybe &operator=(const Maybe &other);
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();
}
void destroy() {
ref().~T();
constructed = false;
}
void destroyIfConstructed() {
if (!empty())
destroy();
}
};
} /* namespace mozilla */
#endif /* __cplusplus */
#endif /* mozilla_Util_h_ */