mirror of
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358 lines
9.1 KiB
C++
358 lines
9.1 KiB
C++
/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
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* vim: set ts=8 sw=4 et tw=99 ft=cpp:
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*
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* ***** BEGIN LICENSE BLOCK *****
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* Version: MPL 1.1/GPL 2.0/LGPL 2.1
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*
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* The contents of this file are subject to the Mozilla Public License Version
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* 1.1 (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at:
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* http://www.mozilla.org/MPL/
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*
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* Software distributed under the License is distributed on an "AS IS" basis,
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* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
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* for the specific language governing rights and limitations under the
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* License.
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*
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* The Original Code is Mozilla Code.
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*
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* The Initial Developer of the Original Code is
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* The Mozilla Foundation
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* Portions created by the Initial Developer are Copyright (C) 2011
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* the Initial Developer. All Rights Reserved.
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*
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* Contributor(s):
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*
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* Alternatively, the contents of this file may be used under the terms of
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* either the GNU General Public License Version 2 or later (the "GPL"), or
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* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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* in which case the provisions of the GPL or the LGPL are applicable instead
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* of those above. If you wish to allow use of your version of this file only
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* under the terms of either the GPL or the LGPL, and not to allow others to
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* use your version of this file under the terms of the MPL, indicate your
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* decision by deleting the provisions above and replace them with the notice
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* and other provisions required by the GPL or the LGPL. If you do not delete
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* the provisions above, a recipient may use your version of this file under
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* the terms of any one of the MPL, the GPL or the LGPL.
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*
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* ***** END LICENSE BLOCK ***** */
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#ifndef mozilla_Util_h_
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#define mozilla_Util_h_
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#include "mozilla/Assertions.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/Types.h"
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#ifdef __cplusplus
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namespace mozilla {
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/**
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* DebugOnly contains a value of type T, but only in debug builds. In
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* release builds, it does not contain a value. This helper is
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* intended to be used along with ASSERT()-style macros, allowing one
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* to write
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*
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* DebugOnly<bool> check = Func();
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* ASSERT(check);
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*
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* more concisely than declaring |check| conditional on #ifdef DEBUG,
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* but also without allocating storage space for |check| in release
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* builds.
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*
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* DebugOnly instances can only be coerced to T in debug builds; in
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* release builds, they don't have a value so type coercion is not
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* well defined.
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*/
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template <typename T>
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struct DebugOnly
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{
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#ifdef DEBUG
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T value;
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DebugOnly() {}
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DebugOnly(const T& other) : value(other) {}
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DebugOnly& operator=(const T& rhs) {
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value = rhs;
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return *this;
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}
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void operator++(int) {
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value++;
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}
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void operator--(int) {
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value--;
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}
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operator T&() { return value; }
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operator const T&() const { return value; }
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T& operator->() { return value; }
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#else
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DebugOnly() {}
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DebugOnly(const T&) {}
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DebugOnly& operator=(const T&) { return *this; }
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void operator++(int) {}
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void operator--(int) {}
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#endif
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/*
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* DebugOnly must always have a destructor or else it will
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* generate "unused variable" warnings, exactly what it's intended
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* to avoid!
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*/
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~DebugOnly() {}
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};
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/*
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* This class, and the corresponding macro MOZ_ALIGNOF, figure out how many
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* bytes of alignment a given type needs.
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*/
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template<class T>
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struct AlignmentFinder
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{
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private:
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struct Aligner
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{
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char c;
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T t;
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};
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public:
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static const int alignment = sizeof(Aligner) - sizeof(T);
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};
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#define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment
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/*
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* Declare the MOZ_ALIGNED_DECL macro for declaring aligned types.
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*
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* For instance,
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*
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* MOZ_ALIGNED_DECL(char arr[2], 8);
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*
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* will declare a two-character array |arr| aligned to 8 bytes.
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*/
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#if defined(__GNUC__)
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# define MOZ_ALIGNED_DECL(_type, _align) \
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_type __attribute__((aligned(_align)))
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#elif defined(_MSC_VER)
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# define MOZ_ALIGNED_DECL(_type, _align) \
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__declspec(align(_align)) _type
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#else
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# warning "We don't know how to align variables on this compiler."
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# define MOZ_ALIGNED_DECL(_type, _align) _type
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#endif
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/*
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* AlignedElem<N> is a structure whose alignment is guaranteed to be at least N bytes.
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*
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* We support 1, 2, 4, 8, and 16-bit alignment.
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*/
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template<size_t align>
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struct AlignedElem;
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/*
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* We have to specialize this template because GCC doesn't like __attribute__((aligned(foo))) where
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* foo is a template parameter.
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*/
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template<>
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struct AlignedElem<1>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 1);
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};
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template<>
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struct AlignedElem<2>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 2);
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};
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template<>
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struct AlignedElem<4>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 4);
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};
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template<>
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struct AlignedElem<8>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 8);
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};
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template<>
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struct AlignedElem<16>
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{
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MOZ_ALIGNED_DECL(uint8_t elem, 16);
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};
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/*
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* This utility pales in comparison to Boost's aligned_storage. The utility
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* simply assumes that uint64_t is enough alignment for anyone. This may need
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* to be extended one day...
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*
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* As an important side effect, pulling the storage into this template is
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* enough obfuscation to confuse gcc's strict-aliasing analysis into not giving
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* false negatives when we cast from the char buffer to whatever type we've
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* constructed using the bytes.
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*/
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template <size_t nbytes>
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struct AlignedStorage
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{
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union U {
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char bytes[nbytes];
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uint64_t _;
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} u;
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const void *addr() const { return u.bytes; }
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void *addr() { return u.bytes; }
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};
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template <class T>
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struct AlignedStorage2
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{
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union U {
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char bytes[sizeof(T)];
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uint64_t _;
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} u;
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const T *addr() const { return (const T *)u.bytes; }
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T *addr() { return (T *)(void *)u.bytes; }
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};
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/*
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* Small utility for lazily constructing objects without using dynamic storage.
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* When a Maybe<T> is constructed, it is |empty()|, i.e., no value of T has
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* been constructed and no T destructor will be called when the Maybe<T> is
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* destroyed. Upon calling |construct|, a T object will be constructed with the
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* given arguments and that object will be destroyed when the owning Maybe<T>
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* is destroyed.
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*
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* N.B. GCC seems to miss some optimizations with Maybe and may generate extra
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* branches/loads/stores. Use with caution on hot paths.
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*/
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template <class T>
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class Maybe
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{
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AlignedStorage2<T> storage;
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bool constructed;
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T &asT() { return *storage.addr(); }
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explicit Maybe(const Maybe &other);
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const Maybe &operator=(const Maybe &other);
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public:
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Maybe() { constructed = false; }
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~Maybe() { if (constructed) asT().~T(); }
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bool empty() const { return !constructed; }
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void construct() {
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MOZ_ASSERT(!constructed);
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new(storage.addr()) T();
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constructed = true;
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}
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template <class T1>
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void construct(const T1 &t1) {
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MOZ_ASSERT(!constructed);
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new(storage.addr()) T(t1);
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constructed = true;
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}
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template <class T1, class T2>
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void construct(const T1 &t1, const T2 &t2) {
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MOZ_ASSERT(!constructed);
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new(storage.addr()) T(t1, t2);
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constructed = true;
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}
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template <class T1, class T2, class T3>
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void construct(const T1 &t1, const T2 &t2, const T3 &t3) {
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MOZ_ASSERT(!constructed);
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new(storage.addr()) T(t1, t2, t3);
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constructed = true;
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}
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template <class T1, class T2, class T3, class T4>
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void construct(const T1 &t1, const T2 &t2, const T3 &t3, const T4 &t4) {
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MOZ_ASSERT(!constructed);
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new(storage.addr()) T(t1, t2, t3, t4);
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constructed = true;
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}
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T *addr() {
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MOZ_ASSERT(constructed);
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return &asT();
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}
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T &ref() {
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MOZ_ASSERT(constructed);
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return asT();
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}
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const T &ref() const {
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MOZ_ASSERT(constructed);
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return const_cast<Maybe *>(this)->asT();
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}
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void destroy() {
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ref().~T();
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constructed = false;
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}
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void destroyIfConstructed() {
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if (!empty())
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destroy();
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}
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};
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/*
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* Safely subtract two pointers when it is known that end >= begin. This avoids
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* the common compiler bug that if (size_t(end) - size_t(begin)) has the MSB
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* set, the unsigned subtraction followed by right shift will produce -1, or
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* size_t(-1), instead of the real difference.
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*/
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template <class T>
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MOZ_ALWAYS_INLINE size_t
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PointerRangeSize(T* begin, T* end)
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{
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MOZ_ASSERT(end >= begin);
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return (size_t(end) - size_t(begin)) / sizeof(T);
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}
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/*
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* Compute the length of an array with constant length. (Use of this method
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* with a non-array pointer will not compile.)
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*
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* Beware of the implicit trailing '\0' when using this with string constants.
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*/
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template<typename T, size_t N>
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size_t
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ArrayLength(T (&arr)[N])
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{
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return N;
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}
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/*
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* Compute the address one past the last element of a constant-length array.
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*
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* Beware of the implicit trailing '\0' when using this with string constants.
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*/
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template<typename T, size_t N>
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T*
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ArrayEnd(T (&arr)[N])
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{
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return arr + ArrayLength(arr);
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}
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} /* namespace mozilla */
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#endif /* __cplusplus */
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#endif /* mozilla_Util_h_ */
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