mirror of
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284 lines
6.8 KiB
C++
284 lines
6.8 KiB
C++
/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/*
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* Miscellaneous uncategorized functionality. Please add new functionality to
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* new headers, or to other appropriate existing headers, not here.
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*/
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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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* 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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class AlignmentFinder
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{
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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 size_t 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
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* 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 reinterpret_cast<const T*>(u.bytes); }
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T* addr() { return static_cast<T*>(static_cast<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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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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private:
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Maybe(const Maybe& other) MOZ_DELETE;
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const Maybe& operator=(const Maybe& other) MOZ_DELETE;
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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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MOZ_CONSTEXPR 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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MOZ_CONSTEXPR 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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/*
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* MOZ_ARRAY_LENGTH() is an alternative to mozilla::ArrayLength() for C files
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* that can't use C++ template functions and for MOZ_STATIC_ASSERT() calls that
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* can't call ArrayLength() when it is not a C++11 constexpr function.
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*/
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#ifdef MOZ_HAVE_CXX11_CONSTEXPR
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# define MOZ_ARRAY_LENGTH(array) mozilla::ArrayLength(array)
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#else
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# define MOZ_ARRAY_LENGTH(array) (sizeof(array)/sizeof((array)[0]))
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#endif
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#endif /* mozilla_Util_h_ */
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