js/public/TemplateLib.h

   1 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
   2  * vim: set ts=8 sw=4 et tw=99 ft=cpp:
   3  *
   4  * This Source Code Form is subject to the terms of the Mozilla Public
   5  * License, v. 2.0. If a copy of the MPL was not distributed with this
   6  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   7
   8 #ifndef js_template_lib_h__
   9 #define js_template_lib_h__
  10
  11 #include "jstypes.h"
  12
  13 /*
  14  * Library of reusable template meta-functions (that is, functions on types and
  15  * compile-time values). Meta-functions are placed inside the 'tl' namespace to
  16  * avoid conflict with non-meta functions that logically have the same name
  17  * (e.g., js::tl::Min vs. js::Min).
  18  */
  19
  20 namespace js {
  21 namespace tl {
  22
  23 /* Compute min/max/clamp. */
  24 template <size_t i, size_t j> struct Min {
  25     static const size_t result = i < j ? i : j;
  26 };
  27 template <size_t i, size_t j> struct Max {
  28     static const size_t result = i > j ? i : j;
  29 };
  30 template <size_t i, size_t min, size_t max> struct Clamp {
  31     static const size_t result = i < min ? min : (i > max ? max : i);
  32 };
  33
  34 /* Compute x^y. */
  35 template <size_t x, size_t y> struct Pow {
  36     static const size_t result = x * Pow<x, y - 1>::result;
  37 };
  38 template <size_t x> struct Pow<x,0> {
  39     static const size_t result = 1;
  40 };
  41
  42 /* Compute floor(log2(i)). */
  43 template <size_t i> struct FloorLog2 {
  44     static const size_t result = 1 + FloorLog2<i / 2>::result;
  45 };
  46 template <> struct FloorLog2<0> { /* Error */ };
  47 template <> struct FloorLog2<1> { static const size_t result = 0; };
  48
  49 /* Compute ceiling(log2(i)). */
  50 template <size_t i> struct CeilingLog2 {
  51     static const size_t result = FloorLog2<2 * i - 1>::result;
  52 };
  53
  54 /* Round up to the nearest power of 2. */
  55 template <size_t i> struct RoundUpPow2 {
  56     static const size_t result = size_t(1) << CeilingLog2<i>::result;
  57 };
  58 template <> struct RoundUpPow2<0> {
  59     static const size_t result = 1;
  60 };
  61
  62 /* Compute the number of bits in the given unsigned type. */
  63 template <class T> struct BitSize {
  64     static const size_t result = sizeof(T) * JS_BITS_PER_BYTE;
  65 };
  66
  67 /* Allow Assertions by only including the 'result' typedef if 'true'. */
  68 template <bool> struct StaticAssert {};
  69 template <> struct StaticAssert<true> { typedef int result; };
  70
  71 /* Boolean test for whether two types are the same. */
  72 template <class T, class U> struct IsSameType {
  73     static const bool result = false;
  74 };
  75 template <class T> struct IsSameType<T,T> {
  76     static const bool result = true;
  77 };
  78
  79 /*
  80  * Produce an N-bit mask, where N <= BitSize<size_t>::result.  Handle the
  81  * language-undefined edge case when N = BitSize<size_t>::result.
  82  */
  83 template <size_t N> struct NBitMask {
  84     typedef typename StaticAssert<N < BitSize<size_t>::result>::result _;
  85     static const size_t result = (size_t(1) << N) - 1;
  86 };
  87 template <> struct NBitMask<BitSize<size_t>::result> {
  88     static const size_t result = size_t(-1);
  89 };
  90
  91 /*
  92  * For the unsigned integral type size_t, compute a mask M for N such that
  93  * for all X, !(X & M) implies X * N will not overflow (w.r.t size_t)
  94  */
  95 template <size_t N> struct MulOverflowMask {
  96     static const size_t result =
  97         ~NBitMask<BitSize<size_t>::result - CeilingLog2<N>::result>::result;
  98 };
  99 template <> struct MulOverflowMask<0> { /* Error */ };
 100 template <> struct MulOverflowMask<1> { static const size_t result = 0; };
 101
 102 /*
 103  * Generate a mask for T such that if (X & sUnsafeRangeSizeMask), an X-sized
 104  * array of T's is big enough to cause a ptrdiff_t overflow when subtracting
 105  * a pointer to the end of the array from the beginning.
 106  */
 107 template <class T> struct UnsafeRangeSizeMask {
 108     /*
 109      * The '2' factor means the top bit is clear, sizeof(T) converts from
 110      * units of elements to bytes.
 111      */
 112     static const size_t result = MulOverflowMask<2 * sizeof(T)>::result;
 113 };
 114
 115 /* Return T stripped of any const-ness. */
 116 template <class T> struct StripConst          { typedef T result; };
 117 template <class T> struct StripConst<const T> { typedef T result; };
 118
 119 /*
 120  * Traits class for identifying POD types. Until C++0x, there is no automatic
 121  * way to detect PODs, so for the moment it is done manually.
 122  */
 123 template <class T> struct IsPodType                 { static const bool result = false; };
 124 template <> struct IsPodType<char>                  { static const bool result = true; };
 125 template <> struct IsPodType<signed char>           { static const bool result = true; };
 126 template <> struct IsPodType<unsigned char>         { static const bool result = true; };
 127 template <> struct IsPodType<short>                 { static const bool result = true; };
 128 template <> struct IsPodType<unsigned short>        { static const bool result = true; };
 129 template <> struct IsPodType<int>                   { static const bool result = true; };
 130 template <> struct IsPodType<unsigned int>          { static const bool result = true; };
 131 template <> struct IsPodType<long>                  { static const bool result = true; };
 132 template <> struct IsPodType<unsigned long>         { static const bool result = true; };
 133 template <> struct IsPodType<long long>             { static const bool result = true; };
 134 template <> struct IsPodType<unsigned long long>    { static const bool result = true; };
 135 template <> struct IsPodType<bool>                  { static const bool result = true; };
 136 template <> struct IsPodType<float>                 { static const bool result = true; };
 137 template <> struct IsPodType<double>                { static const bool result = true; };
 138 template <> struct IsPodType<wchar_t>               { static const bool result = true; };
 139 template <typename T> struct IsPodType<T *>         { static const bool result = true; };
 140
 141 template <bool cond, typename T, T v1, T v2> struct If        { static const T result = v1; };
 142 template <typename T, T v1, T v2> struct If<false, T, v1, v2> { static const T result = v2; };
 143
 144 template <class T> struct IsPointerType             { static const bool result = false; };
 145 template <class T> struct IsPointerType<T *>        { static const bool result = true; };
 146
 147 /*
 148  * Traits class for identifying types that are implicitly barriered.
 149  */
 150 template <class T> struct IsRelocatableHeapType { static const bool result = true; };
 151
 152 } /* namespace tl */
 153 } /* namespace js */
 154
 155 #endif  /* js_template_lib_h__ */