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1 | =head1 LIBECB |
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2 | |
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3 | You suck, we don't(tm) |
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4 | |
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5 | =head2 ABOUT THE HEADER |
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6 | |
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7 | - how to include it |
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8 | - it includes inttypes.h |
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9 | - no .a |
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10 | - whats a bool |
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11 | - function mean macro or function |
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12 | - macro means untyped |
1 | |
13 | |
2 | =head2 GCC ATTRIBUTES |
14 | =head2 GCC ATTRIBUTES |
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15 | |
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16 | blabla where to put, what others |
3 | |
17 | |
4 | =over 4 |
18 | =over 4 |
5 | |
19 | |
6 | =item ecb_attribute ((attrs...)) |
20 | =item ecb_attribute ((attrs...)) |
7 | |
21 | |
8 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and |
22 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and |
9 | to nothing on other compilers, so the effect is that only GCC sees these. |
23 | to nothing on other compilers, so the effect is that only GCC sees these. |
10 | |
24 | |
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25 | =item ecb_unused |
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26 | |
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27 | Marks a function or a variable as "unused", which simply suppresses a |
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28 | warning by GCC when it detects it as unused. This is useful when you e.g. |
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29 | declare a variable but do not always use it: |
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30 | |
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31 | { |
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32 | int var ecb_unused; |
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33 | |
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34 | #ifdef SOMECONDITION |
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35 | var = ...; |
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36 | return var; |
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37 | #else |
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38 | return 0; |
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39 | #endif |
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40 | } |
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41 | |
11 | =item ecb_noinline |
42 | =item ecb_noinline |
12 | |
43 | |
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44 | Prevent a function from being inlined - it might be optimised away, but |
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45 | not inlined into other functions. This is useful if you know your function |
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46 | is rarely called and large enough for inlining not to be helpful. |
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47 | |
13 | =item ecb_noreturn |
48 | =item ecb_noreturn |
14 | |
49 | |
15 | =item ecb_unused |
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16 | |
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17 | =item ecb_const |
50 | =item ecb_const |
18 | |
51 | |
19 | =item ecb_pure |
52 | =item ecb_pure |
20 | |
53 | |
21 | =item ecb_hot |
54 | =item ecb_hot |
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… | |
28 | |
61 | |
29 | =head2 OPTIMISATION HINTS |
62 | =head2 OPTIMISATION HINTS |
30 | |
63 | |
31 | =over 4 |
64 | =over 4 |
32 | |
65 | |
33 | =item bool ecb_is_constant(expr) |
66 | =item bool ecb_is_constant(expr) [MACRO] |
34 | |
67 | |
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68 | Returns true iff the expression can be deduced to be a compile-time |
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69 | constant, and false otherwise. |
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70 | |
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71 | For example, when you have a C<rndm16> function that returns a 16 bit |
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72 | random number, and you have a function that maps this to a range from |
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73 | 0..n-1, then you could use this inline function in a header file: |
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74 | |
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75 | ecb_inline uint32_t |
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76 | rndm (uint32_t n) |
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77 | { |
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78 | return (n * (uint32_t)rndm16 ()) >> 16; |
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79 | } |
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80 | |
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81 | However, for powers of two, you could use a normal mask, but that is only |
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82 | worth it if, at compile time, you can detect this case. This is the case |
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83 | when the passed number is a constant and also a power of two (C<n & (n - |
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84 | 1) == 0>): |
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85 | |
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86 | ecb_inline uint32_t |
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87 | rndm (uint32_t n) |
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88 | { |
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89 | return is_constant (n) && !(n & (n - 1)) |
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90 | ? rndm16 () & (num - 1) |
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91 | : (n * (uint32_t)rndm16 ()) >> 16; |
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92 | } |
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93 | |
35 | =item bool ecb_expect(expr,value) |
94 | =item bool ecb_expect (expr, value) [MACRO] |
36 | |
95 | |
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96 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
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97 | the C<expr> evaluates to C<value> a lot, which can be used for static |
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98 | branch optimisations. |
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99 | |
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100 | Usually, you want to use the more intuitive C<ecb_likely> and |
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101 | C<ecb_unlikely> functions instead. |
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102 | |
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103 | =item bool ecb_likely (bool) [MACRO] |
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104 | |
37 | =item bool ecb_unlikely(bool) |
105 | =item bool ecb_unlikely (bool) [MACRO] |
38 | |
106 | |
39 | =item bool ecb_likely(bool) |
107 | These two functions expect a expression that is true or false and return |
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108 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
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109 | other conditional statement, it will not change the program: |
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110 | |
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111 | /* these two do the same thing */ |
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112 | if (some_condition) ...; |
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113 | if (ecb_likely (some_condition)) ...; |
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114 | |
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115 | However, by using C<ecb_likely>, you tell the compiler that the condition |
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116 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
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117 | true). |
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118 | |
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119 | For example, when you check for a null pointer and expect this to be a |
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120 | rare, exceptional, case, then use C<ecb_unlikely>: |
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121 | |
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122 | void my_free (void *ptr) |
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123 | { |
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124 | if (ecb_unlikely (ptr == 0)) |
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125 | return; |
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126 | } |
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127 | |
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128 | Consequent use of these functions to mark away exceptional cases or to |
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129 | tell the compiler what the hot path through a function is can increase |
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130 | performance considerably. |
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131 | |
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132 | A very good example is in a function that reserves more space for some |
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133 | memory block (for example, inside an implementation of a string stream) - |
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134 | each time something is added, you have to check for a buffer overrun, but |
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135 | you expect that most checks will turn out to be false: |
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136 | |
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137 | /* make sure we have "size" extra room in our buffer */ |
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138 | ecb_inline void |
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139 | reserve (int size) |
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140 | { |
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141 | if (ecb_unlikely (current + size > end)) |
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142 | real_reserve_method (size); /* presumably noinline */ |
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143 | } |
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144 | |
41 | =item bool ecb_assume(cond) |
145 | =item bool ecb_assume (cond) [MACRO] |
42 | |
146 | |
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147 | Try to tell the compiler that some condition is true, even if it's not |
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148 | obvious. |
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149 | |
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150 | This can be used to teach the compiler about invariants or other |
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151 | conditions that might improve code generation, but which are impossible to |
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152 | deduce form the code itself. |
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153 | |
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154 | For example, the example reservation function from the C<ecb_unlikely> |
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155 | description could be written thus (only C<ecb_assume> was added): |
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156 | |
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157 | ecb_inline void |
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158 | reserve (int size) |
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159 | { |
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160 | if (ecb_unlikely (current + size > end)) |
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161 | real_reserve_method (size); /* presumably noinline */ |
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162 | |
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163 | ecb_assume (current + size <= end); |
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164 | } |
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165 | |
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166 | If you then call this function twice, like this: |
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167 | |
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168 | reserve (10); |
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169 | reserve (1); |
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170 | |
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171 | Then the compiler I<might> be able to optimise out the second call |
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172 | completely, as it knows that C<< current + 1 > end >> is false and the |
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173 | call will never be executed. |
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174 | |
43 | =item bool ecb_unreachable() |
175 | =item bool ecb_unreachable () |
44 | |
176 | |
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177 | This function does nothing itself, except tell the compiler that it will |
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178 | never be executed. Apart from suppressing a warning in some cases, this |
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179 | function can be used to implement C<ecb_assume> or similar functions. |
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180 | |
45 | =item bool ecb_prefetch(addr,rw,locality) |
181 | =item bool ecb_prefetch (addr, rw, locality) [MACRO] |
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182 | |
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183 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
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184 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
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185 | C<0> means that there will only be one access later, C<3> means that |
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186 | the data will likely be accessed very often, and values in between mean |
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187 | something... in between. The memory pointed to by the address does not |
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188 | need to be accessible (it could be a null pointer for example), but C<rw> |
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189 | and C<locality> must be compile-time constants. |
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190 | |
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191 | An obvious way to use this is to prefetch some data far away, in a big |
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192 | array you loop over. This prefetches memory some 128 array elements later, |
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193 | in the hope that it will be ready when the CPU arrives at that location. |
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194 | |
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195 | int sum = 0; |
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196 | |
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197 | for (i = 0; i < N; ++i) |
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198 | { |
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199 | sum += arr [i] |
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200 | ecb_prefetch (arr + i + 128, 0, 0); |
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201 | } |
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202 | |
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203 | It's hard to predict how far to prefetch, and most CPUs that can prefetch |
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204 | are often good enough to predict this kind of behaviour themselves. It |
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205 | gets more interesting with linked lists, especially when you do some fair |
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206 | processing on each list element: |
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207 | |
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208 | for (node *n = start; n; n = n->next) |
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209 | { |
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210 | ecb_prefetch (n->next, 0, 0); |
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211 | ... do medium amount of work with *n |
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212 | } |
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213 | |
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214 | After processing the node, (part of) the next node might already be in |
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215 | cache. |
46 | |
216 | |
47 | =back |
217 | =back |
48 | |
218 | |
49 | =head2 BIT FIDDLING / BITSTUFFS |
219 | =head2 BIT FIDDLING / BITSTUFFS |
50 | |
220 | |
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221 | =over 4 |
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222 | |
51 | bool ecb_big_endian (); |
223 | =item bool ecb_big_endian () |
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224 | |
52 | bool ecb_little_endian (); |
225 | =item bool ecb_little_endian () |
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226 | |
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227 | These two functions return true if the byte order is big endian |
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228 | (most-significant byte first) or little endian (least-significant byte |
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229 | first) respectively. |
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230 | |
53 | int ecb_ctz32 (uint32_t x); |
231 | =item int ecb_ctz32 (uint32_t x) |
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232 | |
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233 | Returns the index of the least significant bit set in C<x> (or |
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234 | equivalently the number of bits set to 0 before the least significant |
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235 | bit set), starting from 0. If C<x> is 0 the result is undefined. A |
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236 | common use case is to compute the integer binary logarithm, i.e., |
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237 | floor(log2(n)). For example: |
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238 | |
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239 | ecb_ctz32(3) = 1 |
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240 | ecb_ctz32(6) = 2 |
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241 | |
54 | int ecb_popcount32 (uint32_t x); |
242 | =item int ecb_popcount32 (uint32_t x) |
55 | uint32_t ecb_bswap32 (uint32_t x); |
243 | |
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244 | Returns the number of bits set to 1 in C<x>. For example: |
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245 | |
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246 | ecb_popcount32(7) = 3 |
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247 | ecb_popcount32(255) = 8 |
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248 | |
56 | uint32_t ecb_bswap16 (uint32_t x); |
249 | =item uint32_t ecb_bswap16 (uint32_t x) |
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250 | |
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251 | =item uint32_t ecb_bswap32 (uint32_t x) |
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252 | |
57 | uint32_t ecb_rotr32 (uint32_t x, unsigned int count); |
253 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
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254 | |
58 | uint32_t ecb_rotl32 (uint32_t x, unsigned int count); |
255 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
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256 | |
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257 | These two functions return the value of C<x> after shifting all the bits |
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258 | by C<count> positions to the right or left respectively. |
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259 | |
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260 | =back |
59 | |
261 | |
60 | =head2 ARITHMETIC |
262 | =head2 ARITHMETIC |
61 | |
263 | |
62 | x = ecb_mod (m, n) |
264 | =over 4 |
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265 | |
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266 | =item x = ecb_mod (m, n) [MACRO] |
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267 | |
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268 | Returns the positive remainder of the modulo operation between C<m> |
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269 | and C<n>. |
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270 | |
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271 | =back |
63 | |
272 | |
64 | =head2 UTILITY |
273 | =head2 UTILITY |
65 | |
274 | |
66 | ecb_array_length (name) |
275 | =over 4 |
67 | |
276 | |
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277 | =item element_count = ecb_array_length (name) [MACRO] |
68 | |
278 | |
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279 | =back |
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280 | |
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281 | |