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1 | =head1 LIBECB - e-C-Builtins |
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2 | |
1 | =head1 LIBECB |
3 | =head2 ABOUT LIBECB |
2 | |
4 | |
3 | You suck, we don't(tm) |
5 | Libecb is currently a simple header file that doesn't require any |
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6 | configuration to use or include in your project. |
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7 | |
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8 | It's part of the e-suite of libraries, other members of which include |
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9 | libev and libeio. |
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10 | |
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11 | Its homepage can be found here: |
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12 | |
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13 | http://software.schmorp.de/pkg/libecb |
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14 | |
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15 | It mainly provides a number of wrappers around GCC built-ins, together |
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16 | with replacement functions for other compilers. In addition to this, |
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17 | it provides a number of other lowlevel C utilities, such as endianness |
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18 | detection, byte swapping or bit rotations. |
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19 | |
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20 | More might come. |
4 | |
21 | |
5 | =head2 ABOUT THE HEADER |
22 | =head2 ABOUT THE HEADER |
6 | |
23 | |
7 | - how to include it |
24 | At the moment, all you have to do is copy F<ecb.h> somewhere where your |
8 | - it includes inttypes.h |
25 | compiler can find it and include it: |
9 | - no .a |
26 | |
10 | - whats a bool |
27 | #include <ecb.h> |
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28 | |
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29 | The header should work fine for both C and C++ compilation, and gives you |
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30 | all of F<inttypes.h> in addition to the ECB symbols. |
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31 | |
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32 | There are currently no object files to link to - future versions might |
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33 | come with an (optional) object code library to link against, to reduce |
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34 | code size or gain access to additional features. |
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35 | |
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36 | It also currently includes everything from F<inttypes.h>. |
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37 | |
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38 | =head2 ABOUT THIS MANUAL / CONVENTIONS |
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39 | |
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40 | This manual mainly describes each (public) function available after |
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41 | including the F<ecb.h> header. The header might define other symbols than |
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42 | these, but these are not part of the public API, and not supported in any |
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43 | way. |
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44 | |
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45 | When the manual mentions a "function" then this could be defined either as |
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46 | as inline function, a macro, or an external symbol. |
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47 | |
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48 | When functions use a concrete standard type, such as C<int> or |
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49 | C<uint32_t>, then the corresponding function works only with that type. If |
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50 | only a generic name is used (C<expr>, C<cond>, C<value> and so on), then |
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51 | the corresponding function relies on C to implement the correct types, and |
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52 | is usually implemented as a macro. Specifically, a "bool" in this manual |
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53 | refers to any kind of boolean value, not a specific type. |
11 | |
54 | |
12 | =head2 GCC ATTRIBUTES |
55 | =head2 GCC ATTRIBUTES |
13 | |
56 | |
14 | blabla where to put, what others |
57 | blabla where to put, what others |
15 | |
58 | |
16 | =over 4 |
59 | =over 4 |
17 | |
60 | |
18 | =item ecb_attribute ((attrs...)) |
61 | =item ecb_attribute ((attrs...)) |
19 | |
62 | |
20 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and |
63 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to |
21 | to nothing on other compilers, so the effect is that only GCC sees these. |
64 | nothing on other compilers, so the effect is that only GCC sees these. |
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65 | |
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66 | Example: use the C<deprecated> attribute on a function. |
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67 | |
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68 | ecb_attribute((__deprecated__)) void |
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69 | do_not_use_me_anymore (void); |
22 | |
70 | |
23 | =item ecb_unused |
71 | =item ecb_unused |
24 | |
72 | |
25 | Marks a function or a variable as "unused", which simply suppresses a |
73 | Marks a function or a variable as "unused", which simply suppresses a |
26 | warning by GCC when it detects it as unused. This is useful when you e.g. |
74 | warning by GCC when it detects it as unused. This is useful when you e.g. |
27 | declare a variable but do not always use it: |
75 | declare a variable but do not always use it: |
28 | |
76 | |
29 | { |
77 | { |
30 | int var ecb_unused; |
78 | int var ecb_unused; |
31 | |
79 | |
32 | #ifdef SOMECONDITION |
80 | #ifdef SOMECONDITION |
33 | var = ...; |
81 | var = ...; |
34 | return var; |
82 | return var; |
35 | #else |
83 | #else |
36 | return 0; |
84 | return 0; |
37 | #endif |
85 | #endif |
38 | } |
86 | } |
39 | |
87 | |
40 | =item ecb_noinline |
88 | =item ecb_noinline |
41 | |
89 | |
42 | Prevent a function from being inlined - it might be optimsied away, but |
90 | Prevent a function from being inlined - it might be optimised away, but |
43 | not inlined into other functions. This is useful if you know your function |
91 | not inlined into other functions. This is useful if you know your function |
44 | is rarely called and large enough for inlining not to be helpful. |
92 | is rarely called and large enough for inlining not to be helpful. |
45 | |
93 | |
46 | =item ecb_noreturn |
94 | =item ecb_noreturn |
47 | |
95 | |
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66 | Returns true iff the expression can be deduced to be a compile-time |
114 | Returns true iff the expression can be deduced to be a compile-time |
67 | constant, and false otherwise. |
115 | constant, and false otherwise. |
68 | |
116 | |
69 | For example, when you have a C<rndm16> function that returns a 16 bit |
117 | For example, when you have a C<rndm16> function that returns a 16 bit |
70 | random number, and you have a function that maps this to a range from |
118 | random number, and you have a function that maps this to a range from |
71 | 0..n-1, then you could use this inline fucntion in a header file: |
119 | 0..n-1, then you could use this inline function in a header file: |
72 | |
120 | |
73 | ecb_inline uint32_t |
121 | ecb_inline uint32_t |
74 | rndm (uint32_t n) |
122 | rndm (uint32_t n) |
75 | { |
123 | { |
76 | return n * (uint32_t)rndm16 ()) >> 16; |
124 | return (n * (uint32_t)rndm16 ()) >> 16; |
77 | } |
125 | } |
78 | |
126 | |
79 | However, for powers of two, you could use a normal mask, but that is only |
127 | However, for powers of two, you could use a normal mask, but that is only |
80 | worth it if, at compile time, you can detect this case. This is the case |
128 | worth it if, at compile time, you can detect this case. This is the case |
81 | when the passed number is a constant and also a power of two (C<n & (n - |
129 | when the passed number is a constant and also a power of two (C<n & (n - |
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84 | ecb_inline uint32_t |
132 | ecb_inline uint32_t |
85 | rndm (uint32_t n) |
133 | rndm (uint32_t n) |
86 | { |
134 | { |
87 | return is_constant (n) && !(n & (n - 1)) |
135 | return is_constant (n) && !(n & (n - 1)) |
88 | ? rndm16 () & (num - 1) |
136 | ? rndm16 () & (num - 1) |
89 | : (uint32_t)rndm16 ()) >> 16; |
137 | : (n * (uint32_t)rndm16 ()) >> 16; |
90 | } |
138 | } |
91 | |
139 | |
92 | =item bool ecb_expect(expr,value) |
140 | =item bool ecb_expect (expr, value) |
93 | |
141 | |
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142 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
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143 | the C<expr> evaluates to C<value> a lot, which can be used for static |
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144 | branch optimisations. |
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145 | |
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146 | Usually, you want to use the more intuitive C<ecb_likely> and |
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147 | C<ecb_unlikely> functions instead. |
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148 | |
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149 | =item bool ecb_likely (cond) |
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150 | |
94 | =item bool ecb_unlikely(bool) |
151 | =item bool ecb_unlikely (cond) |
95 | |
152 | |
96 | =item bool ecb_likely(bool) |
153 | These two functions expect a expression that is true or false and return |
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154 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
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155 | other conditional statement, it will not change the program: |
97 | |
156 | |
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157 | /* these two do the same thing */ |
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158 | if (some_condition) ...; |
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159 | if (ecb_likely (some_condition)) ...; |
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160 | |
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161 | However, by using C<ecb_likely>, you tell the compiler that the condition |
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162 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
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163 | true). |
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164 | |
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165 | For example, when you check for a null pointer and expect this to be a |
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166 | rare, exceptional, case, then use C<ecb_unlikely>: |
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167 | |
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168 | void my_free (void *ptr) |
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169 | { |
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170 | if (ecb_unlikely (ptr == 0)) |
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171 | return; |
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172 | } |
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173 | |
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174 | Consequent use of these functions to mark away exceptional cases or to |
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175 | tell the compiler what the hot path through a function is can increase |
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176 | performance considerably. |
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177 | |
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178 | A very good example is in a function that reserves more space for some |
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179 | memory block (for example, inside an implementation of a string stream) - |
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180 | each time something is added, you have to check for a buffer overrun, but |
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181 | you expect that most checks will turn out to be false: |
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182 | |
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183 | /* make sure we have "size" extra room in our buffer */ |
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184 | ecb_inline void |
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185 | reserve (int size) |
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186 | { |
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187 | if (ecb_unlikely (current + size > end)) |
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188 | real_reserve_method (size); /* presumably noinline */ |
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189 | } |
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190 | |
98 | =item bool ecb_assume(cond) |
191 | =item bool ecb_assume (cond) |
99 | |
192 | |
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193 | Try to tell the compiler that some condition is true, even if it's not |
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194 | obvious. |
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195 | |
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196 | This can be used to teach the compiler about invariants or other |
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197 | conditions that might improve code generation, but which are impossible to |
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198 | deduce form the code itself. |
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199 | |
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200 | For example, the example reservation function from the C<ecb_unlikely> |
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201 | description could be written thus (only C<ecb_assume> was added): |
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202 | |
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203 | ecb_inline void |
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204 | reserve (int size) |
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205 | { |
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206 | if (ecb_unlikely (current + size > end)) |
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207 | real_reserve_method (size); /* presumably noinline */ |
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208 | |
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209 | ecb_assume (current + size <= end); |
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210 | } |
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211 | |
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212 | If you then call this function twice, like this: |
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213 | |
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214 | reserve (10); |
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215 | reserve (1); |
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216 | |
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217 | Then the compiler I<might> be able to optimise out the second call |
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218 | completely, as it knows that C<< current + 1 > end >> is false and the |
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219 | call will never be executed. |
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220 | |
100 | =item bool ecb_unreachable() |
221 | =item bool ecb_unreachable () |
101 | |
222 | |
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223 | This function does nothing itself, except tell the compiler that it will |
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224 | never be executed. Apart from suppressing a warning in some cases, this |
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225 | function can be used to implement C<ecb_assume> or similar functions. |
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226 | |
102 | =item bool ecb_prefetch(addr,rw,locality) |
227 | =item bool ecb_prefetch (addr, rw, locality) |
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228 | |
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229 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
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230 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
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231 | C<0> means that there will only be one access later, C<3> means that |
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232 | the data will likely be accessed very often, and values in between mean |
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233 | something... in between. The memory pointed to by the address does not |
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234 | need to be accessible (it could be a null pointer for example), but C<rw> |
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235 | and C<locality> must be compile-time constants. |
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236 | |
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237 | An obvious way to use this is to prefetch some data far away, in a big |
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238 | array you loop over. This prefetches memory some 128 array elements later, |
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239 | in the hope that it will be ready when the CPU arrives at that location. |
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240 | |
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241 | int sum = 0; |
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242 | |
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243 | for (i = 0; i < N; ++i) |
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244 | { |
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245 | sum += arr [i] |
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246 | ecb_prefetch (arr + i + 128, 0, 0); |
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247 | } |
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248 | |
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249 | It's hard to predict how far to prefetch, and most CPUs that can prefetch |
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250 | are often good enough to predict this kind of behaviour themselves. It |
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251 | gets more interesting with linked lists, especially when you do some fair |
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252 | processing on each list element: |
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253 | |
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254 | for (node *n = start; n; n = n->next) |
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255 | { |
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256 | ecb_prefetch (n->next, 0, 0); |
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257 | ... do medium amount of work with *n |
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258 | } |
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259 | |
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260 | After processing the node, (part of) the next node might already be in |
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261 | cache. |
103 | |
262 | |
104 | =back |
263 | =back |
105 | |
264 | |
106 | =head2 BIT FIDDLING / BITSTUFFS |
265 | =head2 BIT FIDDLING / BITSTUFFS |
107 | |
266 | |
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109 | |
268 | |
110 | =item bool ecb_big_endian () |
269 | =item bool ecb_big_endian () |
111 | |
270 | |
112 | =item bool ecb_little_endian () |
271 | =item bool ecb_little_endian () |
113 | |
272 | |
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273 | These two functions return true if the byte order is big endian |
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274 | (most-significant byte first) or little endian (least-significant byte |
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275 | first) respectively. |
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276 | |
114 | =item int ecb_ctz32 (uint32_t x) |
277 | =item int ecb_ctz32 (uint32_t x) |
115 | |
278 | |
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279 | Returns the index of the least significant bit set in C<x> (or |
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280 | equivalently the number of bits set to 0 before the least significant |
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281 | bit set), starting from 0. If C<x> is 0 the result is undefined. A |
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282 | common use case is to compute the integer binary logarithm, i.e., |
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283 | floor(log2(n)). For example: |
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284 | |
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285 | ecb_ctz32 (3) = 0 |
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286 | ecb_ctz32 (6) = 1 |
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287 | |
116 | =item int ecb_popcount32 (uint32_t x) |
288 | =item int ecb_popcount32 (uint32_t x) |
117 | |
289 | |
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290 | Returns the number of bits set to 1 in C<x>. For example: |
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291 | |
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292 | ecb_popcount32 (7) = 3 |
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293 | ecb_popcount32 (255) = 8 |
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294 | |
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295 | =item uint32_t ecb_bswap16 (uint32_t x) |
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296 | |
118 | =item uint32_t ecb_bswap32 (uint32_t x) |
297 | =item uint32_t ecb_bswap32 (uint32_t x) |
119 | |
298 | |
120 | =item uint32_t ecb_bswap16 (uint32_t x) |
299 | These two functions return the value of the 16-bit (32-bit) variable |
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300 | C<x> after reversing the order of bytes. |
121 | |
301 | |
122 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
302 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
123 | |
303 | |
124 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
304 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
125 | |
305 | |
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306 | These two functions return the value of C<x> after shifting all the bits |
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307 | by C<count> positions to the right or left respectively. |
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308 | |
126 | =back |
309 | =back |
127 | |
310 | |
128 | =head2 ARITHMETIC |
311 | =head2 ARITHMETIC |
129 | |
312 | |
130 | =over 4 |
313 | =over 4 |
131 | |
314 | |
132 | =item x = ecb_mod (m, n) [MACRO] |
315 | =item x = ecb_mod (m, n) |
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316 | |
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317 | Returns the positive remainder of the modulo operation between C<m> and |
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318 | C<n>. Unlike the C modulo operator C<%>, this function ensures that the |
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319 | return value is always positive). |
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320 | |
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321 | C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be |
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322 | negatable, that is, both C<m> and C<-m> must be representable in its |
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323 | type. |
133 | |
324 | |
134 | =back |
325 | =back |
135 | |
326 | |
136 | =head2 UTILITY |
327 | =head2 UTILITY |
137 | |
328 | |
138 | =over 4 |
329 | =over 4 |
139 | |
330 | |
140 | =item ecb_array_length (name) [MACRO] |
331 | =item element_count = ecb_array_length (name) [MACRO] |
141 | |
332 | |
142 | =back |
333 | Returns the number of elements in the array C<name>. For example: |
143 | |
334 | |
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335 | int primes[] = { 2, 3, 5, 7, 11 }; |
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336 | int sum = 0; |
144 | |
337 | |
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338 | for (i = 0; i < ecb_array_length (primes); i++) |
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339 | sum += primes [i]; |
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340 | |
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341 | =back |
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342 | |
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343 | |