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3 | =head2 ABOUT LIBECB |
3 | =head2 ABOUT LIBECB |
4 | |
4 | |
5 | Libecb is currently a simple header file that doesn't require any |
5 | Libecb is currently a simple header file that doesn't require any |
6 | configuration to use or include in your project. |
6 | configuration to use or include in your project. |
7 | |
7 | |
8 | It's part of the e-suite of libraries, other memembers of which include |
8 | It's part of the e-suite of libraries, other members of which include |
9 | libev and libeio. |
9 | libev and libeio. |
10 | |
10 | |
11 | Its homepage can be found here: |
11 | Its homepage can be found here: |
12 | |
12 | |
13 | http://software.schmorp.de/pkg/libecb |
13 | http://software.schmorp.de/pkg/libecb |
14 | |
14 | |
15 | It mainly provides a number of wrappers around GCC built-ins, together |
15 | It mainly provides a number of wrappers around GCC built-ins, together |
16 | with replacement functions for other compilers. In addition to this, |
16 | with replacement functions for other compilers. In addition to this, |
17 | it provides a number of other lowlevel C utilities, such endienness |
17 | it provides a number of other lowlevel C utilities, such as endianness |
18 | detection, byte swapping or bit rotations. |
18 | detection, byte swapping or bit rotations. |
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19 | |
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20 | Or in other words, things that should be built into any standard C system, |
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21 | but aren't, implemented as efficient as possible with GCC, and still |
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22 | correct with other compilers. |
19 | |
23 | |
20 | More might come. |
24 | More might come. |
21 | |
25 | |
22 | =head2 ABOUT THE HEADER |
26 | =head2 ABOUT THE HEADER |
23 | |
27 | |
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27 | #include <ecb.h> |
31 | #include <ecb.h> |
28 | |
32 | |
29 | The header should work fine for both C and C++ compilation, and gives you |
33 | The header should work fine for both C and C++ compilation, and gives you |
30 | all of F<inttypes.h> in addition to the ECB symbols. |
34 | all of F<inttypes.h> in addition to the ECB symbols. |
31 | |
35 | |
32 | There are currently no objetc files to link to - future versions might |
36 | There are currently no object files to link to - future versions might |
33 | come with an (optional) object code library to link against, to reduce |
37 | come with an (optional) object code library to link against, to reduce |
34 | code size or gain access to additional features. |
38 | code size or gain access to additional features. |
35 | |
39 | |
36 | It also currently includes everything from F<inttypes.h>. |
40 | It also currently includes everything from F<inttypes.h>. |
37 | |
41 | |
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52 | is usually implemented as a macro. Specifically, a "bool" in this manual |
56 | is usually implemented as a macro. Specifically, a "bool" in this manual |
53 | refers to any kind of boolean value, not a specific type. |
57 | refers to any kind of boolean value, not a specific type. |
54 | |
58 | |
55 | =head2 GCC ATTRIBUTES |
59 | =head2 GCC ATTRIBUTES |
56 | |
60 | |
57 | blabla where to put, what others |
61 | A major part of libecb deals with GCC attributes. These are additional |
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62 | attributes that you can assign to functions, variables and sometimes even |
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63 | types - much like C<const> or C<volatile> in C. |
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64 | |
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65 | While GCC allows declarations to show up in many surprising places, |
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66 | but not in many expected places, the safest way is to put attribute |
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67 | declarations before the whole declaration: |
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68 | |
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69 | ecb_const int mysqrt (int a); |
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70 | ecb_unused int i; |
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71 | |
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72 | For variables, it is often nicer to put the attribute after the name, and |
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73 | avoid multiple declarations using commas: |
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74 | |
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75 | int i ecb_unused; |
58 | |
76 | |
59 | =over 4 |
77 | =over 4 |
60 | |
78 | |
61 | =item ecb_attribute ((attrs...)) |
79 | =item ecb_attribute ((attrs...)) |
62 | |
80 | |
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91 | not inlined into other functions. This is useful if you know your function |
109 | not inlined into other functions. This is useful if you know your function |
92 | is rarely called and large enough for inlining not to be helpful. |
110 | is rarely called and large enough for inlining not to be helpful. |
93 | |
111 | |
94 | =item ecb_noreturn |
112 | =item ecb_noreturn |
95 | |
113 | |
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114 | Marks a function as "not returning, ever". Some typical functions that |
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115 | don't return are C<exit> or C<abort> (which really works hard to not |
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116 | return), and now you can make your own: |
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117 | |
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118 | ecb_noreturn void |
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119 | my_abort (const char *errline) |
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120 | { |
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121 | puts (errline); |
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122 | abort (); |
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123 | } |
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124 | |
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125 | In this case, the compiler would probably be smart enough to deduce it on |
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126 | its own, so this is mainly useful for declarations. |
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127 | |
96 | =item ecb_const |
128 | =item ecb_const |
97 | |
129 | |
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130 | Declares that the function only depends on the values of its arguments, |
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131 | much like a mathematical function. It specifically does not read or write |
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132 | any memory any arguments might point to, global variables, or call any |
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133 | non-const functions. It also must not have any side effects. |
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134 | |
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135 | Such a function can be optimised much more aggressively by the compiler - |
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136 | for example, multiple calls with the same arguments can be optimised into |
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137 | a single call, which wouldn't be possible if the compiler would have to |
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138 | expect any side effects. |
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139 | |
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140 | It is best suited for functions in the sense of mathematical functions, |
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141 | such as a function returning the square root of its input argument. |
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142 | |
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143 | Not suited would be a function that calculates the hash of some memory |
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144 | area you pass in, prints some messages or looks at a global variable to |
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145 | decide on rounding. |
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146 | |
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147 | See C<ecb_pure> for a slightly less restrictive class of functions. |
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148 | |
98 | =item ecb_pure |
149 | =item ecb_pure |
99 | |
150 | |
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151 | Similar to C<ecb_const>, declares a function that has no side |
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152 | effects. Unlike C<ecb_const>, the function is allowed to examine global |
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153 | variables and any other memory areas (such as the ones passed to it via |
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154 | pointers). |
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155 | |
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156 | While these functions cannot be optimised as aggressively as C<ecb_const> |
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157 | functions, they can still be optimised away in many occasions, and the |
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158 | compiler has more freedom in moving calls to them around. |
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159 | |
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160 | Typical examples for such functions would be C<strlen> or C<memcmp>. A |
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161 | function that calculates the MD5 sum of some input and updates some MD5 |
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162 | state passed as argument would I<NOT> be pure, however, as it would modify |
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163 | some memory area that is not the return value. |
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164 | |
100 | =item ecb_hot |
165 | =item ecb_hot |
101 | |
166 | |
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167 | This declares a function as "hot" with regards to the cache - the function |
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168 | is used so often, that it is very beneficial to keep it in the cache if |
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169 | possible. |
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170 | |
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171 | The compiler reacts by trying to place hot functions near to each other in |
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172 | memory. |
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173 | |
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174 | Whether a function is hot or not often depends on the whole program, |
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175 | and less on the function itself. C<ecb_cold> is likely more useful in |
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176 | practise. |
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177 | |
102 | =item ecb_cold |
178 | =item ecb_cold |
103 | |
179 | |
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180 | The opposite of C<ecb_hot> - declares a function as "cold" with regards to |
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181 | the cache, or in other words, this function is not called often, or not at |
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182 | speed-critical times, and keeping it in the cache might be a waste of said |
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183 | cache. |
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184 | |
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185 | In addition to placing cold functions together (or at least away from hot |
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186 | functions), this knowledge can be used in other ways, for example, the |
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187 | function will be optimised for size, as opposed to speed, and codepaths |
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188 | leading to calls to those functions can automatically be marked as if |
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189 | C<ecb_expect_false> had been used to reach them. |
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190 | |
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191 | Good examples for such functions would be error reporting functions, or |
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192 | functions only called in exceptional or rare cases. |
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193 | |
104 | =item ecb_artificial |
194 | =item ecb_artificial |
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195 | |
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196 | Declares the function as "artificial", in this case meaning that this |
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197 | function is not really mean to be a function, but more like an accessor |
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198 | - many methods in C++ classes are mere accessor functions, and having a |
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199 | crash reported in such a method, or single-stepping through them, is not |
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200 | usually so helpful, especially when it's inlined to just a few instructions. |
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201 | |
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202 | Marking them as artificial will instruct the debugger about just this, |
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203 | leading to happier debugging and thus happier lives. |
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204 | |
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205 | Example: in some kind of smart-pointer class, mark the pointer accessor as |
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206 | artificial, so that the whole class acts more like a pointer and less like |
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207 | some C++ abstraction monster. |
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208 | |
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209 | template<typename T> |
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210 | struct my_smart_ptr |
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211 | { |
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212 | T *value; |
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213 | |
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214 | ecb_artificial |
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215 | operator T *() |
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216 | { |
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217 | return value; |
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218 | } |
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219 | }; |
105 | |
220 | |
106 | =back |
221 | =back |
107 | |
222 | |
108 | =head2 OPTIMISATION HINTS |
223 | =head2 OPTIMISATION HINTS |
109 | |
224 | |
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141 | |
256 | |
142 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
257 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
143 | the C<expr> evaluates to C<value> a lot, which can be used for static |
258 | the C<expr> evaluates to C<value> a lot, which can be used for static |
144 | branch optimisations. |
259 | branch optimisations. |
145 | |
260 | |
146 | Usually, you want to use the more intuitive C<ecb_likely> and |
261 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
147 | C<ecb_unlikely> functions instead. |
262 | C<ecb_expect_false> functions instead. |
148 | |
263 | |
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264 | =item bool ecb_expect_true (cond) |
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265 | |
149 | =item bool ecb_likely (cond) |
266 | =item bool ecb_expect_false (cond) |
150 | |
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151 | =item bool ecb_unlikely (cond) |
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152 | |
267 | |
153 | These two functions expect a expression that is true or false and return |
268 | These two functions expect a expression that is true or false and return |
154 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
269 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
155 | other conditional statement, it will not change the program: |
270 | other conditional statement, it will not change the program: |
156 | |
271 | |
157 | /* these two do the same thing */ |
272 | /* these two do the same thing */ |
158 | if (some_condition) ...; |
273 | if (some_condition) ...; |
159 | if (ecb_likely (some_condition)) ...; |
274 | if (ecb_expect_true (some_condition)) ...; |
160 | |
275 | |
161 | However, by using C<ecb_likely>, you tell the compiler that the condition |
276 | However, by using C<ecb_expect_true>, you tell the compiler that the |
162 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
277 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
163 | true). |
278 | unlikely to be true). |
164 | |
279 | |
165 | For example, when you check for a null pointer and expect this to be a |
280 | For example, when you check for a null pointer and expect this to be a |
166 | rare, exceptional, case, then use C<ecb_unlikely>: |
281 | rare, exceptional, case, then use C<ecb_expect_false>: |
167 | |
282 | |
168 | void my_free (void *ptr) |
283 | void my_free (void *ptr) |
169 | { |
284 | { |
170 | if (ecb_unlikely (ptr == 0)) |
285 | if (ecb_expect_false (ptr == 0)) |
171 | return; |
286 | return; |
172 | } |
287 | } |
173 | |
288 | |
174 | Consequent use of these functions to mark away exceptional cases or to |
289 | Consequent use of these functions to mark away exceptional cases or to |
175 | tell the compiler what the hot path through a function is can increase |
290 | tell the compiler what the hot path through a function is can increase |
176 | performance considerably. |
291 | performance considerably. |
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292 | |
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293 | You might know these functions under the name C<likely> and C<unlikely> |
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294 | - while these are common aliases, we find that the expect name is easier |
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295 | to understand when quickly skimming code. If you wish, you can use |
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296 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
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297 | C<ecb_expect_false> - these are simply aliases. |
177 | |
298 | |
178 | A very good example is in a function that reserves more space for some |
299 | A very good example is in a function that reserves more space for some |
179 | memory block (for example, inside an implementation of a string stream) - |
300 | memory block (for example, inside an implementation of a string stream) - |
180 | each time something is added, you have to check for a buffer overrun, but |
301 | each time something is added, you have to check for a buffer overrun, but |
181 | you expect that most checks will turn out to be false: |
302 | you expect that most checks will turn out to be false: |
182 | |
303 | |
183 | /* make sure we have "size" extra room in our buffer */ |
304 | /* make sure we have "size" extra room in our buffer */ |
184 | ecb_inline void |
305 | ecb_inline void |
185 | reserve (int size) |
306 | reserve (int size) |
186 | { |
307 | { |
187 | if (ecb_unlikely (current + size > end)) |
308 | if (ecb_expect_false (current + size > end)) |
188 | real_reserve_method (size); /* presumably noinline */ |
309 | real_reserve_method (size); /* presumably noinline */ |
189 | } |
310 | } |
190 | |
311 | |
191 | =item bool ecb_assume (cond) |
312 | =item bool ecb_assume (cond) |
192 | |
313 | |
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195 | |
316 | |
196 | This can be used to teach the compiler about invariants or other |
317 | This can be used to teach the compiler about invariants or other |
197 | conditions that might improve code generation, but which are impossible to |
318 | conditions that might improve code generation, but which are impossible to |
198 | deduce form the code itself. |
319 | deduce form the code itself. |
199 | |
320 | |
200 | For example, the example reservation function from the C<ecb_unlikely> |
321 | For example, the example reservation function from the C<ecb_expect_false> |
201 | description could be written thus (only C<ecb_assume> was added): |
322 | description could be written thus (only C<ecb_assume> was added): |
202 | |
323 | |
203 | ecb_inline void |
324 | ecb_inline void |
204 | reserve (int size) |
325 | reserve (int size) |
205 | { |
326 | { |
206 | if (ecb_unlikely (current + size > end)) |
327 | if (ecb_expect_false (current + size > end)) |
207 | real_reserve_method (size); /* presumably noinline */ |
328 | real_reserve_method (size); /* presumably noinline */ |
208 | |
329 | |
209 | ecb_assume (current + size <= end); |
330 | ecb_assume (current + size <= end); |
210 | } |
331 | } |
211 | |
332 | |
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272 | |
393 | |
273 | These two functions return true if the byte order is big endian |
394 | These two functions return true if the byte order is big endian |
274 | (most-significant byte first) or little endian (least-significant byte |
395 | (most-significant byte first) or little endian (least-significant byte |
275 | first) respectively. |
396 | first) respectively. |
276 | |
397 | |
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398 | On systems that are neither, their return values are unspecified. |
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399 | |
277 | =item int ecb_ctz32 (uint32_t x) |
400 | =item int ecb_ctz32 (uint32_t x) |
278 | |
401 | |
279 | Returns the index of the least significant bit set in C<x> (or |
402 | Returns the index of the least significant bit set in C<x> (or |
280 | equivalently the number of bits set to 0 before the least significant |
403 | equivalently the number of bits set to 0 before the least significant bit |
281 | bit set), starting from 0. If C<x> is 0 the result is undefined. A |
404 | set), starting from 0. If C<x> is 0 the result is undefined. A common use |
282 | common use case is to compute the integer binary logarithm, i.e., |
405 | case is to compute the integer binary logarithm, i.e., C<floor (log2 |
283 | floor(log2(n)). For example: |
406 | (n))>. For example: |
284 | |
407 | |
285 | ecb_ctz32 (3) = 0 |
408 | ecb_ctz32 (3) = 0 |
286 | ecb_ctz32 (6) = 1 |
409 | ecb_ctz32 (6) = 1 |
287 | |
410 | |
288 | =item int ecb_popcount32 (uint32_t x) |
411 | =item int ecb_popcount32 (uint32_t x) |
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294 | |
417 | |
295 | =item uint32_t ecb_bswap16 (uint32_t x) |
418 | =item uint32_t ecb_bswap16 (uint32_t x) |
296 | |
419 | |
297 | =item uint32_t ecb_bswap32 (uint32_t x) |
420 | =item uint32_t ecb_bswap32 (uint32_t x) |
298 | |
421 | |
299 | These two functions return the value of the 16-bit (32-bit) variable |
422 | These two functions return the value of the 16-bit (32-bit) value C<x> |
300 | C<x> after reversing the order of bytes. |
423 | after reversing the order of bytes (0x11223344 becomes 0x44332211). |
301 | |
424 | |
302 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
425 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
303 | |
426 | |
304 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
427 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
305 | |
428 | |
306 | These two functions return the value of C<x> after shifting all the bits |
429 | These two functions return the value of C<x> after rotating all the bits |
307 | by C<count> positions to the right or left respectively. |
430 | by C<count> positions to the right or left respectively. |
308 | |
431 | |
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432 | Current GCC versions understand these functions and usually compile them |
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433 | to "optimal" code (e.g. a single C<roll> on x86). |
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434 | |
309 | =back |
435 | =back |
310 | |
436 | |
311 | =head2 ARITHMETIC |
437 | =head2 ARITHMETIC |
312 | |
438 | |
313 | =over 4 |
439 | =over 4 |
314 | |
440 | |
315 | =item x = ecb_mod (m, n) |
441 | =item x = ecb_mod (m, n) |
316 | |
442 | |
317 | Returns the positive remainder of the modulo operation between C<m> and |
443 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
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444 | of the division operation between C<m> and C<n>, using floored |
318 | C<n>. Unlike the C moduloe operator C<%>, this function ensures that the |
445 | division. Unlike the C remainder operator C<%>, this function ensures that |
319 | return value is always positive). |
446 | the return value is always positive and that the two numbers I<m> and |
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447 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
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448 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
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449 | in the language. |
320 | |
450 | |
321 | C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be |
451 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
322 | negatable, that is, both C<m> and C<-m> must be representable in its |
452 | negatable, that is, both C<m> and C<-m> must be representable in its |
323 | type. |
453 | type (this typically includes the minimum signed integer value, the same |
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454 | limitation as for C</> and C<%> in C). |
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455 | |
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456 | Current GCC versions compile this into an efficient branchless sequence on |
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457 | many systems. |
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458 | |
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459 | For example, when you want to rotate forward through the members of an |
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460 | array for increasing C<m> (which might be negative), then you should use |
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461 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
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462 | change direction for negative values: |
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463 | |
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464 | for (m = -100; m <= 100; ++m) |
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465 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
324 | |
466 | |
325 | =back |
467 | =back |
326 | |
468 | |
327 | =head2 UTILITY |
469 | =head2 UTILITY |
328 | |
470 | |
329 | =over 4 |
471 | =over 4 |
330 | |
472 | |
331 | =item element_count = ecb_array_length (name) [MACRO] |
473 | =item element_count = ecb_array_length (name) |
332 | |
474 | |
333 | Returns the number of elements in the array C<name>. For example: |
475 | Returns the number of elements in the array C<name>. For example: |
334 | |
476 | |
335 | int primes[] = { 2, 3, 5, 7, 11 }; |
477 | int primes[] = { 2, 3, 5, 7, 11 }; |
336 | int sum = 0; |
478 | int sum = 0; |