… | |
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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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50 | only a generic name is used (C<expr>, C<cond>, C<value> and so on), then |
54 | only a generic name is used (C<expr>, C<cond>, C<value> and so on), then |
51 | the corresponding function relies on C to implement the correct types, and |
55 | the corresponding function relies on C to implement the correct types, and |
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 | |
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59 | =head2 TYPES / TYPE SUPPORT |
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60 | |
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61 | ecb.h makes sure that the following types are defined (in the expected way): |
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62 | |
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63 | int8_t uint8_t int16_t uint16_t |
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64 | int32_t uint32_t int64_t uint64_t |
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65 | intptr_t uintptr_t ptrdiff_t |
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66 | |
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67 | The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this |
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68 | platform (currently C<4> or C<8>) and can be used in preprocessor |
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69 | expressions. |
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70 | |
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71 | =head2 LANGUAGE/COMPILER VERSIONS |
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72 | |
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73 | All the following symbols expand to an expression that can be tested in |
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74 | preprocessor instructions as well as treated as a boolean (use C<!!> to |
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75 | ensure it's either C<0> or C<1> if you need that). |
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76 | |
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77 | =over 4 |
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78 | |
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79 | =item ECB_C |
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80 | |
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81 | True if the implementation defines the C<__STDC__> macro to a true value, |
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82 | which is typically true for both C and C++ compilers. |
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83 | |
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84 | =item ECB_C99 |
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85 | |
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86 | True if the implementation claims to be C99 compliant. |
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87 | |
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88 | =item ECB_C11 |
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89 | |
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90 | True if the implementation claims to be C11 compliant. |
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91 | |
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92 | =item ECB_CPP |
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93 | |
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94 | True if the implementation defines the C<__cplusplus__> macro to a true |
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95 | value, which is typically true for C++ compilers. |
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96 | |
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97 | =item ECB_CPP98 |
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98 | |
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99 | True if the implementation claims to be compliant to ISO/IEC 14882:1998 |
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100 | (the first C++ ISO standard) or any later version. Typically true for all |
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101 | C++ compilers. |
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102 | |
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103 | =item ECB_CPP11 |
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104 | |
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105 | True if the implementation claims to be compliant to ISO/IEC 14882:2011 |
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106 | (C++11) or any later version. |
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107 | |
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108 | =item ECB_GCC_VERSION(major,minor) |
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109 | |
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110 | Expands to a true value (suitable for testing in by the preprocessor) |
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111 | if the compiler used is GNU C and the version is the given version, or |
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112 | higher. |
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113 | |
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114 | This macro tries to return false on compilers that claim to be GCC |
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115 | compatible but aren't. |
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116 | |
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117 | =back |
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118 | |
55 | =head2 GCC ATTRIBUTES |
119 | =head2 GCC ATTRIBUTES |
56 | |
120 | |
57 | blabla where to put, what others |
121 | A major part of libecb deals with GCC attributes. These are additional |
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122 | attributes that you can assign to functions, variables and sometimes even |
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123 | types - much like C<const> or C<volatile> in C. |
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124 | |
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125 | While GCC allows declarations to show up in many surprising places, |
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126 | but not in many expected places, the safest way is to put attribute |
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127 | declarations before the whole declaration: |
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128 | |
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129 | ecb_const int mysqrt (int a); |
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130 | ecb_unused int i; |
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131 | |
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132 | For variables, it is often nicer to put the attribute after the name, and |
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133 | avoid multiple declarations using commas: |
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134 | |
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135 | int i ecb_unused; |
58 | |
136 | |
59 | =over 4 |
137 | =over 4 |
60 | |
138 | |
61 | =item ecb_attribute ((attrs...)) |
139 | =item ecb_attribute ((attrs...)) |
62 | |
140 | |
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83 | #else |
161 | #else |
84 | return 0; |
162 | return 0; |
85 | #endif |
163 | #endif |
86 | } |
164 | } |
87 | |
165 | |
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166 | =item ecb_inline |
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167 | |
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168 | This is not actually an attribute, but you use it like one. It expands |
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169 | either to C<static inline> or to just C<static>, if inline isn't |
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170 | supported. It should be used to declare functions that should be inlined, |
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171 | for code size or speed reasons. |
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172 | |
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173 | Example: inline this function, it surely will reduce codesize. |
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174 | |
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175 | ecb_inline int |
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176 | negmul (int a, int b) |
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177 | { |
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178 | return - (a * b); |
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179 | } |
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180 | |
88 | =item ecb_noinline |
181 | =item ecb_noinline |
89 | |
182 | |
90 | Prevent a function from being inlined - it might be optimised away, but |
183 | Prevent a function from being inlined - it might be optimised away, but |
91 | not inlined into other functions. This is useful if you know your function |
184 | 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. |
185 | is rarely called and large enough for inlining not to be helpful. |
93 | |
186 | |
94 | =item ecb_noreturn |
187 | =item ecb_noreturn |
95 | |
188 | |
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189 | Marks a function as "not returning, ever". Some typical functions that |
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190 | don't return are C<exit> or C<abort> (which really works hard to not |
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191 | return), and now you can make your own: |
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192 | |
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193 | ecb_noreturn void |
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194 | my_abort (const char *errline) |
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195 | { |
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196 | puts (errline); |
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197 | abort (); |
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198 | } |
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199 | |
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200 | In this case, the compiler would probably be smart enough to deduce it on |
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201 | its own, so this is mainly useful for declarations. |
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202 | |
96 | =item ecb_const |
203 | =item ecb_const |
97 | |
204 | |
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205 | Declares that the function only depends on the values of its arguments, |
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206 | much like a mathematical function. It specifically does not read or write |
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207 | any memory any arguments might point to, global variables, or call any |
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208 | non-const functions. It also must not have any side effects. |
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209 | |
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210 | Such a function can be optimised much more aggressively by the compiler - |
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211 | for example, multiple calls with the same arguments can be optimised into |
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212 | a single call, which wouldn't be possible if the compiler would have to |
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213 | expect any side effects. |
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214 | |
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215 | It is best suited for functions in the sense of mathematical functions, |
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216 | such as a function returning the square root of its input argument. |
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217 | |
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218 | Not suited would be a function that calculates the hash of some memory |
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219 | area you pass in, prints some messages or looks at a global variable to |
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220 | decide on rounding. |
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221 | |
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222 | See C<ecb_pure> for a slightly less restrictive class of functions. |
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223 | |
98 | =item ecb_pure |
224 | =item ecb_pure |
99 | |
225 | |
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226 | Similar to C<ecb_const>, declares a function that has no side |
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227 | effects. Unlike C<ecb_const>, the function is allowed to examine global |
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228 | variables and any other memory areas (such as the ones passed to it via |
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229 | pointers). |
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230 | |
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231 | While these functions cannot be optimised as aggressively as C<ecb_const> |
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232 | functions, they can still be optimised away in many occasions, and the |
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233 | compiler has more freedom in moving calls to them around. |
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234 | |
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235 | Typical examples for such functions would be C<strlen> or C<memcmp>. A |
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236 | function that calculates the MD5 sum of some input and updates some MD5 |
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237 | state passed as argument would I<NOT> be pure, however, as it would modify |
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238 | some memory area that is not the return value. |
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239 | |
100 | =item ecb_hot |
240 | =item ecb_hot |
101 | |
241 | |
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242 | This declares a function as "hot" with regards to the cache - the function |
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243 | is used so often, that it is very beneficial to keep it in the cache if |
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244 | possible. |
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245 | |
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246 | The compiler reacts by trying to place hot functions near to each other in |
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247 | memory. |
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248 | |
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249 | Whether a function is hot or not often depends on the whole program, |
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250 | and less on the function itself. C<ecb_cold> is likely more useful in |
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251 | practise. |
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252 | |
102 | =item ecb_cold |
253 | =item ecb_cold |
103 | |
254 | |
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255 | The opposite of C<ecb_hot> - declares a function as "cold" with regards to |
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256 | the cache, or in other words, this function is not called often, or not at |
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257 | speed-critical times, and keeping it in the cache might be a waste of said |
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258 | cache. |
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259 | |
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260 | In addition to placing cold functions together (or at least away from hot |
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261 | functions), this knowledge can be used in other ways, for example, the |
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262 | function will be optimised for size, as opposed to speed, and codepaths |
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263 | leading to calls to those functions can automatically be marked as if |
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264 | C<ecb_expect_false> had been used to reach them. |
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265 | |
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266 | Good examples for such functions would be error reporting functions, or |
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267 | functions only called in exceptional or rare cases. |
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268 | |
104 | =item ecb_artificial |
269 | =item ecb_artificial |
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270 | |
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271 | Declares the function as "artificial", in this case meaning that this |
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272 | function is not really mean to be a function, but more like an accessor |
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273 | - many methods in C++ classes are mere accessor functions, and having a |
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274 | crash reported in such a method, or single-stepping through them, is not |
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275 | usually so helpful, especially when it's inlined to just a few instructions. |
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276 | |
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277 | Marking them as artificial will instruct the debugger about just this, |
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278 | leading to happier debugging and thus happier lives. |
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279 | |
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280 | Example: in some kind of smart-pointer class, mark the pointer accessor as |
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281 | artificial, so that the whole class acts more like a pointer and less like |
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282 | some C++ abstraction monster. |
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283 | |
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284 | template<typename T> |
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285 | struct my_smart_ptr |
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286 | { |
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287 | T *value; |
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288 | |
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289 | ecb_artificial |
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290 | operator T *() |
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291 | { |
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292 | return value; |
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293 | } |
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294 | }; |
105 | |
295 | |
106 | =back |
296 | =back |
107 | |
297 | |
108 | =head2 OPTIMISATION HINTS |
298 | =head2 OPTIMISATION HINTS |
109 | |
299 | |
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141 | |
331 | |
142 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
332 | 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 |
333 | the C<expr> evaluates to C<value> a lot, which can be used for static |
144 | branch optimisations. |
334 | branch optimisations. |
145 | |
335 | |
146 | Usually, you want to use the more intuitive C<ecb_likely> and |
336 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
147 | C<ecb_unlikely> functions instead. |
337 | C<ecb_expect_false> functions instead. |
148 | |
338 | |
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339 | =item bool ecb_expect_true (cond) |
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340 | |
149 | =item bool ecb_likely (cond) |
341 | =item bool ecb_expect_false (cond) |
150 | |
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151 | =item bool ecb_unlikely (cond) |
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152 | |
342 | |
153 | These two functions expect a expression that is true or false and return |
343 | 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 |
344 | 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: |
345 | other conditional statement, it will not change the program: |
156 | |
346 | |
157 | /* these two do the same thing */ |
347 | /* these two do the same thing */ |
158 | if (some_condition) ...; |
348 | if (some_condition) ...; |
159 | if (ecb_likely (some_condition)) ...; |
349 | if (ecb_expect_true (some_condition)) ...; |
160 | |
350 | |
161 | However, by using C<ecb_likely>, you tell the compiler that the condition |
351 | 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 |
352 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
163 | true). |
353 | unlikely to be true). |
164 | |
354 | |
165 | For example, when you check for a null pointer and expect this to be a |
355 | For example, when you check for a null pointer and expect this to be a |
166 | rare, exceptional, case, then use C<ecb_unlikely>: |
356 | rare, exceptional, case, then use C<ecb_expect_false>: |
167 | |
357 | |
168 | void my_free (void *ptr) |
358 | void my_free (void *ptr) |
169 | { |
359 | { |
170 | if (ecb_unlikely (ptr == 0)) |
360 | if (ecb_expect_false (ptr == 0)) |
171 | return; |
361 | return; |
172 | } |
362 | } |
173 | |
363 | |
174 | Consequent use of these functions to mark away exceptional cases or to |
364 | 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 |
365 | tell the compiler what the hot path through a function is can increase |
176 | performance considerably. |
366 | performance considerably. |
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367 | |
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368 | You might know these functions under the name C<likely> and C<unlikely> |
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369 | - while these are common aliases, we find that the expect name is easier |
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370 | to understand when quickly skimming code. If you wish, you can use |
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371 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
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372 | C<ecb_expect_false> - these are simply aliases. |
177 | |
373 | |
178 | A very good example is in a function that reserves more space for some |
374 | 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) - |
375 | 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 |
376 | 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: |
377 | you expect that most checks will turn out to be false: |
182 | |
378 | |
183 | /* make sure we have "size" extra room in our buffer */ |
379 | /* make sure we have "size" extra room in our buffer */ |
184 | ecb_inline void |
380 | ecb_inline void |
185 | reserve (int size) |
381 | reserve (int size) |
186 | { |
382 | { |
187 | if (ecb_unlikely (current + size > end)) |
383 | if (ecb_expect_false (current + size > end)) |
188 | real_reserve_method (size); /* presumably noinline */ |
384 | real_reserve_method (size); /* presumably noinline */ |
189 | } |
385 | } |
190 | |
386 | |
191 | =item bool ecb_assume (cond) |
387 | =item bool ecb_assume (cond) |
192 | |
388 | |
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195 | |
391 | |
196 | This can be used to teach the compiler about invariants or other |
392 | This can be used to teach the compiler about invariants or other |
197 | conditions that might improve code generation, but which are impossible to |
393 | conditions that might improve code generation, but which are impossible to |
198 | deduce form the code itself. |
394 | deduce form the code itself. |
199 | |
395 | |
200 | For example, the example reservation function from the C<ecb_unlikely> |
396 | For example, the example reservation function from the C<ecb_expect_false> |
201 | description could be written thus (only C<ecb_assume> was added): |
397 | description could be written thus (only C<ecb_assume> was added): |
202 | |
398 | |
203 | ecb_inline void |
399 | ecb_inline void |
204 | reserve (int size) |
400 | reserve (int size) |
205 | { |
401 | { |
206 | if (ecb_unlikely (current + size > end)) |
402 | if (ecb_expect_false (current + size > end)) |
207 | real_reserve_method (size); /* presumably noinline */ |
403 | real_reserve_method (size); /* presumably noinline */ |
208 | |
404 | |
209 | ecb_assume (current + size <= end); |
405 | ecb_assume (current + size <= end); |
210 | } |
406 | } |
211 | |
407 | |
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260 | After processing the node, (part of) the next node might already be in |
456 | After processing the node, (part of) the next node might already be in |
261 | cache. |
457 | cache. |
262 | |
458 | |
263 | =back |
459 | =back |
264 | |
460 | |
265 | =head2 BIT FIDDLING / BITSTUFFS |
461 | =head2 BIT FIDDLING / BIT WIZARDRY |
266 | |
462 | |
267 | =over 4 |
463 | =over 4 |
268 | |
464 | |
269 | =item bool ecb_big_endian () |
465 | =item bool ecb_big_endian () |
270 | |
466 | |
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… | |
272 | |
468 | |
273 | These two functions return true if the byte order is big endian |
469 | These two functions return true if the byte order is big endian |
274 | (most-significant byte first) or little endian (least-significant byte |
470 | (most-significant byte first) or little endian (least-significant byte |
275 | first) respectively. |
471 | first) respectively. |
276 | |
472 | |
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473 | On systems that are neither, their return values are unspecified. |
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474 | |
277 | =item int ecb_ctz32 (uint32_t x) |
475 | =item int ecb_ctz32 (uint32_t x) |
278 | |
476 | |
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477 | =item int ecb_ctz64 (uint64_t x) |
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478 | |
279 | Returns the index of the least significant bit set in C<x> (or |
479 | 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 |
480 | 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 |
481 | set), starting from 0. If C<x> is 0 the result is undefined. |
282 | common use case is to compute the integer binary logarithm, i.e., |
482 | |
283 | floor(log2(n)). For example: |
483 | For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>. |
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484 | |
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485 | For example: |
284 | |
486 | |
285 | ecb_ctz32 (3) = 0 |
487 | ecb_ctz32 (3) = 0 |
286 | ecb_ctz32 (6) = 1 |
488 | ecb_ctz32 (6) = 1 |
287 | |
489 | |
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490 | =item bool ecb_is_pot32 (uint32_t x) |
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491 | |
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492 | =item bool ecb_is_pot64 (uint32_t x) |
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493 | |
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494 | Return true iff C<x> is a power of two or C<x == 0>. |
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495 | |
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496 | For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>. |
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497 | |
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498 | =item int ecb_ld32 (uint32_t x) |
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499 | |
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500 | =item int ecb_ld64 (uint64_t x) |
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501 | |
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502 | Returns the index of the most significant bit set in C<x>, or the number |
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503 | of digits the number requires in binary (so that C<< 2**ld <= x < |
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504 | 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is |
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505 | to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for |
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506 | example to see how many bits a certain number requires to be encoded. |
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507 | |
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508 | This function is similar to the "count leading zero bits" function, except |
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509 | that that one returns how many zero bits are "in front" of the number (in |
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510 | the given data type), while C<ecb_ld> returns how many bits the number |
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511 | itself requires. |
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512 | |
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513 | For smaller types than C<uint32_t> you can safely use C<ecb_ld32>. |
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514 | |
288 | =item int ecb_popcount32 (uint32_t x) |
515 | =item int ecb_popcount32 (uint32_t x) |
289 | |
516 | |
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517 | =item int ecb_popcount64 (uint64_t x) |
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518 | |
290 | Returns the number of bits set to 1 in C<x>. For example: |
519 | Returns the number of bits set to 1 in C<x>. |
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520 | |
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521 | For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>. |
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522 | |
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523 | For example: |
291 | |
524 | |
292 | ecb_popcount32 (7) = 3 |
525 | ecb_popcount32 (7) = 3 |
293 | ecb_popcount32 (255) = 8 |
526 | ecb_popcount32 (255) = 8 |
294 | |
527 | |
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528 | =item uint8_t ecb_bitrev8 (uint8_t x) |
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529 | |
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530 | =item uint16_t ecb_bitrev16 (uint16_t x) |
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531 | |
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532 | =item uint32_t ecb_bitrev32 (uint32_t x) |
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533 | |
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534 | Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1 |
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535 | and so on. |
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536 | |
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537 | Example: |
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538 | |
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539 | ecb_bitrev8 (0xa7) = 0xea |
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540 | ecb_bitrev32 (0xffcc4411) = 0x882233ff |
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541 | |
295 | =item uint32_t ecb_bswap16 (uint32_t x) |
542 | =item uint32_t ecb_bswap16 (uint32_t x) |
296 | |
543 | |
297 | =item uint32_t ecb_bswap32 (uint32_t x) |
544 | =item uint32_t ecb_bswap32 (uint32_t x) |
298 | |
545 | |
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546 | =item uint64_t ecb_bswap64 (uint64_t x) |
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547 | |
299 | These two functions return the value of the 16-bit (32-bit) variable |
548 | These functions return the value of the 16-bit (32-bit, 64-bit) value |
300 | C<x> after reversing the order of bytes. |
549 | C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in |
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550 | C<ecb_bswap32>). |
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551 | |
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552 | =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count) |
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553 | |
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554 | =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count) |
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555 | |
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556 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
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557 | |
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558 | =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count) |
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559 | |
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560 | =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count) |
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561 | |
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562 | =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count) |
301 | |
563 | |
302 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
564 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
303 | |
565 | |
304 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
566 | =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count) |
305 | |
567 | |
306 | These two functions return the value of C<x> after shifting all the bits |
568 | These two families of functions return the value of C<x> after rotating |
307 | by C<count> positions to the right or left respectively. |
569 | all the bits by C<count> positions to the right (C<ecb_rotr>) or left |
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570 | (C<ecb_rotl>). |
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571 | |
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572 | Current GCC versions understand these functions and usually compile them |
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573 | to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on |
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574 | x86). |
308 | |
575 | |
309 | =back |
576 | =back |
310 | |
577 | |
311 | =head2 ARITHMETIC |
578 | =head2 ARITHMETIC |
312 | |
579 | |
313 | =over 4 |
580 | =over 4 |
314 | |
581 | |
315 | =item x = ecb_mod (m, n) |
582 | =item x = ecb_mod (m, n) |
316 | |
583 | |
317 | Returns the positive remainder of the modulo operation between C<m> and |
584 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
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585 | 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 |
586 | division. Unlike the C remainder operator C<%>, this function ensures that |
319 | return value is always positive). |
587 | the return value is always positive and that the two numbers I<m> and |
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588 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
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589 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
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590 | in the language. |
320 | |
591 | |
321 | C<n> must be strictly positive (i.e. C<< >1 >>), while C<m> must be |
592 | 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 |
593 | negatable, that is, both C<m> and C<-m> must be representable in its |
323 | type. |
594 | type (this typically excludes the minimum signed integer value, the same |
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595 | limitation as for C</> and C<%> in C). |
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596 | |
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597 | Current GCC versions compile this into an efficient branchless sequence on |
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598 | almost all CPUs. |
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599 | |
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600 | For example, when you want to rotate forward through the members of an |
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601 | array for increasing C<m> (which might be negative), then you should use |
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602 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
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603 | change direction for negative values: |
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604 | |
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605 | for (m = -100; m <= 100; ++m) |
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606 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
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607 | |
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608 | =item x = ecb_div_rd (val, div) |
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609 | |
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610 | =item x = ecb_div_ru (val, div) |
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611 | |
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612 | Returns C<val> divided by C<div> rounded down or up, respectively. |
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613 | C<val> and C<div> must have integer types and C<div> must be strictly |
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614 | positive. Note that these functions are implemented with macros in C |
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615 | and with function templates in C++. |
324 | |
616 | |
325 | =back |
617 | =back |
326 | |
618 | |
327 | =head2 UTILITY |
619 | =head2 UTILITY |
328 | |
620 | |
329 | =over 4 |
621 | =over 4 |
330 | |
622 | |
331 | =item element_count = ecb_array_length (name) [MACRO] |
623 | =item element_count = ecb_array_length (name) |
332 | |
624 | |
333 | Returns the number of elements in the array C<name>. For example: |
625 | Returns the number of elements in the array C<name>. For example: |
334 | |
626 | |
335 | int primes[] = { 2, 3, 5, 7, 11 }; |
627 | int primes[] = { 2, 3, 5, 7, 11 }; |
336 | int sum = 0; |
628 | int sum = 0; |
… | |
… | |
338 | for (i = 0; i < ecb_array_length (primes); i++) |
630 | for (i = 0; i < ecb_array_length (primes); i++) |
339 | sum += primes [i]; |
631 | sum += primes [i]; |
340 | |
632 | |
341 | =back |
633 | =back |
342 | |
634 | |
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635 | =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF |
343 | |
636 | |
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637 | These symbols need to be defined before including F<ecb.h> the first time. |
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638 | |
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639 | =over 4 |
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640 | |
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641 | =item ECB_NO_THRADS |
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642 | |
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643 | If F<ecb.h> is never used from multiple threads, then this symbol can |
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644 | be defined, in which case memory fences (and similar constructs) are |
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645 | completely removed, leading to more efficient code and fewer dependencies. |
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646 | |
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647 | Setting this symbol to a true value implies C<ECB_NO_SMP>. |
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648 | |
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649 | =item ECB_NO_SMP |
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650 | |
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651 | The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from |
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652 | multiple threads, but never concurrently (e.g. if the system the program |
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653 | runs on has only a single CPU with a single core, no hyperthreading and so |
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654 | on), then this symbol can be defined, leading to more efficient code and |
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655 | fewer dependencies. |
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656 | |
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657 | =back |
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658 | |
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659 | |