… | |
… | |
54 | 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 |
55 | 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 |
56 | 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 |
57 | refers to any kind of boolean value, not a specific type. |
57 | refers to any kind of boolean value, not a specific type. |
58 | |
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_ |
|
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64 | int16_t uint16_t |
|
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65 | int32_t uint32_ |
|
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66 | int64_t uint64_t |
|
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67 | int_fast8_t uint_fast8_t |
|
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68 | int_fast16_t uint_fast16_t |
|
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69 | int_fast32_t uint_fast32_t |
|
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70 | int_fast64_t uint_fast64_t |
|
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71 | intptr_t uintptr_t |
|
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72 | |
|
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73 | The macro C<ECB_PTRSIZE> is defined to the size of a pointer on this |
|
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74 | platform (currently C<4> or C<8>) and can be used in preprocessor |
|
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75 | expressions. |
|
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76 | |
|
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77 | For C<ptrdiff_t> and C<size_t> use C<stddef.h>/C<cstddef>. |
|
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78 | |
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79 | =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS |
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80 | |
|
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81 | All the following symbols expand to an expression that can be tested in |
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82 | preprocessor instructions as well as treated as a boolean (use C<!!> to |
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83 | ensure it's either C<0> or C<1> if you need that). |
|
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84 | |
|
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85 | =over 4 |
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86 | |
|
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87 | =item ECB_C |
|
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88 | |
|
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89 | True if the implementation defines the C<__STDC__> macro to a true value, |
|
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90 | while not claiming to be C++, i..e C, but not C++. |
|
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91 | |
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92 | =item ECB_C99 |
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93 | |
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94 | True if the implementation claims to be compliant to C99 (ISO/IEC |
|
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95 | 9899:1999) or any later version, while not claiming to be C++. |
|
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96 | |
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97 | Note that later versions (ECB_C11) remove core features again (for |
|
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98 | example, variable length arrays). |
|
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99 | |
|
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100 | =item ECB_C11, ECB_C17 |
|
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101 | |
|
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102 | True if the implementation claims to be compliant to C11/C17 (ISO/IEC |
|
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103 | 9899:2011, :20187) or any later version, while not claiming to be C++. |
|
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104 | |
|
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105 | =item ECB_CPP |
|
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106 | |
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107 | True if the implementation defines the C<__cplusplus__> macro to a true |
|
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108 | value, which is typically true for C++ compilers. |
|
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109 | |
|
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110 | =item ECB_CPP11, ECB_CPP14, ECB_CPP17 |
|
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111 | |
|
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112 | True if the implementation claims to be compliant to C++11/C++14/C++17 |
|
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113 | (ISO/IEC 14882:2011, :2014, :2017) or any later version. |
|
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114 | |
|
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115 | Note that many C++20 features will likely have their own feature test |
|
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116 | macros (see e.g. L<http://eel.is/c++draft/cpp.predefined#1.8>). |
|
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117 | |
|
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118 | =item ECB_OPTIMIZE_SIZE |
|
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119 | |
|
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120 | Is C<1> when the compiler optimizes for size, C<0> otherwise. This symbol |
|
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121 | can also be defined before including F<ecb.h>, in which case it will be |
|
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122 | unchanged. |
|
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123 | |
|
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124 | =item ECB_GCC_VERSION (major, minor) |
|
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125 | |
|
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126 | Expands to a true value (suitable for testing by the preprocessor) if the |
|
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127 | compiler used is GNU C and the version is the given version, or higher. |
|
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128 | |
|
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129 | This macro tries to return false on compilers that claim to be GCC |
|
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130 | compatible but aren't. |
|
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131 | |
|
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132 | =item ECB_EXTERN_C |
|
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133 | |
|
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134 | Expands to C<extern "C"> in C++, and a simple C<extern> in C. |
|
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135 | |
|
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136 | This can be used to declare a single external C function: |
|
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137 | |
|
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138 | ECB_EXTERN_C int printf (const char *format, ...); |
|
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139 | |
|
|
140 | =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END |
|
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141 | |
|
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142 | These two macros can be used to wrap multiple C<extern "C"> definitions - |
|
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143 | they expand to nothing in C. |
|
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144 | |
|
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145 | They are most useful in header files: |
|
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146 | |
|
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147 | ECB_EXTERN_C_BEG |
|
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148 | |
|
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149 | int mycfun1 (int x); |
|
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150 | int mycfun2 (int x); |
|
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151 | |
|
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152 | ECB_EXTERN_C_END |
|
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153 | |
|
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154 | =item ECB_STDFP |
|
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155 | |
|
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156 | If this evaluates to a true value (suitable for testing by the |
|
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157 | preprocessor), then C<float> and C<double> use IEEE 754 single/binary32 |
|
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158 | and double/binary64 representations internally I<and> the endianness of |
|
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159 | both types match the endianness of C<uint32_t> and C<uint64_t>. |
|
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160 | |
|
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161 | This means you can just copy the bits of a C<float> (or C<double>) to an |
|
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162 | C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation |
|
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163 | without having to think about format or endianness. |
|
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164 | |
|
|
165 | This is true for basically all modern platforms, although F<ecb.h> might |
|
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166 | not be able to deduce this correctly everywhere and might err on the safe |
|
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167 | side. |
|
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168 | |
|
|
169 | =item ECB_AMD64, ECB_AMD64_X32 |
|
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170 | |
|
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171 | These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32 |
|
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172 | ABI, respectively, and undefined elsewhere. |
|
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173 | |
|
|
174 | The designers of the new X32 ABI for some inexplicable reason decided to |
|
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175 | make it look exactly like amd64, even though it's completely incompatible |
|
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176 | to that ABI, breaking about every piece of software that assumed that |
|
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177 | C<__x86_64> stands for, well, the x86-64 ABI, making these macros |
|
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178 | necessary. |
|
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179 | |
|
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180 | =back |
|
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181 | |
|
|
182 | =head2 MACRO TRICKERY |
|
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183 | |
|
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184 | =over 4 |
|
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185 | |
|
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186 | =item ECB_CONCAT (a, b) |
|
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187 | |
|
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188 | Expands any macros in C<a> and C<b>, then concatenates the result to form |
|
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189 | a single token. This is mainly useful to form identifiers from components, |
|
|
190 | e.g.: |
|
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191 | |
|
|
192 | #define S1 str |
|
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193 | #define S2 cpy |
|
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194 | |
|
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195 | ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src); |
|
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196 | |
|
|
197 | =item ECB_STRINGIFY (arg) |
|
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198 | |
|
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199 | Expands any macros in C<arg> and returns the stringified version of |
|
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200 | it. This is mainly useful to get the contents of a macro in string form, |
|
|
201 | e.g.: |
|
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202 | |
|
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203 | #define SQL_LIMIT 100 |
|
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204 | sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT)); |
|
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205 | |
|
|
206 | =item ECB_STRINGIFY_EXPR (expr) |
|
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207 | |
|
|
208 | Like C<ECB_STRINGIFY>, but additionally evaluates C<expr> to make sure it |
|
|
209 | is a valid expression. This is useful to catch typos or cases where the |
|
|
210 | macro isn't available: |
|
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211 | |
|
|
212 | #include <errno.h> |
|
|
213 | |
|
|
214 | ECB_STRINGIFY (EDOM); // "33" (on my system at least) |
|
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215 | ECB_STRINGIFY_EXPR (EDOM); // "33" |
|
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216 | |
|
|
217 | // now imagine we had a typo: |
|
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218 | |
|
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219 | ECB_STRINGIFY (EDAM); // "EDAM" |
|
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220 | ECB_STRINGIFY_EXPR (EDAM); // error: EDAM undefined |
|
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221 | |
|
|
222 | =back |
|
|
223 | |
59 | =head2 GCC ATTRIBUTES |
224 | =head2 ATTRIBUTES |
60 | |
225 | |
61 | A major part of libecb deals with GCC attributes. These are additional |
226 | A major part of libecb deals with additional attributes that can be |
62 | attributes that you can assign to functions, variables and sometimes even |
227 | assigned to functions, variables and sometimes even types - much like |
63 | types - much like C<const> or C<volatile> in C. |
228 | C<const> or C<volatile> in C. They are implemented using either GCC |
64 | |
229 | attributes or other compiler/language specific features. Attributes |
65 | While GCC allows declarations to show up in many surprising places, |
|
|
66 | but not in many expected places, the safest way is to put attribute |
|
|
67 | declarations before the whole declaration: |
230 | declarations must be put before the whole declaration: |
68 | |
231 | |
69 | ecb_const int mysqrt (int a); |
232 | ecb_const int mysqrt (int a); |
70 | ecb_unused int i; |
233 | ecb_unused int i; |
71 | |
234 | |
72 | For variables, it is often nicer to put the attribute after the name, and |
|
|
73 | avoid multiple declarations using commas: |
|
|
74 | |
|
|
75 | int i ecb_unused; |
|
|
76 | |
|
|
77 | =over 4 |
235 | =over 4 |
78 | |
|
|
79 | =item ecb_attribute ((attrs...)) |
|
|
80 | |
|
|
81 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to |
|
|
82 | nothing on other compilers, so the effect is that only GCC sees these. |
|
|
83 | |
|
|
84 | Example: use the C<deprecated> attribute on a function. |
|
|
85 | |
|
|
86 | ecb_attribute((__deprecated__)) void |
|
|
87 | do_not_use_me_anymore (void); |
|
|
88 | |
236 | |
89 | =item ecb_unused |
237 | =item ecb_unused |
90 | |
238 | |
91 | Marks a function or a variable as "unused", which simply suppresses a |
239 | Marks a function or a variable as "unused", which simply suppresses a |
92 | warning by GCC when it detects it as unused. This is useful when you e.g. |
240 | warning by GCC when it detects it as unused. This is useful when you e.g. |
93 | declare a variable but do not always use it: |
241 | declare a variable but do not always use it: |
94 | |
242 | |
95 | { |
243 | { |
96 | int var ecb_unused; |
244 | ecb_unused int var; |
97 | |
245 | |
98 | #ifdef SOMECONDITION |
246 | #ifdef SOMECONDITION |
99 | var = ...; |
247 | var = ...; |
100 | return var; |
248 | return var; |
101 | #else |
249 | #else |
102 | return 0; |
250 | return 0; |
103 | #endif |
251 | #endif |
104 | } |
252 | } |
105 | |
253 | |
|
|
254 | =item ecb_deprecated |
|
|
255 | |
|
|
256 | Similar to C<ecb_unused>, but marks a function, variable or type as |
|
|
257 | deprecated. This makes some compilers warn when the type is used. |
|
|
258 | |
|
|
259 | =item ecb_deprecated_message (message) |
|
|
260 | |
|
|
261 | Same as C<ecb_deprecated>, but if possible, the specified diagnostic is |
|
|
262 | used instead of a generic depreciation message when the object is being |
|
|
263 | used. |
|
|
264 | |
|
|
265 | =item ecb_inline |
|
|
266 | |
|
|
267 | Expands either to (a compiler-specific equivalent of) C<static inline> or |
|
|
268 | to just C<static>, if inline isn't supported. It should be used to declare |
|
|
269 | functions that should be inlined, for code size or speed reasons. |
|
|
270 | |
|
|
271 | Example: inline this function, it surely will reduce codesize. |
|
|
272 | |
|
|
273 | ecb_inline int |
|
|
274 | negmul (int a, int b) |
|
|
275 | { |
|
|
276 | return - (a * b); |
|
|
277 | } |
|
|
278 | |
106 | =item ecb_noinline |
279 | =item ecb_noinline |
107 | |
280 | |
108 | Prevent a function from being inlined - it might be optimised away, but |
281 | Prevents a function from being inlined - it might be optimised away, but |
109 | not inlined into other functions. This is useful if you know your function |
282 | not inlined into other functions. This is useful if you know your function |
110 | is rarely called and large enough for inlining not to be helpful. |
283 | is rarely called and large enough for inlining not to be helpful. |
111 | |
284 | |
112 | =item ecb_noreturn |
285 | =item ecb_noreturn |
113 | |
286 | |
… | |
… | |
123 | } |
296 | } |
124 | |
297 | |
125 | In this case, the compiler would probably be smart enough to deduce it on |
298 | In this case, the compiler would probably be smart enough to deduce it on |
126 | its own, so this is mainly useful for declarations. |
299 | its own, so this is mainly useful for declarations. |
127 | |
300 | |
|
|
301 | =item ecb_restrict |
|
|
302 | |
|
|
303 | Expands to the C<restrict> keyword or equivalent on compilers that support |
|
|
304 | them, and to nothing on others. Must be specified on a pointer type or |
|
|
305 | an array index to indicate that the memory doesn't alias with any other |
|
|
306 | restricted pointer in the same scope. |
|
|
307 | |
|
|
308 | Example: multiply a vector, and allow the compiler to parallelise the |
|
|
309 | loop, because it knows it doesn't overwrite input values. |
|
|
310 | |
|
|
311 | void |
|
|
312 | multiply (ecb_restrict float *src, |
|
|
313 | ecb_restrict float *dst, |
|
|
314 | int len, float factor) |
|
|
315 | { |
|
|
316 | int i; |
|
|
317 | |
|
|
318 | for (i = 0; i < len; ++i) |
|
|
319 | dst [i] = src [i] * factor; |
|
|
320 | } |
|
|
321 | |
128 | =item ecb_const |
322 | =item ecb_const |
129 | |
323 | |
130 | Declares that the function only depends on the values of its arguments, |
324 | Declares that the function only depends on the values of its arguments, |
131 | much like a mathematical function. It specifically does not read or write |
325 | much like a mathematical function. It specifically does not read or write |
132 | any memory any arguments might point to, global variables, or call any |
326 | any memory any arguments might point to, global variables, or call any |
… | |
… | |
184 | |
378 | |
185 | In addition to placing cold functions together (or at least away from hot |
379 | In addition to placing cold functions together (or at least away from hot |
186 | functions), this knowledge can be used in other ways, for example, the |
380 | functions), this knowledge can be used in other ways, for example, the |
187 | function will be optimised for size, as opposed to speed, and codepaths |
381 | function will be optimised for size, as opposed to speed, and codepaths |
188 | leading to calls to those functions can automatically be marked as if |
382 | leading to calls to those functions can automatically be marked as if |
189 | C<ecb_unlikely> had been used to reach them. |
383 | C<ecb_expect_false> had been used to reach them. |
190 | |
384 | |
191 | Good examples for such functions would be error reporting functions, or |
385 | Good examples for such functions would be error reporting functions, or |
192 | functions only called in exceptional or rare cases. |
386 | functions only called in exceptional or rare cases. |
193 | |
387 | |
194 | =item ecb_artificial |
388 | =item ecb_artificial |
195 | |
389 | |
196 | Declares the function as "artificial", in this case meaning that this |
390 | Declares the function as "artificial", in this case meaning that this |
197 | function is not really mean to be a function, but more like an accessor |
391 | function is not really meant to be a function, but more like an accessor |
198 | - many methods in C++ classes are mere accessor functions, and having a |
392 | - many methods in C++ classes are mere accessor functions, and having a |
199 | crash reported in such a method, or single-stepping through them, is not |
393 | crash reported in such a method, or single-stepping through them, is not |
200 | usually so helpful, especially when it's inlined to just a few instructions. |
394 | usually so helpful, especially when it's inlined to just a few instructions. |
201 | |
395 | |
202 | Marking them as artificial will instruct the debugger about just this, |
396 | Marking them as artificial will instruct the debugger about just this, |
… | |
… | |
222 | |
416 | |
223 | =head2 OPTIMISATION HINTS |
417 | =head2 OPTIMISATION HINTS |
224 | |
418 | |
225 | =over 4 |
419 | =over 4 |
226 | |
420 | |
227 | =item bool ecb_is_constant(expr) |
421 | =item bool ecb_is_constant (expr) |
228 | |
422 | |
229 | Returns true iff the expression can be deduced to be a compile-time |
423 | Returns true iff the expression can be deduced to be a compile-time |
230 | constant, and false otherwise. |
424 | constant, and false otherwise. |
231 | |
425 | |
232 | For example, when you have a C<rndm16> function that returns a 16 bit |
426 | For example, when you have a C<rndm16> function that returns a 16 bit |
… | |
… | |
250 | return is_constant (n) && !(n & (n - 1)) |
444 | return is_constant (n) && !(n & (n - 1)) |
251 | ? rndm16 () & (num - 1) |
445 | ? rndm16 () & (num - 1) |
252 | : (n * (uint32_t)rndm16 ()) >> 16; |
446 | : (n * (uint32_t)rndm16 ()) >> 16; |
253 | } |
447 | } |
254 | |
448 | |
255 | =item bool ecb_expect (expr, value) |
449 | =item ecb_expect (expr, value) |
256 | |
450 | |
257 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
451 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
258 | the C<expr> evaluates to C<value> a lot, which can be used for static |
452 | the C<expr> evaluates to C<value> a lot, which can be used for static |
259 | branch optimisations. |
453 | branch optimisations. |
260 | |
454 | |
261 | Usually, you want to use the more intuitive C<ecb_likely> and |
455 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
262 | C<ecb_unlikely> functions instead. |
456 | C<ecb_expect_false> functions instead. |
263 | |
457 | |
|
|
458 | =item bool ecb_expect_true (cond) |
|
|
459 | |
264 | =item bool ecb_likely (cond) |
460 | =item bool ecb_expect_false (cond) |
265 | |
|
|
266 | =item bool ecb_unlikely (cond) |
|
|
267 | |
461 | |
268 | These two functions expect a expression that is true or false and return |
462 | These two functions expect a expression that is true or false and return |
269 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
463 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
270 | other conditional statement, it will not change the program: |
464 | other conditional statement, it will not change the program: |
271 | |
465 | |
272 | /* these two do the same thing */ |
466 | /* these two do the same thing */ |
273 | if (some_condition) ...; |
467 | if (some_condition) ...; |
274 | if (ecb_likely (some_condition)) ...; |
468 | if (ecb_expect_true (some_condition)) ...; |
275 | |
469 | |
276 | However, by using C<ecb_likely>, you tell the compiler that the condition |
470 | However, by using C<ecb_expect_true>, you tell the compiler that the |
277 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
471 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
278 | true). |
472 | unlikely to be true). |
279 | |
473 | |
280 | For example, when you check for a null pointer and expect this to be a |
474 | For example, when you check for a null pointer and expect this to be a |
281 | rare, exceptional, case, then use C<ecb_unlikely>: |
475 | rare, exceptional, case, then use C<ecb_expect_false>: |
282 | |
476 | |
283 | void my_free (void *ptr) |
477 | void my_free (void *ptr) |
284 | { |
478 | { |
285 | if (ecb_unlikely (ptr == 0)) |
479 | if (ecb_expect_false (ptr == 0)) |
286 | return; |
480 | return; |
287 | } |
481 | } |
288 | |
482 | |
289 | Consequent use of these functions to mark away exceptional cases or to |
483 | Consequent use of these functions to mark away exceptional cases or to |
290 | tell the compiler what the hot path through a function is can increase |
484 | tell the compiler what the hot path through a function is can increase |
291 | performance considerably. |
485 | performance considerably. |
|
|
486 | |
|
|
487 | You might know these functions under the name C<likely> and C<unlikely> |
|
|
488 | - while these are common aliases, we find that the expect name is easier |
|
|
489 | to understand when quickly skimming code. If you wish, you can use |
|
|
490 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
|
|
491 | C<ecb_expect_false> - these are simply aliases. |
292 | |
492 | |
293 | A very good example is in a function that reserves more space for some |
493 | A very good example is in a function that reserves more space for some |
294 | memory block (for example, inside an implementation of a string stream) - |
494 | memory block (for example, inside an implementation of a string stream) - |
295 | each time something is added, you have to check for a buffer overrun, but |
495 | each time something is added, you have to check for a buffer overrun, but |
296 | you expect that most checks will turn out to be false: |
496 | you expect that most checks will turn out to be false: |
297 | |
497 | |
298 | /* make sure we have "size" extra room in our buffer */ |
498 | /* make sure we have "size" extra room in our buffer */ |
299 | ecb_inline void |
499 | ecb_inline void |
300 | reserve (int size) |
500 | reserve (int size) |
301 | { |
501 | { |
302 | if (ecb_unlikely (current + size > end)) |
502 | if (ecb_expect_false (current + size > end)) |
303 | real_reserve_method (size); /* presumably noinline */ |
503 | real_reserve_method (size); /* presumably noinline */ |
304 | } |
504 | } |
305 | |
505 | |
306 | =item bool ecb_assume (cond) |
506 | =item ecb_assume (cond) |
307 | |
507 | |
308 | Try to tell the compiler that some condition is true, even if it's not |
508 | Tries to tell the compiler that some condition is true, even if it's not |
309 | obvious. |
509 | obvious. This is not a function, but a statement: it cannot be used in |
|
|
510 | another expression. |
310 | |
511 | |
311 | This can be used to teach the compiler about invariants or other |
512 | This can be used to teach the compiler about invariants or other |
312 | conditions that might improve code generation, but which are impossible to |
513 | conditions that might improve code generation, but which are impossible to |
313 | deduce form the code itself. |
514 | deduce form the code itself. |
314 | |
515 | |
315 | For example, the example reservation function from the C<ecb_unlikely> |
516 | For example, the example reservation function from the C<ecb_expect_false> |
316 | description could be written thus (only C<ecb_assume> was added): |
517 | description could be written thus (only C<ecb_assume> was added): |
317 | |
518 | |
318 | ecb_inline void |
519 | ecb_inline void |
319 | reserve (int size) |
520 | reserve (int size) |
320 | { |
521 | { |
321 | if (ecb_unlikely (current + size > end)) |
522 | if (ecb_expect_false (current + size > end)) |
322 | real_reserve_method (size); /* presumably noinline */ |
523 | real_reserve_method (size); /* presumably noinline */ |
323 | |
524 | |
324 | ecb_assume (current + size <= end); |
525 | ecb_assume (current + size <= end); |
325 | } |
526 | } |
326 | |
527 | |
… | |
… | |
331 | |
532 | |
332 | Then the compiler I<might> be able to optimise out the second call |
533 | Then the compiler I<might> be able to optimise out the second call |
333 | completely, as it knows that C<< current + 1 > end >> is false and the |
534 | completely, as it knows that C<< current + 1 > end >> is false and the |
334 | call will never be executed. |
535 | call will never be executed. |
335 | |
536 | |
336 | =item bool ecb_unreachable () |
537 | =item ecb_unreachable () |
337 | |
538 | |
338 | This function does nothing itself, except tell the compiler that it will |
539 | This function does nothing itself, except tell the compiler that it will |
339 | never be executed. Apart from suppressing a warning in some cases, this |
540 | never be executed. Apart from suppressing a warning in some cases, this |
340 | function can be used to implement C<ecb_assume> or similar functions. |
541 | function can be used to implement C<ecb_assume> or similar functionality. |
341 | |
542 | |
342 | =item bool ecb_prefetch (addr, rw, locality) |
543 | =item ecb_prefetch (addr, rw, locality) |
343 | |
544 | |
344 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
545 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
345 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
546 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
346 | C<0> means that there will only be one access later, C<3> means that |
547 | C<0> means that there will only be one access later, C<3> means that |
347 | the data will likely be accessed very often, and values in between mean |
548 | the data will likely be accessed very often, and values in between mean |
348 | something... in between. The memory pointed to by the address does not |
549 | something... in between. The memory pointed to by the address does not |
349 | need to be accessible (it could be a null pointer for example), but C<rw> |
550 | need to be accessible (it could be a null pointer for example), but C<rw> |
350 | and C<locality> must be compile-time constants. |
551 | and C<locality> must be compile-time constants. |
351 | |
552 | |
|
|
553 | This is a statement, not a function: you cannot use it as part of an |
|
|
554 | expression. |
|
|
555 | |
352 | An obvious way to use this is to prefetch some data far away, in a big |
556 | An obvious way to use this is to prefetch some data far away, in a big |
353 | array you loop over. This prefetches memory some 128 array elements later, |
557 | array you loop over. This prefetches memory some 128 array elements later, |
354 | in the hope that it will be ready when the CPU arrives at that location. |
558 | in the hope that it will be ready when the CPU arrives at that location. |
355 | |
559 | |
356 | int sum = 0; |
560 | int sum = 0; |
… | |
… | |
375 | After processing the node, (part of) the next node might already be in |
579 | After processing the node, (part of) the next node might already be in |
376 | cache. |
580 | cache. |
377 | |
581 | |
378 | =back |
582 | =back |
379 | |
583 | |
380 | =head2 BIT FIDDLING / BITSTUFFS |
584 | =head2 BIT FIDDLING / BIT WIZARDRY |
381 | |
585 | |
382 | =over 4 |
586 | =over 4 |
383 | |
587 | |
384 | =item bool ecb_big_endian () |
588 | =item bool ecb_big_endian () |
385 | |
589 | |
… | |
… | |
391 | |
595 | |
392 | On systems that are neither, their return values are unspecified. |
596 | On systems that are neither, their return values are unspecified. |
393 | |
597 | |
394 | =item int ecb_ctz32 (uint32_t x) |
598 | =item int ecb_ctz32 (uint32_t x) |
395 | |
599 | |
|
|
600 | =item int ecb_ctz64 (uint64_t x) |
|
|
601 | |
|
|
602 | =item int ecb_ctz (T x) [C++] |
|
|
603 | |
396 | Returns the index of the least significant bit set in C<x> (or |
604 | Returns the index of the least significant bit set in C<x> (or |
397 | equivalently the number of bits set to 0 before the least significant bit |
605 | equivalently the number of bits set to 0 before the least significant bit |
398 | set), starting from 0. If C<x> is 0 the result is undefined. A common use |
606 | set), starting from 0. If C<x> is 0 the result is undefined. |
399 | case is to compute the integer binary logarithm, i.e., C<floor (log2 |
607 | |
|
|
608 | For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>. |
|
|
609 | |
|
|
610 | The overloaded C++ C<ecb_ctz> function supports C<uint8_t>, C<uint16_t>, |
|
|
611 | C<uint32_t> and C<uint64_t> types. |
|
|
612 | |
400 | (n))>. For example: |
613 | For example: |
401 | |
614 | |
402 | ecb_ctz32 (3) = 0 |
615 | ecb_ctz32 (3) = 0 |
403 | ecb_ctz32 (6) = 1 |
616 | ecb_ctz32 (6) = 1 |
404 | |
617 | |
|
|
618 | =item bool ecb_is_pot32 (uint32_t x) |
|
|
619 | |
|
|
620 | =item bool ecb_is_pot64 (uint32_t x) |
|
|
621 | |
|
|
622 | =item bool ecb_is_pot (T x) [C++] |
|
|
623 | |
|
|
624 | Returns true iff C<x> is a power of two or C<x == 0>. |
|
|
625 | |
|
|
626 | For smaller types than C<uint32_t> you can safely use C<ecb_is_pot32>. |
|
|
627 | |
|
|
628 | The overloaded C++ C<ecb_is_pot> function supports C<uint8_t>, C<uint16_t>, |
|
|
629 | C<uint32_t> and C<uint64_t> types. |
|
|
630 | |
|
|
631 | =item int ecb_ld32 (uint32_t x) |
|
|
632 | |
|
|
633 | =item int ecb_ld64 (uint64_t x) |
|
|
634 | |
|
|
635 | =item int ecb_ld64 (T x) [C++] |
|
|
636 | |
|
|
637 | Returns the index of the most significant bit set in C<x>, or the number |
|
|
638 | of digits the number requires in binary (so that C<< 2**ld <= x < |
|
|
639 | 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is |
|
|
640 | to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for |
|
|
641 | example to see how many bits a certain number requires to be encoded. |
|
|
642 | |
|
|
643 | This function is similar to the "count leading zero bits" function, except |
|
|
644 | that that one returns how many zero bits are "in front" of the number (in |
|
|
645 | the given data type), while C<ecb_ld> returns how many bits the number |
|
|
646 | itself requires. |
|
|
647 | |
|
|
648 | For smaller types than C<uint32_t> you can safely use C<ecb_ld32>. |
|
|
649 | |
|
|
650 | The overloaded C++ C<ecb_ld> function supports C<uint8_t>, C<uint16_t>, |
|
|
651 | C<uint32_t> and C<uint64_t> types. |
|
|
652 | |
405 | =item int ecb_popcount32 (uint32_t x) |
653 | =item int ecb_popcount32 (uint32_t x) |
406 | |
654 | |
|
|
655 | =item int ecb_popcount64 (uint64_t x) |
|
|
656 | |
|
|
657 | =item int ecb_popcount (T x) [C++] |
|
|
658 | |
407 | Returns the number of bits set to 1 in C<x>. For example: |
659 | Returns the number of bits set to 1 in C<x>. |
|
|
660 | |
|
|
661 | For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>. |
|
|
662 | |
|
|
663 | The overloaded C++ C<ecb_popcount> function supports C<uint8_t>, C<uint16_t>, |
|
|
664 | C<uint32_t> and C<uint64_t> types. |
|
|
665 | |
|
|
666 | For example: |
408 | |
667 | |
409 | ecb_popcount32 (7) = 3 |
668 | ecb_popcount32 (7) = 3 |
410 | ecb_popcount32 (255) = 8 |
669 | ecb_popcount32 (255) = 8 |
411 | |
670 | |
|
|
671 | =item uint8_t ecb_bitrev8 (uint8_t x) |
|
|
672 | |
|
|
673 | =item uint16_t ecb_bitrev16 (uint16_t x) |
|
|
674 | |
|
|
675 | =item uint32_t ecb_bitrev32 (uint32_t x) |
|
|
676 | |
|
|
677 | =item T ecb_bitrev (T x) [C++] |
|
|
678 | |
|
|
679 | Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1 |
|
|
680 | and so on. |
|
|
681 | |
|
|
682 | The overloaded C++ C<ecb_bitrev> function supports C<uint8_t>, C<uint16_t> and C<uint32_t> types. |
|
|
683 | |
|
|
684 | Example: |
|
|
685 | |
|
|
686 | ecb_bitrev8 (0xa7) = 0xea |
|
|
687 | ecb_bitrev32 (0xffcc4411) = 0x882233ff |
|
|
688 | |
|
|
689 | =item T ecb_bitrev (T x) [C++] |
|
|
690 | |
|
|
691 | Overloaded C++ bitrev function. |
|
|
692 | |
|
|
693 | C<T> must be one of C<uint8_t>, C<uint16_t> or C<uint32_t>. |
|
|
694 | |
412 | =item uint32_t ecb_bswap16 (uint32_t x) |
695 | =item uint32_t ecb_bswap16 (uint32_t x) |
413 | |
696 | |
414 | =item uint32_t ecb_bswap32 (uint32_t x) |
697 | =item uint32_t ecb_bswap32 (uint32_t x) |
415 | |
698 | |
|
|
699 | =item uint64_t ecb_bswap64 (uint64_t x) |
|
|
700 | |
|
|
701 | =item T ecb_bswap (T x) |
|
|
702 | |
416 | These two functions return the value of the 16-bit (32-bit) value C<x> |
703 | These functions return the value of the 16-bit (32-bit, 64-bit) value |
417 | after reversing the order of bytes (0x11223344 becomes 0x44332211). |
704 | C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in |
|
|
705 | C<ecb_bswap32>). |
|
|
706 | |
|
|
707 | The overloaded C++ C<ecb_bswap> function supports C<uint8_t>, C<uint16_t>, |
|
|
708 | C<uint32_t> and C<uint64_t> types. |
|
|
709 | |
|
|
710 | =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count) |
|
|
711 | |
|
|
712 | =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count) |
|
|
713 | |
|
|
714 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
|
|
715 | |
|
|
716 | =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count) |
|
|
717 | |
|
|
718 | =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count) |
|
|
719 | |
|
|
720 | =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count) |
418 | |
721 | |
419 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
722 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
420 | |
723 | |
421 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
724 | =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count) |
422 | |
725 | |
423 | These two functions return the value of C<x> after rotating all the bits |
726 | These two families of functions return the value of C<x> after rotating |
424 | by C<count> positions to the right or left respectively. |
727 | all the bits by C<count> positions to the right (C<ecb_rotr>) or left |
|
|
728 | (C<ecb_rotl>). |
425 | |
729 | |
426 | Current GCC versions understand these functions and usually compile them |
730 | Current GCC versions understand these functions and usually compile them |
427 | to "optimal" code (e.g. a single C<roll> on x86). |
731 | to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on |
|
|
732 | x86). |
|
|
733 | |
|
|
734 | =item T ecb_rotl (T x, unsigned int count) [C++] |
|
|
735 | |
|
|
736 | =item T ecb_rotr (T x, unsigned int count) [C++] |
|
|
737 | |
|
|
738 | Overloaded C++ rotl/rotr functions. |
|
|
739 | |
|
|
740 | C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>. |
|
|
741 | |
|
|
742 | =back |
|
|
743 | |
|
|
744 | =head2 HOST ENDIANNESS CONVERSION |
|
|
745 | |
|
|
746 | =over 4 |
|
|
747 | |
|
|
748 | =item uint_fast16_t ecb_be_u16_to_host (uint_fast16_t v) |
|
|
749 | |
|
|
750 | =item uint_fast32_t ecb_be_u32_to_host (uint_fast32_t v) |
|
|
751 | |
|
|
752 | =item uint_fast64_t ecb_be_u64_to_host (uint_fast64_t v) |
|
|
753 | |
|
|
754 | =item uint_fast16_t ecb_le_u16_to_host (uint_fast16_t v) |
|
|
755 | |
|
|
756 | =item uint_fast32_t ecb_le_u32_to_host (uint_fast32_t v) |
|
|
757 | |
|
|
758 | =item uint_fast64_t ecb_le_u64_to_host (uint_fast64_t v) |
|
|
759 | |
|
|
760 | Convert an unsigned 16, 32 or 64 bit value from big or little endian to host byte order. |
|
|
761 | |
|
|
762 | The naming convention is C<ecb_>(C<be>|C<le>)C<_u>C<16|32|64>C<_to_host>, |
|
|
763 | where C<be> and C<le> stand for big endian and little endian, respectively. |
|
|
764 | |
|
|
765 | =item uint_fast16_t ecb_host_to_be_u16 (uint_fast16_t v) |
|
|
766 | |
|
|
767 | =item uint_fast32_t ecb_host_to_be_u32 (uint_fast32_t v) |
|
|
768 | |
|
|
769 | =item uint_fast64_t ecb_host_to_be_u64 (uint_fast64_t v) |
|
|
770 | |
|
|
771 | =item uint_fast16_t ecb_host_to_le_u16 (uint_fast16_t v) |
|
|
772 | |
|
|
773 | =item uint_fast32_t ecb_host_to_le_u32 (uint_fast32_t v) |
|
|
774 | |
|
|
775 | =item uint_fast64_t ecb_host_to_le_u64 (uint_fast64_t v) |
|
|
776 | |
|
|
777 | Like above, but converts I<from> host byte order to the specified |
|
|
778 | endianness. |
|
|
779 | |
|
|
780 | =back |
|
|
781 | |
|
|
782 | In C++ the following additional template functions are supported: |
|
|
783 | |
|
|
784 | =over 4 |
|
|
785 | |
|
|
786 | =item T ecb_be_to_host (T v) |
|
|
787 | |
|
|
788 | =item T ecb_le_to_host (T v) |
|
|
789 | |
|
|
790 | =item T ecb_host_to_be (T v) |
|
|
791 | |
|
|
792 | =item T ecb_host_to_le (T v) |
|
|
793 | |
|
|
794 | These functions work like their C counterparts, above, but use templates, |
|
|
795 | which make them useful in generic code. |
|
|
796 | |
|
|
797 | C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t> |
|
|
798 | (so unlike their C counterparts, there is a version for C<uint8_t>, which |
|
|
799 | again can be useful in generic code). |
|
|
800 | |
|
|
801 | =head2 UNALIGNED LOAD/STORE |
|
|
802 | |
|
|
803 | These function load or store unaligned multi-byte values. |
|
|
804 | |
|
|
805 | =over 4 |
|
|
806 | |
|
|
807 | =item uint_fast16_t ecb_peek_u16_u (const void *ptr) |
|
|
808 | |
|
|
809 | =item uint_fast32_t ecb_peek_u32_u (const void *ptr) |
|
|
810 | |
|
|
811 | =item uint_fast64_t ecb_peek_u64_u (const void *ptr) |
|
|
812 | |
|
|
813 | These functions load an unaligned, unsigned 16, 32 or 64 bit value from |
|
|
814 | memory. |
|
|
815 | |
|
|
816 | =item uint_fast16_t ecb_peek_be_u16_u (const void *ptr) |
|
|
817 | |
|
|
818 | =item uint_fast32_t ecb_peek_be_u32_u (const void *ptr) |
|
|
819 | |
|
|
820 | =item uint_fast64_t ecb_peek_be_u64_u (const void *ptr) |
|
|
821 | |
|
|
822 | =item uint_fast16_t ecb_peek_le_u16_u (const void *ptr) |
|
|
823 | |
|
|
824 | =item uint_fast32_t ecb_peek_le_u32_u (const void *ptr) |
|
|
825 | |
|
|
826 | =item uint_fast64_t ecb_peek_le_u64_u (const void *ptr) |
|
|
827 | |
|
|
828 | Like above, but additionally convert from big endian (C<be>) or little |
|
|
829 | endian (C<le>) byte order to host byte order while doing so. |
|
|
830 | |
|
|
831 | =item ecb_poke_u16_u (void *ptr, uint16_t v) |
|
|
832 | |
|
|
833 | =item ecb_poke_u32_u (void *ptr, uint32_t v) |
|
|
834 | |
|
|
835 | =item ecb_poke_u64_u (void *ptr, uint64_t v) |
|
|
836 | |
|
|
837 | These functions store an unaligned, unsigned 16, 32 or 64 bit value to |
|
|
838 | memory. |
|
|
839 | |
|
|
840 | =item ecb_poke_be_u16_u (void *ptr, uint_fast16_t v) |
|
|
841 | |
|
|
842 | =item ecb_poke_be_u32_u (void *ptr, uint_fast32_t v) |
|
|
843 | |
|
|
844 | =item ecb_poke_be_u64_u (void *ptr, uint_fast64_t v) |
|
|
845 | |
|
|
846 | =item ecb_poke_le_u16_u (void *ptr, uint_fast16_t v) |
|
|
847 | |
|
|
848 | =item ecb_poke_le_u32_u (void *ptr, uint_fast32_t v) |
|
|
849 | |
|
|
850 | =item ecb_poke_le_u64_u (void *ptr, uint_fast64_t v) |
|
|
851 | |
|
|
852 | Like above, but additionally convert from host byte order to big endian |
|
|
853 | (C<be>) or little endian (C<le>) byte order while doing so. |
|
|
854 | |
|
|
855 | =back |
|
|
856 | |
|
|
857 | In C++ the following additional template functions are supported: |
|
|
858 | |
|
|
859 | =over 4 |
|
|
860 | |
|
|
861 | =item T ecb_peek<T> (const void *ptr) |
|
|
862 | |
|
|
863 | =item T ecb_peek_be<T> (const void *ptr) |
|
|
864 | |
|
|
865 | =item T ecb_peek_le<T> (const void *ptr) |
|
|
866 | |
|
|
867 | =item T ecb_peek_u<T> (const void *ptr) |
|
|
868 | |
|
|
869 | =item T ecb_peek_be_u<T> (const void *ptr) |
|
|
870 | |
|
|
871 | =item T ecb_peek_le_u<T> (const void *ptr) |
|
|
872 | |
|
|
873 | Similarly to their C counterparts, these functions load an unsigned 8, 16, |
|
|
874 | 32 or 64 bit value from memory, with optional conversion from big/little |
|
|
875 | endian. |
|
|
876 | |
|
|
877 | Since the type cannot be deduced, it has to be specified explicitly, e.g. |
|
|
878 | |
|
|
879 | uint_fast16_t v = ecb_peek<uint16_t> (ptr); |
|
|
880 | |
|
|
881 | C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>. |
|
|
882 | |
|
|
883 | Unlike their C counterparts, these functions support 8 bit quantities |
|
|
884 | (C<uint8_t>) and also have an aligned version (without the C<_u> prefix), |
|
|
885 | all of which hopefully makes them more useful in generic code. |
|
|
886 | |
|
|
887 | =item ecb_poke (void *ptr, T v) |
|
|
888 | |
|
|
889 | =item ecb_poke_be (void *ptr, T v) |
|
|
890 | |
|
|
891 | =item ecb_poke_le (void *ptr, T v) |
|
|
892 | |
|
|
893 | =item ecb_poke_u (void *ptr, T v) |
|
|
894 | |
|
|
895 | =item ecb_poke_be_u (void *ptr, T v) |
|
|
896 | |
|
|
897 | =item ecb_poke_le_u (void *ptr, T v) |
|
|
898 | |
|
|
899 | Again, similarly to their C counterparts, these functions store an |
|
|
900 | unsigned 8, 16, 32 or z64 bit value to memory, with optional conversion to |
|
|
901 | big/little endian. |
|
|
902 | |
|
|
903 | C<T> must be one of C<uint8_t>, C<uint16_t>, C<uint32_t> or C<uint64_t>. |
|
|
904 | |
|
|
905 | Unlike their C counterparts, these functions support 8 bit quantities |
|
|
906 | (C<uint8_t>) and also have an aligned version (without the C<_u> prefix), |
|
|
907 | all of which hopefully makes them more useful in generic code. |
|
|
908 | |
|
|
909 | =back |
|
|
910 | |
|
|
911 | =head2 FLOATING POINT FIDDLING |
|
|
912 | |
|
|
913 | =over 4 |
|
|
914 | |
|
|
915 | =item ECB_INFINITY [-UECB_NO_LIBM] |
|
|
916 | |
|
|
917 | Evaluates to positive infinity if supported by the platform, otherwise to |
|
|
918 | a truly huge number. |
|
|
919 | |
|
|
920 | =item ECB_NAN [-UECB_NO_LIBM] |
|
|
921 | |
|
|
922 | Evaluates to a quiet NAN if supported by the platform, otherwise to |
|
|
923 | C<ECB_INFINITY>. |
|
|
924 | |
|
|
925 | =item float ecb_ldexpf (float x, int exp) [-UECB_NO_LIBM] |
|
|
926 | |
|
|
927 | Same as C<ldexpf>, but always available. |
|
|
928 | |
|
|
929 | =item uint32_t ecb_float_to_binary16 (float x) [-UECB_NO_LIBM] |
|
|
930 | |
|
|
931 | =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM] |
|
|
932 | |
|
|
933 | =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM] |
|
|
934 | |
|
|
935 | These functions each take an argument in the native C<float> or C<double> |
|
|
936 | type and return the IEEE 754 bit representation of it (binary16/half, |
|
|
937 | binary32/single or binary64/double precision). |
|
|
938 | |
|
|
939 | The bit representation is just as IEEE 754 defines it, i.e. the sign bit |
|
|
940 | will be the most significant bit, followed by exponent and mantissa. |
|
|
941 | |
|
|
942 | This function should work even when the native floating point format isn't |
|
|
943 | IEEE compliant, of course at a speed and code size penalty, and of course |
|
|
944 | also within reasonable limits (it tries to convert NaNs, infinities and |
|
|
945 | denormals, but will likely convert negative zero to positive zero). |
|
|
946 | |
|
|
947 | On all modern platforms (where C<ECB_STDFP> is true), the compiler should |
|
|
948 | be able to optimise away this function completely. |
|
|
949 | |
|
|
950 | These functions can be helpful when serialising floats to the network - you |
|
|
951 | can serialise the return value like a normal uint16_t/uint32_t/uint64_t. |
|
|
952 | |
|
|
953 | Another use for these functions is to manipulate floating point values |
|
|
954 | directly. |
|
|
955 | |
|
|
956 | Silly example: toggle the sign bit of a float. |
|
|
957 | |
|
|
958 | /* On gcc-4.7 on amd64, */ |
|
|
959 | /* this results in a single add instruction to toggle the bit, and 4 extra */ |
|
|
960 | /* instructions to move the float value to an integer register and back. */ |
|
|
961 | |
|
|
962 | x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U) |
|
|
963 | |
|
|
964 | =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM] |
|
|
965 | |
|
|
966 | =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM] |
|
|
967 | |
|
|
968 | =item double ecb_binary64_to_double (uint64_t x) [-UECB_NO_LIBM] |
|
|
969 | |
|
|
970 | The reverse operation of the previous function - takes the bit |
|
|
971 | representation of an IEEE binary16, binary32 or binary64 number (half, |
|
|
972 | single or double precision) and converts it to the native C<float> or |
|
|
973 | C<double> format. |
|
|
974 | |
|
|
975 | This function should work even when the native floating point format isn't |
|
|
976 | IEEE compliant, of course at a speed and code size penalty, and of course |
|
|
977 | also within reasonable limits (it tries to convert normals and denormals, |
|
|
978 | and might be lucky for infinities, and with extraordinary luck, also for |
|
|
979 | negative zero). |
|
|
980 | |
|
|
981 | On all modern platforms (where C<ECB_STDFP> is true), the compiler should |
|
|
982 | be able to optimise away this function completely. |
|
|
983 | |
|
|
984 | =item uint16_t ecb_binary32_to_binary16 (uint32_t x) |
|
|
985 | |
|
|
986 | =item uint32_t ecb_binary16_to_binary32 (uint16_t x) |
|
|
987 | |
|
|
988 | Convert a IEEE binary32/single precision to binary16/half format, and vice |
|
|
989 | versa, handling all details (round-to-nearest-even, subnormals, infinity |
|
|
990 | and NaNs) correctly. |
|
|
991 | |
|
|
992 | These are functions are available under C<-DECB_NO_LIBM>, since |
|
|
993 | they do not rely on the platform floating point format. The |
|
|
994 | C<ecb_float_to_binary16> and C<ecb_binary16_to_float> functions are |
|
|
995 | usually what you want. |
428 | |
996 | |
429 | =back |
997 | =back |
430 | |
998 | |
431 | =head2 ARITHMETIC |
999 | =head2 ARITHMETIC |
432 | |
1000 | |
… | |
… | |
442 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
1010 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
443 | in the language. |
1011 | in the language. |
444 | |
1012 | |
445 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
1013 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
446 | negatable, that is, both C<m> and C<-m> must be representable in its |
1014 | negatable, that is, both C<m> and C<-m> must be representable in its |
447 | type (this typically includes the minimum signed integer value, the same |
1015 | type (this typically excludes the minimum signed integer value, the same |
448 | limitation as for C</> and C<%> in C). |
1016 | limitation as for C</> and C<%> in C). |
449 | |
1017 | |
450 | Current GCC versions compile this into an efficient branchless sequence on |
1018 | Current GCC versions compile this into an efficient branchless sequence on |
451 | many systems. |
1019 | almost all CPUs. |
452 | |
1020 | |
453 | For example, when you want to rotate forward through the members of an |
1021 | For example, when you want to rotate forward through the members of an |
454 | array for increasing C<m> (which might be negative), then you should use |
1022 | array for increasing C<m> (which might be negative), then you should use |
455 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
1023 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
456 | change direction for negative values: |
1024 | change direction for negative values: |
457 | |
1025 | |
458 | for (m = -100; m <= 100; ++m) |
1026 | for (m = -100; m <= 100; ++m) |
459 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
1027 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
460 | |
1028 | |
|
|
1029 | =item x = ecb_div_rd (val, div) |
|
|
1030 | |
|
|
1031 | =item x = ecb_div_ru (val, div) |
|
|
1032 | |
|
|
1033 | Returns C<val> divided by C<div> rounded down or up, respectively. |
|
|
1034 | C<val> and C<div> must have integer types and C<div> must be strictly |
|
|
1035 | positive. Note that these functions are implemented with macros in C |
|
|
1036 | and with function templates in C++. |
|
|
1037 | |
461 | =back |
1038 | =back |
462 | |
1039 | |
463 | =head2 UTILITY |
1040 | =head2 UTILITY |
464 | |
1041 | |
465 | =over 4 |
1042 | =over 4 |
… | |
… | |
474 | for (i = 0; i < ecb_array_length (primes); i++) |
1051 | for (i = 0; i < ecb_array_length (primes); i++) |
475 | sum += primes [i]; |
1052 | sum += primes [i]; |
476 | |
1053 | |
477 | =back |
1054 | =back |
478 | |
1055 | |
|
|
1056 | =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF |
479 | |
1057 | |
|
|
1058 | These symbols need to be defined before including F<ecb.h> the first time. |
|
|
1059 | |
|
|
1060 | =over 4 |
|
|
1061 | |
|
|
1062 | =item ECB_NO_THREADS |
|
|
1063 | |
|
|
1064 | If F<ecb.h> is never used from multiple threads, then this symbol can |
|
|
1065 | be defined, in which case memory fences (and similar constructs) are |
|
|
1066 | completely removed, leading to more efficient code and fewer dependencies. |
|
|
1067 | |
|
|
1068 | Setting this symbol to a true value implies C<ECB_NO_SMP>. |
|
|
1069 | |
|
|
1070 | =item ECB_NO_SMP |
|
|
1071 | |
|
|
1072 | The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from |
|
|
1073 | multiple threads, but never concurrently (e.g. if the system the program |
|
|
1074 | runs on has only a single CPU with a single core, no hyperthreading and so |
|
|
1075 | on), then this symbol can be defined, leading to more efficient code and |
|
|
1076 | fewer dependencies. |
|
|
1077 | |
|
|
1078 | =item ECB_NO_LIBM |
|
|
1079 | |
|
|
1080 | When defined to C<1>, do not export any functions that might introduce |
|
|
1081 | dependencies on the math library (usually called F<-lm>) - these are |
|
|
1082 | marked with [-UECB_NO_LIBM]. |
|
|
1083 | |
|
|
1084 | =back |
|
|
1085 | |
|
|
1086 | =head1 UNDOCUMENTED FUNCTIONALITY |
|
|
1087 | |
|
|
1088 | F<ecb.h> is full of undocumented functionality as well, some of which is |
|
|
1089 | intended to be internal-use only, some of which we forgot to document, and |
|
|
1090 | some of which we hide because we are not sure we will keep the interface |
|
|
1091 | stable. |
|
|
1092 | |
|
|
1093 | While you are welcome to rummage around and use whatever you find useful |
|
|
1094 | (we can't stop you), keep in mind that we will change undocumented |
|
|
1095 | functionality in incompatible ways without thinking twice, while we are |
|
|
1096 | considerably more conservative with documented things. |
|
|
1097 | |
|
|
1098 | =head1 AUTHORS |
|
|
1099 | |
|
|
1100 | C<libecb> is designed and maintained by: |
|
|
1101 | |
|
|
1102 | Emanuele Giaquinta <e.giaquinta@glauco.it> |
|
|
1103 | Marc Alexander Lehmann <schmorp@schmorp.de> |
|
|
1104 | |
|
|
1105 | |