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1 | =head1 LIBECB - e-C-Builtins |
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2 | |
1 | =head1 LIBECB |
3 | =head2 ABOUT LIBECB |
2 | |
4 | |
3 | You suck, we don't(tm) |
5 | Libecb is currently a simple header file that doesn't require any |
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6 | configuration to use or include in your project. |
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7 | |
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8 | It's part of the e-suite of libraries, other members of which include |
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9 | libev and libeio. |
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10 | |
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11 | Its homepage can be found here: |
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12 | |
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13 | http://software.schmorp.de/pkg/libecb |
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14 | |
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15 | It mainly provides a number of wrappers around GCC built-ins, together |
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16 | with replacement functions for other compilers. In addition to this, |
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17 | it provides a number of other lowlevel C utilities, such as endianness |
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18 | detection, byte swapping or bit rotations. |
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19 | |
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20 | 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. |
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23 | |
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24 | More might come. |
4 | |
25 | |
5 | =head2 ABOUT THE HEADER |
26 | =head2 ABOUT THE HEADER |
6 | |
27 | |
7 | - how to include it |
28 | At the moment, all you have to do is copy F<ecb.h> somewhere where your |
8 | - it includes inttypes.h |
29 | compiler can find it and include it: |
9 | - no .a |
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10 | - whats a bool |
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11 | |
30 | |
12 | =head2 GCC ATTRIBUTES |
31 | #include <ecb.h> |
13 | |
32 | |
14 | blabla where to put, what others |
33 | The header should work fine for both C and C++ compilation, and gives you |
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34 | all of F<inttypes.h> in addition to the ECB symbols. |
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35 | |
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36 | There are currently no object files to link to - future versions might |
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37 | come with an (optional) object code library to link against, to reduce |
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38 | code size or gain access to additional features. |
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39 | |
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40 | It also currently includes everything from F<inttypes.h>. |
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41 | |
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42 | =head2 ABOUT THIS MANUAL / CONVENTIONS |
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43 | |
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44 | This manual mainly describes each (public) function available after |
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45 | including the F<ecb.h> header. The header might define other symbols than |
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46 | these, but these are not part of the public API, and not supported in any |
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47 | way. |
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48 | |
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49 | When the manual mentions a "function" then this could be defined either as |
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50 | as inline function, a macro, or an external symbol. |
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51 | |
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52 | When functions use a concrete standard type, such as C<int> or |
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53 | C<uint32_t>, then the corresponding function works only with that type. If |
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54 | only a generic name is used (C<expr>, C<cond>, C<value> and so on), then |
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55 | the corresponding function relies on C to implement the correct types, and |
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56 | is usually implemented as a macro. Specifically, a "bool" in this manual |
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57 | refers to any kind of boolean value, not a specific type. |
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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 |
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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 | For C<ptrdiff_t> and C<size_t> use C<stddef.h>. |
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72 | |
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73 | =head2 LANGUAGE/ENVIRONMENT/COMPILER VERSIONS |
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74 | |
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75 | All the following symbols expand to an expression that can be tested in |
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76 | preprocessor instructions as well as treated as a boolean (use C<!!> to |
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77 | ensure it's either C<0> or C<1> if you need that). |
15 | |
78 | |
16 | =over 4 |
79 | =over 4 |
17 | |
80 | |
18 | =item ecb_attribute ((attrs...)) |
81 | =item ECB_C |
19 | |
82 | |
20 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and |
83 | True if the implementation defines the C<__STDC__> macro to a true value, |
21 | to nothing on other compilers, so the effect is that only GCC sees these. |
84 | while not claiming to be C++. |
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85 | |
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86 | =item ECB_C99 |
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87 | |
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88 | True if the implementation claims to be compliant to C99 (ISO/IEC |
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89 | 9899:1999) or any later version, while not claiming to be C++. |
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90 | |
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91 | Note that later versions (ECB_C11) remove core features again (for |
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92 | example, variable length arrays). |
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93 | |
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94 | =item ECB_C11 |
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95 | |
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96 | True if the implementation claims to be compliant to C11 (ISO/IEC |
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97 | 9899:2011) or any later version, while not claiming to be C++. |
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98 | |
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99 | =item ECB_CPP |
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100 | |
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101 | True if the implementation defines the C<__cplusplus__> macro to a true |
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102 | value, which is typically true for C++ compilers. |
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103 | |
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104 | =item ECB_CPP11 |
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105 | |
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106 | True if the implementation claims to be compliant to ISO/IEC 14882:2011 |
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107 | (C++11) or any later version. |
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108 | |
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109 | =item ECB_GCC_VERSION (major, minor) |
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110 | |
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111 | Expands to a true value (suitable for testing in by the preprocessor) |
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112 | if the compiler used is GNU C and the version is the given version, or |
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113 | higher. |
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114 | |
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115 | This macro tries to return false on compilers that claim to be GCC |
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116 | compatible but aren't. |
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117 | |
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118 | =item ECB_EXTERN_C |
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119 | |
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120 | Expands to C<extern "C"> in C++, and a simple C<extern> in C. |
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121 | |
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122 | This can be used to declare a single external C function: |
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123 | |
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124 | ECB_EXTERN_C int printf (const char *format, ...); |
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125 | |
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126 | =item ECB_EXTERN_C_BEG / ECB_EXTERN_C_END |
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127 | |
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128 | These two macros can be used to wrap multiple C<extern "C"> definitions - |
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129 | they expand to nothing in C. |
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130 | |
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131 | They are most useful in header files: |
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132 | |
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133 | ECB_EXTERN_C_BEG |
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134 | |
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135 | int mycfun1 (int x); |
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136 | int mycfun2 (int x); |
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137 | |
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138 | ECB_EXTERN_C_END |
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139 | |
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140 | =item ECB_STDFP |
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141 | |
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142 | If this evaluates to a true value (suitable for testing in by the |
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143 | preprocessor), then C<float> and C<double> use IEEE 754 single/binary32 |
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144 | and double/binary64 representations internally I<and> the endianness of |
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145 | both types match the endianness of C<uint32_t> and C<uint64_t>. |
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146 | |
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147 | This means you can just copy the bits of a C<float> (or C<double>) to an |
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148 | C<uint32_t> (or C<uint64_t>) and get the raw IEEE 754 bit representation |
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149 | without having to think about format or endianness. |
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150 | |
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151 | This is true for basically all modern platforms, although F<ecb.h> might |
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152 | not be able to deduce this correctly everywhere and might err on the safe |
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153 | side. |
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154 | |
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155 | =item ECB_AMD64, ECB_AMD64_X32 |
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156 | |
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157 | These two macros are defined to C<1> on the x86_64/amd64 ABI and the X32 |
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158 | ABI, respectively, and undefined elsewhere. |
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159 | |
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160 | The designers of the new X32 ABI for some inexplicable reason decided to |
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161 | make it look exactly like amd64, even though it's completely incompatible |
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162 | to that ABI, breaking about every piece of software that assumed that |
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163 | C<__x86_64> stands for, well, the x86-64 ABI, making these macros |
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164 | necessary. |
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165 | |
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166 | =back |
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167 | |
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168 | =head2 MACRO TRICKERY |
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169 | |
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170 | =over 4 |
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171 | |
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172 | =item ECB_CONCAT (a, b) |
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173 | |
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174 | Expands any macros in C<a> and C<b>, then concatenates the result to form |
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175 | a single token. This is mainly useful to form identifiers from components, |
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176 | e.g.: |
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177 | |
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178 | #define S1 str |
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179 | #define S2 cpy |
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180 | |
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181 | ECB_CONCAT (S1, S2)(dst, src); // == strcpy (dst, src); |
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182 | |
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183 | =item ECB_STRINGIFY (arg) |
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184 | |
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185 | Expands any macros in C<arg> and returns the stringified version of |
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186 | it. This is mainly useful to get the contents of a macro in string form, |
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187 | e.g.: |
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188 | |
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189 | #define SQL_LIMIT 100 |
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190 | sql_exec ("select * from table limit " ECB_STRINGIFY (SQL_LIMIT)); |
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191 | |
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192 | =back |
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193 | |
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194 | =head2 ATTRIBUTES |
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195 | |
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196 | A major part of libecb deals with additional attributes that can be |
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197 | assigned to functions, variables and sometimes even types - much like |
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198 | C<const> or C<volatile> in C. They are implemented using either GCC |
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199 | attributes or other compiler/language specific features. Attributes |
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200 | declarations must be put before the whole declaration: |
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201 | |
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202 | ecb_const int mysqrt (int a); |
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203 | ecb_unused int i; |
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204 | |
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205 | =over 4 |
22 | |
206 | |
23 | =item ecb_unused |
207 | =item ecb_unused |
24 | |
208 | |
25 | Marks a function or a variable as "unused", which simply suppresses a |
209 | Marks a function or a variable as "unused", which simply suppresses a |
26 | warning by GCC when it detects it as unused. This is useful when you e.g. |
210 | warning by GCC when it detects it as unused. This is useful when you e.g. |
27 | declare a variable but do not always use it: |
211 | declare a variable but do not always use it: |
28 | |
212 | |
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213 | { |
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214 | ecb_unused int var; |
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215 | |
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216 | #ifdef SOMECONDITION |
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217 | var = ...; |
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218 | return var; |
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219 | #else |
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220 | return 0; |
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221 | #endif |
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222 | } |
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223 | |
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224 | =item ecb_deprecated |
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225 | |
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226 | Similar to C<ecb_unused>, but marks a function, variable or type as |
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227 | deprecated. This makes some compilers warn when the type is used. |
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228 | |
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229 | =item ecb_deprecated_message (message) |
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230 | |
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231 | Same as C<ecb_deprecated>, but if possible, supply a diagnostic that is |
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232 | used instead of a generic depreciation message when the object is being |
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233 | used. |
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234 | |
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235 | =item ecb_inline |
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236 | |
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237 | Expands either to C<static inline> or to just C<static>, if inline |
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238 | isn't supported. It should be used to declare functions that should be |
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239 | inlined, for code size or speed reasons. |
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240 | |
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241 | Example: inline this function, it surely will reduce codesize. |
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242 | |
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243 | ecb_inline int |
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244 | negmul (int a, int b) |
29 | { |
245 | { |
30 | int var ecb_unused; |
246 | return - (a * b); |
31 | |
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32 | #ifdef SOMECONDITION |
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33 | var = ...; |
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34 | return var; |
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35 | #else |
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36 | return 0; |
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37 | #endif |
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38 | } |
247 | } |
39 | |
248 | |
40 | =item ecb_noinline |
249 | =item ecb_noinline |
41 | |
250 | |
42 | Prevent a function from being inlined - it might be optimsied away, but |
251 | Prevent a function from being inlined - it might be optimised away, but |
43 | not inlined into other functions. This is useful if you know your function |
252 | not inlined into other functions. This is useful if you know your function |
44 | is rarely called and large enough for inlining not to be helpful. |
253 | is rarely called and large enough for inlining not to be helpful. |
45 | |
254 | |
46 | =item ecb_noreturn |
255 | =item ecb_noreturn |
47 | |
256 | |
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257 | Marks a function as "not returning, ever". Some typical functions that |
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258 | don't return are C<exit> or C<abort> (which really works hard to not |
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259 | return), and now you can make your own: |
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260 | |
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261 | ecb_noreturn void |
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262 | my_abort (const char *errline) |
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263 | { |
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264 | puts (errline); |
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265 | abort (); |
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266 | } |
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267 | |
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268 | In this case, the compiler would probably be smart enough to deduce it on |
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269 | its own, so this is mainly useful for declarations. |
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270 | |
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271 | =item ecb_restrict |
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272 | |
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273 | Expands to the C<restrict> keyword or equivalent on compilers that support |
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274 | them, and to nothing on others. Must be specified on a pointer type or |
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275 | an array index to indicate that the memory doesn't alias with any other |
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276 | restricted pointer in the same scope. |
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277 | |
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278 | Example: multiply a vector, and allow the compiler to parallelise the |
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279 | loop, because it knows it doesn't overwrite input values. |
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280 | |
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281 | void |
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282 | multiply (ecb_restrict float *src, |
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283 | ecb_restrict float *dst, |
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284 | int len, float factor) |
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285 | { |
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286 | int i; |
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287 | |
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288 | for (i = 0; i < len; ++i) |
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289 | dst [i] = src [i] * factor; |
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290 | } |
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291 | |
48 | =item ecb_const |
292 | =item ecb_const |
49 | |
293 | |
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294 | Declares that the function only depends on the values of its arguments, |
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295 | much like a mathematical function. It specifically does not read or write |
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296 | any memory any arguments might point to, global variables, or call any |
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297 | non-const functions. It also must not have any side effects. |
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298 | |
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299 | Such a function can be optimised much more aggressively by the compiler - |
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300 | for example, multiple calls with the same arguments can be optimised into |
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301 | a single call, which wouldn't be possible if the compiler would have to |
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302 | expect any side effects. |
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303 | |
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304 | It is best suited for functions in the sense of mathematical functions, |
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305 | such as a function returning the square root of its input argument. |
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306 | |
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307 | Not suited would be a function that calculates the hash of some memory |
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308 | area you pass in, prints some messages or looks at a global variable to |
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309 | decide on rounding. |
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310 | |
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311 | See C<ecb_pure> for a slightly less restrictive class of functions. |
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312 | |
50 | =item ecb_pure |
313 | =item ecb_pure |
51 | |
314 | |
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315 | Similar to C<ecb_const>, declares a function that has no side |
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316 | effects. Unlike C<ecb_const>, the function is allowed to examine global |
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317 | variables and any other memory areas (such as the ones passed to it via |
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318 | pointers). |
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319 | |
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320 | While these functions cannot be optimised as aggressively as C<ecb_const> |
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321 | functions, they can still be optimised away in many occasions, and the |
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322 | compiler has more freedom in moving calls to them around. |
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323 | |
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324 | Typical examples for such functions would be C<strlen> or C<memcmp>. A |
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325 | function that calculates the MD5 sum of some input and updates some MD5 |
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326 | state passed as argument would I<NOT> be pure, however, as it would modify |
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327 | some memory area that is not the return value. |
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328 | |
52 | =item ecb_hot |
329 | =item ecb_hot |
53 | |
330 | |
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331 | This declares a function as "hot" with regards to the cache - the function |
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332 | is used so often, that it is very beneficial to keep it in the cache if |
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333 | possible. |
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334 | |
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335 | The compiler reacts by trying to place hot functions near to each other in |
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336 | memory. |
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337 | |
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338 | Whether a function is hot or not often depends on the whole program, |
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339 | and less on the function itself. C<ecb_cold> is likely more useful in |
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340 | practise. |
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341 | |
54 | =item ecb_cold |
342 | =item ecb_cold |
55 | |
343 | |
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344 | The opposite of C<ecb_hot> - declares a function as "cold" with regards to |
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345 | the cache, or in other words, this function is not called often, or not at |
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346 | speed-critical times, and keeping it in the cache might be a waste of said |
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347 | cache. |
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348 | |
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349 | In addition to placing cold functions together (or at least away from hot |
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350 | functions), this knowledge can be used in other ways, for example, the |
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351 | function will be optimised for size, as opposed to speed, and codepaths |
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352 | leading to calls to those functions can automatically be marked as if |
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353 | C<ecb_expect_false> had been used to reach them. |
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354 | |
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355 | Good examples for such functions would be error reporting functions, or |
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356 | functions only called in exceptional or rare cases. |
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357 | |
56 | =item ecb_artificial |
358 | =item ecb_artificial |
57 | |
359 | |
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360 | Declares the function as "artificial", in this case meaning that this |
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361 | function is not really meant to be a function, but more like an accessor |
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362 | - many methods in C++ classes are mere accessor functions, and having a |
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363 | crash reported in such a method, or single-stepping through them, is not |
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364 | usually so helpful, especially when it's inlined to just a few instructions. |
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365 | |
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366 | Marking them as artificial will instruct the debugger about just this, |
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367 | leading to happier debugging and thus happier lives. |
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368 | |
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369 | Example: in some kind of smart-pointer class, mark the pointer accessor as |
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370 | artificial, so that the whole class acts more like a pointer and less like |
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371 | some C++ abstraction monster. |
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372 | |
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373 | template<typename T> |
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374 | struct my_smart_ptr |
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375 | { |
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376 | T *value; |
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377 | |
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378 | ecb_artificial |
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379 | operator T *() |
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380 | { |
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381 | return value; |
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382 | } |
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383 | }; |
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384 | |
58 | =back |
385 | =back |
59 | |
386 | |
60 | =head2 OPTIMISATION HINTS |
387 | =head2 OPTIMISATION HINTS |
61 | |
388 | |
62 | =over 4 |
389 | =over 4 |
63 | |
390 | |
64 | =item bool ecb_is_constant(expr) |
391 | =item bool ecb_is_constant (expr) |
65 | |
392 | |
66 | Returns true iff the expression can be deduced to be a compile-time |
393 | Returns true iff the expression can be deduced to be a compile-time |
67 | constant, and false otherwise. |
394 | constant, and false otherwise. |
68 | |
395 | |
69 | For example, when you have a C<rndm16> function that returns a 16 bit |
396 | For example, when you have a C<rndm16> function that returns a 16 bit |
… | |
… | |
87 | return is_constant (n) && !(n & (n - 1)) |
414 | return is_constant (n) && !(n & (n - 1)) |
88 | ? rndm16 () & (num - 1) |
415 | ? rndm16 () & (num - 1) |
89 | : (n * (uint32_t)rndm16 ()) >> 16; |
416 | : (n * (uint32_t)rndm16 ()) >> 16; |
90 | } |
417 | } |
91 | |
418 | |
92 | =item bool ecb_expect(expr,value) |
419 | =item ecb_expect (expr, value) |
93 | |
420 | |
94 | =item bool ecb_unlikely(bool) |
421 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
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422 | the C<expr> evaluates to C<value> a lot, which can be used for static |
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423 | branch optimisations. |
95 | |
424 | |
96 | =item bool ecb_likely(bool) |
425 | Usually, you want to use the more intuitive C<ecb_expect_true> and |
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426 | C<ecb_expect_false> functions instead. |
97 | |
427 | |
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|
428 | =item bool ecb_expect_true (cond) |
|
|
429 | |
|
|
430 | =item bool ecb_expect_false (cond) |
|
|
431 | |
|
|
432 | These two functions expect a expression that is true or false and return |
|
|
433 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
|
|
434 | other conditional statement, it will not change the program: |
|
|
435 | |
|
|
436 | /* these two do the same thing */ |
|
|
437 | if (some_condition) ...; |
|
|
438 | if (ecb_expect_true (some_condition)) ...; |
|
|
439 | |
|
|
440 | However, by using C<ecb_expect_true>, you tell the compiler that the |
|
|
441 | condition is likely to be true (and for C<ecb_expect_false>, that it is |
|
|
442 | unlikely to be true). |
|
|
443 | |
|
|
444 | For example, when you check for a null pointer and expect this to be a |
|
|
445 | rare, exceptional, case, then use C<ecb_expect_false>: |
|
|
446 | |
|
|
447 | void my_free (void *ptr) |
|
|
448 | { |
|
|
449 | if (ecb_expect_false (ptr == 0)) |
|
|
450 | return; |
|
|
451 | } |
|
|
452 | |
|
|
453 | Consequent use of these functions to mark away exceptional cases or to |
|
|
454 | tell the compiler what the hot path through a function is can increase |
|
|
455 | performance considerably. |
|
|
456 | |
|
|
457 | You might know these functions under the name C<likely> and C<unlikely> |
|
|
458 | - while these are common aliases, we find that the expect name is easier |
|
|
459 | to understand when quickly skimming code. If you wish, you can use |
|
|
460 | C<ecb_likely> instead of C<ecb_expect_true> and C<ecb_unlikely> instead of |
|
|
461 | C<ecb_expect_false> - these are simply aliases. |
|
|
462 | |
|
|
463 | A very good example is in a function that reserves more space for some |
|
|
464 | memory block (for example, inside an implementation of a string stream) - |
|
|
465 | each time something is added, you have to check for a buffer overrun, but |
|
|
466 | you expect that most checks will turn out to be false: |
|
|
467 | |
|
|
468 | /* make sure we have "size" extra room in our buffer */ |
|
|
469 | ecb_inline void |
|
|
470 | reserve (int size) |
|
|
471 | { |
|
|
472 | if (ecb_expect_false (current + size > end)) |
|
|
473 | real_reserve_method (size); /* presumably noinline */ |
|
|
474 | } |
|
|
475 | |
98 | =item bool ecb_assume(cond) |
476 | =item ecb_assume (cond) |
99 | |
477 | |
|
|
478 | Try to tell the compiler that some condition is true, even if it's not |
|
|
479 | obvious. |
|
|
480 | |
|
|
481 | This can be used to teach the compiler about invariants or other |
|
|
482 | conditions that might improve code generation, but which are impossible to |
|
|
483 | deduce form the code itself. |
|
|
484 | |
|
|
485 | For example, the example reservation function from the C<ecb_expect_false> |
|
|
486 | description could be written thus (only C<ecb_assume> was added): |
|
|
487 | |
|
|
488 | ecb_inline void |
|
|
489 | reserve (int size) |
|
|
490 | { |
|
|
491 | if (ecb_expect_false (current + size > end)) |
|
|
492 | real_reserve_method (size); /* presumably noinline */ |
|
|
493 | |
|
|
494 | ecb_assume (current + size <= end); |
|
|
495 | } |
|
|
496 | |
|
|
497 | If you then call this function twice, like this: |
|
|
498 | |
|
|
499 | reserve (10); |
|
|
500 | reserve (1); |
|
|
501 | |
|
|
502 | Then the compiler I<might> be able to optimise out the second call |
|
|
503 | completely, as it knows that C<< current + 1 > end >> is false and the |
|
|
504 | call will never be executed. |
|
|
505 | |
100 | =item bool ecb_unreachable() |
506 | =item ecb_unreachable () |
101 | |
507 | |
|
|
508 | This function does nothing itself, except tell the compiler that it will |
|
|
509 | never be executed. Apart from suppressing a warning in some cases, this |
|
|
510 | function can be used to implement C<ecb_assume> or similar functions. |
|
|
511 | |
102 | =item bool ecb_prefetch(addr,rw,locality) |
512 | =item ecb_prefetch (addr, rw, locality) |
|
|
513 | |
|
|
514 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
|
|
515 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
|
|
516 | C<0> means that there will only be one access later, C<3> means that |
|
|
517 | the data will likely be accessed very often, and values in between mean |
|
|
518 | something... in between. The memory pointed to by the address does not |
|
|
519 | need to be accessible (it could be a null pointer for example), but C<rw> |
|
|
520 | and C<locality> must be compile-time constants. |
|
|
521 | |
|
|
522 | An obvious way to use this is to prefetch some data far away, in a big |
|
|
523 | array you loop over. This prefetches memory some 128 array elements later, |
|
|
524 | in the hope that it will be ready when the CPU arrives at that location. |
|
|
525 | |
|
|
526 | int sum = 0; |
|
|
527 | |
|
|
528 | for (i = 0; i < N; ++i) |
|
|
529 | { |
|
|
530 | sum += arr [i] |
|
|
531 | ecb_prefetch (arr + i + 128, 0, 0); |
|
|
532 | } |
|
|
533 | |
|
|
534 | It's hard to predict how far to prefetch, and most CPUs that can prefetch |
|
|
535 | are often good enough to predict this kind of behaviour themselves. It |
|
|
536 | gets more interesting with linked lists, especially when you do some fair |
|
|
537 | processing on each list element: |
|
|
538 | |
|
|
539 | for (node *n = start; n; n = n->next) |
|
|
540 | { |
|
|
541 | ecb_prefetch (n->next, 0, 0); |
|
|
542 | ... do medium amount of work with *n |
|
|
543 | } |
|
|
544 | |
|
|
545 | After processing the node, (part of) the next node might already be in |
|
|
546 | cache. |
103 | |
547 | |
104 | =back |
548 | =back |
105 | |
549 | |
106 | =head2 BIT FIDDLING / BITSTUFFS |
550 | =head2 BIT FIDDLING / BIT WIZARDRY |
107 | |
551 | |
108 | =over 4 |
552 | =over 4 |
109 | |
553 | |
110 | =item bool ecb_big_endian () |
554 | =item bool ecb_big_endian () |
111 | |
555 | |
112 | =item bool ecb_little_endian () |
556 | =item bool ecb_little_endian () |
113 | |
557 | |
|
|
558 | These two functions return true if the byte order is big endian |
|
|
559 | (most-significant byte first) or little endian (least-significant byte |
|
|
560 | first) respectively. |
|
|
561 | |
|
|
562 | On systems that are neither, their return values are unspecified. |
|
|
563 | |
114 | =item int ecb_ctz32 (uint32_t x) |
564 | =item int ecb_ctz32 (uint32_t x) |
115 | |
565 | |
|
|
566 | =item int ecb_ctz64 (uint64_t x) |
|
|
567 | |
|
|
568 | Returns the index of the least significant bit set in C<x> (or |
|
|
569 | equivalently the number of bits set to 0 before the least significant bit |
|
|
570 | set), starting from 0. If C<x> is 0 the result is undefined. |
|
|
571 | |
|
|
572 | For smaller types than C<uint32_t> you can safely use C<ecb_ctz32>. |
|
|
573 | |
|
|
574 | For example: |
|
|
575 | |
|
|
576 | ecb_ctz32 (3) = 0 |
|
|
577 | ecb_ctz32 (6) = 1 |
|
|
578 | |
|
|
579 | =item bool ecb_is_pot32 (uint32_t x) |
|
|
580 | |
|
|
581 | =item bool ecb_is_pot64 (uint32_t x) |
|
|
582 | |
|
|
583 | Return true iff C<x> is a power of two or C<x == 0>. |
|
|
584 | |
|
|
585 | For smaller types then C<uint32_t> you can safely use C<ecb_is_pot32>. |
|
|
586 | |
|
|
587 | =item int ecb_ld32 (uint32_t x) |
|
|
588 | |
|
|
589 | =item int ecb_ld64 (uint64_t x) |
|
|
590 | |
|
|
591 | Returns the index of the most significant bit set in C<x>, or the number |
|
|
592 | of digits the number requires in binary (so that C<< 2**ld <= x < |
|
|
593 | 2**(ld+1) >>). If C<x> is 0 the result is undefined. A common use case is |
|
|
594 | to compute the integer binary logarithm, i.e. C<floor (log2 (n))>, for |
|
|
595 | example to see how many bits a certain number requires to be encoded. |
|
|
596 | |
|
|
597 | This function is similar to the "count leading zero bits" function, except |
|
|
598 | that that one returns how many zero bits are "in front" of the number (in |
|
|
599 | the given data type), while C<ecb_ld> returns how many bits the number |
|
|
600 | itself requires. |
|
|
601 | |
|
|
602 | For smaller types than C<uint32_t> you can safely use C<ecb_ld32>. |
|
|
603 | |
116 | =item int ecb_popcount32 (uint32_t x) |
604 | =item int ecb_popcount32 (uint32_t x) |
117 | |
605 | |
|
|
606 | =item int ecb_popcount64 (uint64_t x) |
|
|
607 | |
|
|
608 | Returns the number of bits set to 1 in C<x>. |
|
|
609 | |
|
|
610 | For smaller types than C<uint32_t> you can safely use C<ecb_popcount32>. |
|
|
611 | |
|
|
612 | For example: |
|
|
613 | |
|
|
614 | ecb_popcount32 (7) = 3 |
|
|
615 | ecb_popcount32 (255) = 8 |
|
|
616 | |
|
|
617 | =item uint8_t ecb_bitrev8 (uint8_t x) |
|
|
618 | |
|
|
619 | =item uint16_t ecb_bitrev16 (uint16_t x) |
|
|
620 | |
|
|
621 | =item uint32_t ecb_bitrev32 (uint32_t x) |
|
|
622 | |
|
|
623 | Reverses the bits in x, i.e. the MSB becomes the LSB, MSB-1 becomes LSB+1 |
|
|
624 | and so on. |
|
|
625 | |
|
|
626 | Example: |
|
|
627 | |
|
|
628 | ecb_bitrev8 (0xa7) = 0xea |
|
|
629 | ecb_bitrev32 (0xffcc4411) = 0x882233ff |
|
|
630 | |
|
|
631 | =item uint32_t ecb_bswap16 (uint32_t x) |
|
|
632 | |
118 | =item uint32_t ecb_bswap32 (uint32_t x) |
633 | =item uint32_t ecb_bswap32 (uint32_t x) |
119 | |
634 | |
120 | =item uint32_t ecb_bswap16 (uint32_t x) |
635 | =item uint64_t ecb_bswap64 (uint64_t x) |
|
|
636 | |
|
|
637 | These functions return the value of the 16-bit (32-bit, 64-bit) value |
|
|
638 | C<x> after reversing the order of bytes (0x11223344 becomes 0x44332211 in |
|
|
639 | C<ecb_bswap32>). |
|
|
640 | |
|
|
641 | =item uint8_t ecb_rotl8 (uint8_t x, unsigned int count) |
|
|
642 | |
|
|
643 | =item uint16_t ecb_rotl16 (uint16_t x, unsigned int count) |
|
|
644 | |
|
|
645 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
|
|
646 | |
|
|
647 | =item uint64_t ecb_rotl64 (uint64_t x, unsigned int count) |
|
|
648 | |
|
|
649 | =item uint8_t ecb_rotr8 (uint8_t x, unsigned int count) |
|
|
650 | |
|
|
651 | =item uint16_t ecb_rotr16 (uint16_t x, unsigned int count) |
121 | |
652 | |
122 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
653 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
123 | |
654 | |
124 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
655 | =item uint64_t ecb_rotr64 (uint64_t x, unsigned int count) |
|
|
656 | |
|
|
657 | These two families of functions return the value of C<x> after rotating |
|
|
658 | all the bits by C<count> positions to the right (C<ecb_rotr>) or left |
|
|
659 | (C<ecb_rotl>). |
|
|
660 | |
|
|
661 | Current GCC versions understand these functions and usually compile them |
|
|
662 | to "optimal" code (e.g. a single C<rol> or a combination of C<shld> on |
|
|
663 | x86). |
125 | |
664 | |
126 | =back |
665 | =back |
127 | |
666 | |
|
|
667 | =head2 FLOATING POINT FIDDLING |
|
|
668 | |
|
|
669 | =over 4 |
|
|
670 | |
|
|
671 | =item ECB_INFINITY |
|
|
672 | |
|
|
673 | Evaluates to positive infinity if supported by the platform, otherwise to |
|
|
674 | a truly huge number. |
|
|
675 | |
|
|
676 | =item ECB_NON |
|
|
677 | |
|
|
678 | Evaluates to a quiet NAN if supported by the platform, otherwise to |
|
|
679 | C<ECB_INFINITY>. |
|
|
680 | |
|
|
681 | =item float ecb_ldexpf (float x, int exp) |
|
|
682 | |
|
|
683 | Same as C<ldexpf>, but always available. |
|
|
684 | |
|
|
685 | =item uint32_t ecb_float_to_binary32 (float x) [-UECB_NO_LIBM] |
|
|
686 | |
|
|
687 | =item uint64_t ecb_double_to_binary64 (double x) [-UECB_NO_LIBM] |
|
|
688 | |
|
|
689 | These functions each take an argument in the native C<float> or C<double> |
|
|
690 | type and return the IEEE 754 bit representation of it. |
|
|
691 | |
|
|
692 | The bit representation is just as IEEE 754 defines it, i.e. the sign bit |
|
|
693 | will be the most significant bit, followed by exponent and mantissa. |
|
|
694 | |
|
|
695 | This function should work even when the native floating point format isn't |
|
|
696 | IEEE compliant, of course at a speed and code size penalty, and of course |
|
|
697 | also within reasonable limits (it tries to convert NaNs, infinities and |
|
|
698 | denormals, but will likely convert negative zero to positive zero). |
|
|
699 | |
|
|
700 | On all modern platforms (where C<ECB_STDFP> is true), the compiler should |
|
|
701 | be able to optimise away this function completely. |
|
|
702 | |
|
|
703 | These functions can be helpful when serialising floats to the network - you |
|
|
704 | can serialise the return value like a normal uint32_t/uint64_t. |
|
|
705 | |
|
|
706 | Another use for these functions is to manipulate floating point values |
|
|
707 | directly. |
|
|
708 | |
|
|
709 | Silly example: toggle the sign bit of a float. |
|
|
710 | |
|
|
711 | /* On gcc-4.7 on amd64, */ |
|
|
712 | /* this results in a single add instruction to toggle the bit, and 4 extra */ |
|
|
713 | /* instructions to move the float value to an integer register and back. */ |
|
|
714 | |
|
|
715 | x = ecb_binary32_to_float (ecb_float_to_binary32 (x) ^ 0x80000000U) |
|
|
716 | |
|
|
717 | =item float ecb_binary16_to_float (uint16_t x) [-UECB_NO_LIBM] |
|
|
718 | |
|
|
719 | =item float ecb_binary32_to_float (uint32_t x) [-UECB_NO_LIBM] |
|
|
720 | |
|
|
721 | =item double ecb_binary32_to_double (uint64_t x) [-UECB_NO_LIBM] |
|
|
722 | |
|
|
723 | The reverse operation of the previous function - takes the bit |
|
|
724 | representation of an IEEE binary16, binary32 or binary64 number and |
|
|
725 | converts it to the native C<float> or C<double> format. |
|
|
726 | |
|
|
727 | This function should work even when the native floating point format isn't |
|
|
728 | IEEE compliant, of course at a speed and code size penalty, and of course |
|
|
729 | also within reasonable limits (it tries to convert normals and denormals, |
|
|
730 | and might be lucky for infinities, and with extraordinary luck, also for |
|
|
731 | negative zero). |
|
|
732 | |
|
|
733 | On all modern platforms (where C<ECB_STDFP> is true), the compiler should |
|
|
734 | be able to optimise away this function completely. |
|
|
735 | |
|
|
736 | =back |
|
|
737 | |
128 | =head2 ARITHMETIC |
738 | =head2 ARITHMETIC |
129 | |
739 | |
130 | =over 4 |
740 | =over 4 |
131 | |
741 | |
132 | =item x = ecb_mod (m, n) [MACRO] |
742 | =item x = ecb_mod (m, n) |
|
|
743 | |
|
|
744 | Returns C<m> modulo C<n>, which is the same as the positive remainder |
|
|
745 | of the division operation between C<m> and C<n>, using floored |
|
|
746 | division. Unlike the C remainder operator C<%>, this function ensures that |
|
|
747 | the return value is always positive and that the two numbers I<m> and |
|
|
748 | I<m' = m + i * n> result in the same value modulo I<n> - in other words, |
|
|
749 | C<ecb_mod> implements the mathematical modulo operation, which is missing |
|
|
750 | in the language. |
|
|
751 | |
|
|
752 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
|
|
753 | negatable, that is, both C<m> and C<-m> must be representable in its |
|
|
754 | type (this typically excludes the minimum signed integer value, the same |
|
|
755 | limitation as for C</> and C<%> in C). |
|
|
756 | |
|
|
757 | Current GCC versions compile this into an efficient branchless sequence on |
|
|
758 | almost all CPUs. |
|
|
759 | |
|
|
760 | For example, when you want to rotate forward through the members of an |
|
|
761 | array for increasing C<m> (which might be negative), then you should use |
|
|
762 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
|
|
763 | change direction for negative values: |
|
|
764 | |
|
|
765 | for (m = -100; m <= 100; ++m) |
|
|
766 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
|
|
767 | |
|
|
768 | =item x = ecb_div_rd (val, div) |
|
|
769 | |
|
|
770 | =item x = ecb_div_ru (val, div) |
|
|
771 | |
|
|
772 | Returns C<val> divided by C<div> rounded down or up, respectively. |
|
|
773 | C<val> and C<div> must have integer types and C<div> must be strictly |
|
|
774 | positive. Note that these functions are implemented with macros in C |
|
|
775 | and with function templates in C++. |
133 | |
776 | |
134 | =back |
777 | =back |
135 | |
778 | |
136 | =head2 UTILITY |
779 | =head2 UTILITY |
137 | |
780 | |
138 | =over 4 |
781 | =over 4 |
139 | |
782 | |
140 | =item ecb_array_length (name) [MACRO] |
783 | =item element_count = ecb_array_length (name) |
|
|
784 | |
|
|
785 | Returns the number of elements in the array C<name>. For example: |
|
|
786 | |
|
|
787 | int primes[] = { 2, 3, 5, 7, 11 }; |
|
|
788 | int sum = 0; |
|
|
789 | |
|
|
790 | for (i = 0; i < ecb_array_length (primes); i++) |
|
|
791 | sum += primes [i]; |
141 | |
792 | |
142 | =back |
793 | =back |
143 | |
794 | |
|
|
795 | =head2 SYMBOLS GOVERNING COMPILATION OF ECB.H ITSELF |
144 | |
796 | |
|
|
797 | These symbols need to be defined before including F<ecb.h> the first time. |
|
|
798 | |
|
|
799 | =over 4 |
|
|
800 | |
|
|
801 | =item ECB_NO_THREADS |
|
|
802 | |
|
|
803 | If F<ecb.h> is never used from multiple threads, then this symbol can |
|
|
804 | be defined, in which case memory fences (and similar constructs) are |
|
|
805 | completely removed, leading to more efficient code and fewer dependencies. |
|
|
806 | |
|
|
807 | Setting this symbol to a true value implies C<ECB_NO_SMP>. |
|
|
808 | |
|
|
809 | =item ECB_NO_SMP |
|
|
810 | |
|
|
811 | The weaker version of C<ECB_NO_THREADS> - if F<ecb.h> is used from |
|
|
812 | multiple threads, but never concurrently (e.g. if the system the program |
|
|
813 | runs on has only a single CPU with a single core, no hyperthreading and so |
|
|
814 | on), then this symbol can be defined, leading to more efficient code and |
|
|
815 | fewer dependencies. |
|
|
816 | |
|
|
817 | =item ECB_NO_LIBM |
|
|
818 | |
|
|
819 | When defined to C<1>, do not export any functions that might introduce |
|
|
820 | dependencies on the math library (usually called F<-lm>) - these are |
|
|
821 | marked with [-UECB_NO_LIBM]. |
|
|
822 | |
|
|
823 | =back |
|
|
824 | |
|
|
825 | |