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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 |
30 | |
10 | - whats a bool |
31 | #include <ecb.h> |
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32 | |
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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. |
11 | |
58 | |
12 | =head2 GCC ATTRIBUTES |
59 | =head2 GCC ATTRIBUTES |
13 | |
60 | |
14 | blabla where to put, what others |
61 | A major part of libecb deals with GCC attributes. These are additional |
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62 | attributes that you cna assign to functions, variables and sometimes even |
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63 | types - much like C<const> or C<volatile> in C. |
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64 | |
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65 | While GCC allows declarations to show up in many surprising places, |
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66 | but not in many expeted places, the safest way is to put attribute |
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67 | declarations before the whole declaration: |
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68 | |
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69 | ecb_const int mysqrt (int a); |
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70 | ecb_unused int i; |
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71 | |
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72 | For variables, it is often nicer to put the attribute after the name, and |
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73 | avoid multiple declarations using commas: |
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74 | |
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75 | int i ecb_unused; |
15 | |
76 | |
16 | =over 4 |
77 | =over 4 |
17 | |
78 | |
18 | =item ecb_attribute ((attrs...)) |
79 | =item ecb_attribute ((attrs...)) |
19 | |
80 | |
20 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and |
81 | A simple wrapper that expands to C<__attribute__((attrs))> on GCC, and to |
21 | to nothing on other compilers, so the effect is that only GCC sees these. |
82 | nothing on other compilers, so the effect is that only GCC sees these. |
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83 | |
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84 | Example: use the C<deprecated> attribute on a function. |
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85 | |
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86 | ecb_attribute((__deprecated__)) void |
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87 | do_not_use_me_anymore (void); |
22 | |
88 | |
23 | =item ecb_unused |
89 | =item ecb_unused |
24 | |
90 | |
25 | Marks a function or a variable as "unused", which simply suppresses a |
91 | 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. |
92 | 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: |
93 | declare a variable but do not always use it: |
28 | |
94 | |
29 | { |
95 | { |
30 | int var ecb_unused; |
96 | int var ecb_unused; |
31 | |
97 | |
32 | #ifdef SOMECONDITION |
98 | #ifdef SOMECONDITION |
33 | var = ...; |
99 | var = ...; |
34 | return var; |
100 | return var; |
35 | #else |
101 | #else |
36 | return 0; |
102 | return 0; |
37 | #endif |
103 | #endif |
38 | } |
104 | } |
39 | |
105 | |
40 | =item ecb_noinline |
106 | =item ecb_noinline |
41 | |
107 | |
42 | Prevent a function from being inlined - it might be optimsied away, but |
108 | 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 |
109 | 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. |
110 | is rarely called and large enough for inlining not to be helpful. |
45 | |
111 | |
46 | =item ecb_noreturn |
112 | =item ecb_noreturn |
47 | |
113 | |
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114 | Marks a function as "not returning, ever". Some typical functions that |
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115 | don't return are C<exit> or C<abort> (which really works hard to not |
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116 | return), and now you can make your own: |
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117 | |
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118 | ecb_noreturn void |
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119 | my_abort (const char *errline) |
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120 | { |
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121 | puts (errline); |
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122 | abort (); |
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123 | } |
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124 | |
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125 | In this case, the compiler would probably be smart enough to deduce it on |
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126 | its own, so this is mainly useful for declarations. |
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127 | |
48 | =item ecb_const |
128 | =item ecb_const |
49 | |
129 | |
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130 | Declares that the function only depends on the values of its arguments, |
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131 | much like a mathematical function. It specifically does not read or write |
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132 | any memory any arguments might point to, global variables, or call any |
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133 | non-const functions. It also must not have any side effects. |
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134 | |
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135 | Such a function can be optimised much more aggressively by the compiler - |
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136 | for example, multiple calls with the same arguments can be optimised into |
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137 | a single call, which wouldn't be possible if the compiler would have to |
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138 | expect any side effects. |
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139 | |
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140 | It is best suited for functions in the sense of mathematical functions, |
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141 | such as a function returning the square root of its input argument. |
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142 | |
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143 | Not suited would be a function that calculates the hash of some memory |
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144 | area you pass in, prints some messages or looks at a global variable to |
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145 | decide on rounding. |
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146 | |
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147 | See C<ecb_pure> for a slightly less restrictive class of functions. |
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148 | |
50 | =item ecb_pure |
149 | =item ecb_pure |
51 | |
150 | |
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151 | Similar to C<ecb_const>, declares a function that has no side |
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152 | effects. Unlike C<ecb_const>, the function is allowed to examine global |
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153 | variables and any other memory areas (such as the ones passed to it via |
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154 | pointers). |
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155 | |
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156 | While these functions cannot be optimised as aggressively as C<ecb_const> |
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157 | functions, they can still be optimised away in many occasions, and the |
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158 | compiler has more freedom in moving calls to them around. |
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159 | |
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160 | Typical examples for such functions would be C<strlen> or C<memcmp>. A |
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161 | function that calculates the MD5 sum of some input and updates some MD5 |
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162 | state passed as argument would I<NOT> be pure, however, as it would modify |
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163 | some memory area that is not the return value. |
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164 | |
52 | =item ecb_hot |
165 | =item ecb_hot |
53 | |
166 | |
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167 | This declares a function as "hot" with regards to the cache - the function |
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168 | is used so often, that it is very beneficial to keep it in the cache if |
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169 | possible. |
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170 | |
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171 | The compiler reacts by trying to place hot functions near to each other in |
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172 | memory. |
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173 | |
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174 | Whether a function is hot or not often depends on the whole program, |
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175 | and less on the function itself. C<ecb_cold> is likely more useful in |
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176 | practise. |
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177 | |
54 | =item ecb_cold |
178 | =item ecb_cold |
55 | |
179 | |
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180 | The opposite of C<ecb_hot> - declares a function as "cold" with regards to |
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181 | the cache, or in other words, this function is not called often, or not at |
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182 | speed-critical times, and keeping it in the cache might be a waste of said |
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183 | cache. |
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184 | |
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185 | In addition to placing cold functions together (or at least away from hot |
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186 | functions), this knowledge can be used in other ways, for example, the |
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187 | function will be optimised for size, as opposed to speed, and codepaths |
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188 | leading to calls to those functions can automatically be marked as if |
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189 | C<ecb_unlikely> had been used to reach them. |
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190 | |
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191 | Good examples for such functions would be error reporting functions, or |
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192 | functions only called in exceptional or rare cases. |
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193 | |
56 | =item ecb_artificial |
194 | =item ecb_artificial |
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195 | |
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196 | Declares the function as "artificial", in this case meaning that this |
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197 | function is not really mean to be a function, but more like an accessor |
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198 | - many methods in C++ classes are mere accessor functions, and having a |
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199 | crash reported in such a method, or single-stepping through them, is not |
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200 | usually so helpful, especially when it's inlined to just a few instructions. |
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201 | |
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202 | Marking them as artificial will instruct the debugger about just this, |
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203 | leading to happier debugging and thus happier lives. |
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204 | |
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205 | Example: in some kind of smart-pointer class, mark the pointer accessor as |
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206 | artificial, so that the whole class acts more like a pointer and less like |
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207 | some C++ abstraction monster. |
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208 | |
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209 | template<typename T> |
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210 | struct my_smart_ptr |
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211 | { |
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212 | T *value; |
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213 | |
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214 | ecb_artificial |
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215 | operator T *() |
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216 | { |
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217 | return value; |
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218 | } |
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219 | }; |
57 | |
220 | |
58 | =back |
221 | =back |
59 | |
222 | |
60 | =head2 OPTIMISATION HINTS |
223 | =head2 OPTIMISATION HINTS |
61 | |
224 | |
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66 | Returns true iff the expression can be deduced to be a compile-time |
229 | Returns true iff the expression can be deduced to be a compile-time |
67 | constant, and false otherwise. |
230 | constant, and false otherwise. |
68 | |
231 | |
69 | For example, when you have a C<rndm16> function that returns a 16 bit |
232 | For example, when you have a C<rndm16> function that returns a 16 bit |
70 | random number, and you have a function that maps this to a range from |
233 | random number, and you have a function that maps this to a range from |
71 | 0..n-1, then you could use this inline fucntion in a header file: |
234 | 0..n-1, then you could use this inline function in a header file: |
72 | |
235 | |
73 | ecb_inline uint32_t |
236 | ecb_inline uint32_t |
74 | rndm (uint32_t n) |
237 | rndm (uint32_t n) |
75 | { |
238 | { |
76 | return n * (uint32_t)rndm16 ()) >> 16; |
239 | return (n * (uint32_t)rndm16 ()) >> 16; |
77 | } |
240 | } |
78 | |
241 | |
79 | However, for powers of two, you could use a normal mask, but that is only |
242 | However, for powers of two, you could use a normal mask, but that is only |
80 | worth it if, at compile time, you can detect this case. This is the case |
243 | worth it if, at compile time, you can detect this case. This is the case |
81 | when the passed number is a constant and also a power of two (C<n & (n - |
244 | when the passed number is a constant and also a power of two (C<n & (n - |
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84 | ecb_inline uint32_t |
247 | ecb_inline uint32_t |
85 | rndm (uint32_t n) |
248 | rndm (uint32_t n) |
86 | { |
249 | { |
87 | return is_constant (n) && !(n & (n - 1)) |
250 | return is_constant (n) && !(n & (n - 1)) |
88 | ? rndm16 () & (num - 1) |
251 | ? rndm16 () & (num - 1) |
89 | : (uint32_t)rndm16 ()) >> 16; |
252 | : (n * (uint32_t)rndm16 ()) >> 16; |
90 | } |
253 | } |
91 | |
254 | |
92 | =item bool ecb_expect(expr,value) |
255 | =item bool ecb_expect (expr, value) |
93 | |
256 | |
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257 | Evaluates C<expr> and returns it. In addition, it tells the compiler that |
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258 | the C<expr> evaluates to C<value> a lot, which can be used for static |
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259 | branch optimisations. |
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260 | |
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261 | Usually, you want to use the more intuitive C<ecb_likely> and |
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262 | C<ecb_unlikely> functions instead. |
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263 | |
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264 | =item bool ecb_likely (cond) |
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265 | |
94 | =item bool ecb_unlikely(bool) |
266 | =item bool ecb_unlikely (cond) |
95 | |
267 | |
96 | =item bool ecb_likely(bool) |
268 | These two functions expect a expression that is true or false and return |
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269 | C<1> or C<0>, respectively, so when used in the condition of an C<if> or |
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270 | other conditional statement, it will not change the program: |
97 | |
271 | |
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272 | /* these two do the same thing */ |
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273 | if (some_condition) ...; |
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274 | if (ecb_likely (some_condition)) ...; |
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275 | |
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276 | However, by using C<ecb_likely>, you tell the compiler that the condition |
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277 | is likely to be true (and for C<ecb_unlikely>, that it is unlikely to be |
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278 | true). |
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279 | |
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280 | For example, when you check for a null pointer and expect this to be a |
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281 | rare, exceptional, case, then use C<ecb_unlikely>: |
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282 | |
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283 | void my_free (void *ptr) |
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284 | { |
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285 | if (ecb_unlikely (ptr == 0)) |
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286 | return; |
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287 | } |
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288 | |
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289 | Consequent use of these functions to mark away exceptional cases or to |
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290 | tell the compiler what the hot path through a function is can increase |
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291 | performance considerably. |
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292 | |
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293 | A very good example is in a function that reserves more space for some |
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294 | memory block (for example, inside an implementation of a string stream) - |
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295 | each time something is added, you have to check for a buffer overrun, but |
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296 | you expect that most checks will turn out to be false: |
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297 | |
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298 | /* make sure we have "size" extra room in our buffer */ |
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299 | ecb_inline void |
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300 | reserve (int size) |
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301 | { |
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302 | if (ecb_unlikely (current + size > end)) |
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303 | real_reserve_method (size); /* presumably noinline */ |
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304 | } |
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305 | |
98 | =item bool ecb_assume(cond) |
306 | =item bool ecb_assume (cond) |
99 | |
307 | |
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308 | Try to tell the compiler that some condition is true, even if it's not |
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309 | obvious. |
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310 | |
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311 | This can be used to teach the compiler about invariants or other |
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312 | conditions that might improve code generation, but which are impossible to |
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313 | deduce form the code itself. |
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314 | |
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315 | For example, the example reservation function from the C<ecb_unlikely> |
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316 | description could be written thus (only C<ecb_assume> was added): |
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317 | |
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318 | ecb_inline void |
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319 | reserve (int size) |
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320 | { |
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321 | if (ecb_unlikely (current + size > end)) |
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322 | real_reserve_method (size); /* presumably noinline */ |
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323 | |
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324 | ecb_assume (current + size <= end); |
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325 | } |
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326 | |
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327 | If you then call this function twice, like this: |
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328 | |
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329 | reserve (10); |
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330 | reserve (1); |
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331 | |
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332 | Then the compiler I<might> be able to optimise out the second call |
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333 | completely, as it knows that C<< current + 1 > end >> is false and the |
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334 | call will never be executed. |
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335 | |
100 | =item bool ecb_unreachable() |
336 | =item bool ecb_unreachable () |
101 | |
337 | |
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338 | This function does nothing itself, except tell the compiler that it will |
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339 | never be executed. Apart from suppressing a warning in some cases, this |
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340 | function can be used to implement C<ecb_assume> or similar functions. |
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341 | |
102 | =item bool ecb_prefetch(addr,rw,locality) |
342 | =item bool ecb_prefetch (addr, rw, locality) |
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343 | |
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344 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
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345 | for either reading (C<rw> = 0) or writing (C<rw> = 1). A C<locality> of |
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346 | C<0> means that there will only be one access later, C<3> means that |
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347 | the data will likely be accessed very often, and values in between mean |
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348 | something... in between. The memory pointed to by the address does not |
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349 | need to be accessible (it could be a null pointer for example), but C<rw> |
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350 | and C<locality> must be compile-time constants. |
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351 | |
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352 | An obvious way to use this is to prefetch some data far away, in a big |
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353 | array you loop over. This prefetches memory some 128 array elements later, |
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354 | in the hope that it will be ready when the CPU arrives at that location. |
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355 | |
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356 | int sum = 0; |
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357 | |
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358 | for (i = 0; i < N; ++i) |
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359 | { |
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360 | sum += arr [i] |
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361 | ecb_prefetch (arr + i + 128, 0, 0); |
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362 | } |
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363 | |
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364 | It's hard to predict how far to prefetch, and most CPUs that can prefetch |
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365 | are often good enough to predict this kind of behaviour themselves. It |
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366 | gets more interesting with linked lists, especially when you do some fair |
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367 | processing on each list element: |
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368 | |
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369 | for (node *n = start; n; n = n->next) |
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370 | { |
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371 | ecb_prefetch (n->next, 0, 0); |
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372 | ... do medium amount of work with *n |
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373 | } |
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374 | |
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375 | After processing the node, (part of) the next node might already be in |
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376 | cache. |
103 | |
377 | |
104 | =back |
378 | =back |
105 | |
379 | |
106 | =head2 BIT FIDDLING / BITSTUFFS |
380 | =head2 BIT FIDDLING / BITSTUFFS |
107 | |
381 | |
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109 | |
383 | |
110 | =item bool ecb_big_endian () |
384 | =item bool ecb_big_endian () |
111 | |
385 | |
112 | =item bool ecb_little_endian () |
386 | =item bool ecb_little_endian () |
113 | |
387 | |
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388 | These two functions return true if the byte order is big endian |
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389 | (most-significant byte first) or little endian (least-significant byte |
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390 | first) respectively. |
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391 | |
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392 | On systems that are neither, their return values are unspecified. |
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393 | |
114 | =item int ecb_ctz32 (uint32_t x) |
394 | =item int ecb_ctz32 (uint32_t x) |
115 | |
395 | |
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396 | Returns the index of the least significant bit set in C<x> (or |
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397 | equivalently the number of bits set to 0 before the least significant bit |
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398 | set), starting from 0. If C<x> is 0 the result is undefined. A common use |
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399 | case is to compute the integer binary logarithm, i.e., C<floor (log2 |
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400 | (n))>. For example: |
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401 | |
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402 | ecb_ctz32 (3) = 0 |
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403 | ecb_ctz32 (6) = 1 |
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404 | |
116 | =item int ecb_popcount32 (uint32_t x) |
405 | =item int ecb_popcount32 (uint32_t x) |
117 | |
406 | |
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407 | Returns the number of bits set to 1 in C<x>. For example: |
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408 | |
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409 | ecb_popcount32 (7) = 3 |
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410 | ecb_popcount32 (255) = 8 |
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411 | |
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412 | =item uint32_t ecb_bswap16 (uint32_t x) |
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413 | |
118 | =item uint32_t ecb_bswap32 (uint32_t x) |
414 | =item uint32_t ecb_bswap32 (uint32_t x) |
119 | |
415 | |
120 | =item uint32_t ecb_bswap16 (uint32_t x) |
416 | These two functions return the value of the 16-bit (32-bit) value C<x> |
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417 | after reversing the order of bytes (0x11223344 becomes 0x44332211). |
121 | |
418 | |
122 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
419 | =item uint32_t ecb_rotr32 (uint32_t x, unsigned int count) |
123 | |
420 | |
124 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
421 | =item uint32_t ecb_rotl32 (uint32_t x, unsigned int count) |
125 | |
422 | |
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423 | These two functions return the value of C<x> after rotating all the bits |
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424 | by C<count> positions to the right or left respectively. |
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425 | |
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426 | Current GCC versions understand these functions and usually compile them |
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427 | to "optimal" code (e.g. a single C<roll> on x86). |
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428 | |
126 | =back |
429 | =back |
127 | |
430 | |
128 | =head2 ARITHMETIC |
431 | =head2 ARITHMETIC |
129 | |
432 | |
130 | =over 4 |
433 | =over 4 |
131 | |
434 | |
132 | =item x = ecb_mod (m, n) [MACRO] |
435 | =item x = ecb_mod (m, n) |
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436 | |
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437 | Returns the positive remainder of the modulo operation between C<m> and |
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438 | C<n>, using floored division. Unlike the C modulo operator C<%>, this |
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439 | function ensures that the return value is always positive and that the two |
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440 | numbers I<m> and I<m' = m + i * n> result in the same value modulo I<n> - |
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441 | the C<%> operator usually has a behaviour change at C<m = 0>. |
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442 | |
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443 | C<n> must be strictly positive (i.e. C<< >= 1 >>), while C<m> must be |
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444 | negatable, that is, both C<m> and C<-m> must be representable in its |
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445 | type. |
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446 | |
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447 | Current GCC versions compile this into an efficient branchless sequence on |
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448 | many systems. |
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449 | |
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450 | For example, when you want to rotate forward through the members of an |
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451 | array for increasing C<m> (which might be negative), then you should use |
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452 | C<ecb_mod>, as the C<%> operator might give either negative results, or |
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453 | change direction for negative values: |
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454 | |
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455 | for (m = -100; m <= 100; ++m) |
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456 | int elem = myarray [ecb_mod (m, ecb_array_length (myarray))]; |
133 | |
457 | |
134 | =back |
458 | =back |
135 | |
459 | |
136 | =head2 UTILITY |
460 | =head2 UTILITY |
137 | |
461 | |
138 | =over 4 |
462 | =over 4 |
139 | |
463 | |
140 | =item ecb_array_length (name) [MACRO] |
464 | =item element_count = ecb_array_length (name) |
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465 | |
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466 | Returns the number of elements in the array C<name>. For example: |
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467 | |
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468 | int primes[] = { 2, 3, 5, 7, 11 }; |
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469 | int sum = 0; |
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470 | |
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471 | for (i = 0; i < ecb_array_length (primes); i++) |
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472 | sum += primes [i]; |
141 | |
473 | |
142 | =back |
474 | =back |
143 | |
475 | |
144 | |
476 | |