| … | |
… | |
| 39 | #endif |
39 | #endif |
| 40 | } |
40 | } |
| 41 | |
41 | |
| 42 | =item ecb_noinline |
42 | =item ecb_noinline |
| 43 | |
43 | |
| 44 | Prevent a function from being inlined - it might be optimsied away, but |
44 | Prevent a function from being inlined - it might be optimised away, but |
| 45 | not inlined into other functions. This is useful if you know your function |
45 | not inlined into other functions. This is useful if you know your function |
| 46 | is rarely called and large enough for inlining not to be helpful. |
46 | is rarely called and large enough for inlining not to be helpful. |
| 47 | |
47 | |
| 48 | =item ecb_noreturn |
48 | =item ecb_noreturn |
| 49 | |
49 | |
| … | |
… | |
| 114 | |
114 | |
| 115 | However, by using C<ecb_likely>, you tell the compiler that the condition |
115 | However, by using C<ecb_likely>, you tell the compiler that the condition |
| 116 | is likely to be true (and for C<ecb_unlikel>, that it is unlikely to be |
116 | is likely to be true (and for C<ecb_unlikel>, that it is unlikely to be |
| 117 | true). |
117 | true). |
| 118 | |
118 | |
| 119 | For example, when you check for a 0-ptr and expect this to be a rare, |
119 | For example, when you check for a null pointer and expect this to be a |
| 120 | exceptional, case, then use C<ecb_unlikely>: |
120 | rare, exceptional, case, then use C<ecb_unlikely>: |
| 121 | |
121 | |
| 122 | void my_free (void *ptr) |
122 | void my_free (void *ptr) |
| 123 | { |
123 | { |
| 124 | if (ecb_unlikely (ptr == 0)) |
124 | if (ecb_unlikely (ptr == 0)) |
| 125 | return; |
125 | return; |
| … | |
… | |
| 129 | tell the compiler what the hot path through a function is can increase |
129 | tell the compiler what the hot path through a function is can increase |
| 130 | performance considerably. |
130 | performance considerably. |
| 131 | |
131 | |
| 132 | A very good example is in a function that reserves more space for some |
132 | A very good example is in a function that reserves more space for some |
| 133 | memory block (for example, inside an implementation of a string stream) - |
133 | memory block (for example, inside an implementation of a string stream) - |
| 134 | eahc time something is added, you have to check for a buffer overrun, but |
134 | each time something is added, you have to check for a buffer overrun, but |
| 135 | you expect that most checks will turn out to be false: |
135 | you expect that most checks will turn out to be false: |
| 136 | |
136 | |
| 137 | /* make sure we have "size" extra room in our buffer */ |
137 | /* make sure we have "size" extra room in our buffer */ |
| 138 | ecb_inline void |
138 | ecb_inline void |
| 139 | reserve (int size) |
139 | reserve (int size) |
| … | |
… | |
| 173 | call will never be executed. |
173 | call will never be executed. |
| 174 | |
174 | |
| 175 | =item bool ecb_unreachable () |
175 | =item bool ecb_unreachable () |
| 176 | |
176 | |
| 177 | This function does nothing itself, except tell the compiler that it will |
177 | This function does nothing itself, except tell the compiler that it will |
| 178 | never be executed. Apart from supressing a warning in some cases, this |
178 | never be executed. Apart from suppressing a warning in some cases, this |
| 179 | function can be used to implement C<ecb_assume> or similar functions. |
179 | function can be used to implement C<ecb_assume> or similar functions. |
| 180 | |
180 | |
| 181 | =item bool ecb_prefetch (addr, rw, locality) [MACRO] |
181 | =item bool ecb_prefetch (addr, rw, locality) [MACRO] |
| 182 | |
182 | |
| 183 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
183 | Tells the compiler to try to prefetch memory at the given C<addr>ess |
| … | |
… | |
| 187 | something... in between. The memory pointed to by the address does not |
187 | something... in between. The memory pointed to by the address does not |
| 188 | need to be accessible (it could be a null pointer for example), but C<rw> |
188 | need to be accessible (it could be a null pointer for example), but C<rw> |
| 189 | and C<locality> must be compile-time constants. |
189 | and C<locality> must be compile-time constants. |
| 190 | |
190 | |
| 191 | An obvious way to use this is to prefetch some data far away, in a big |
191 | An obvious way to use this is to prefetch some data far away, in a big |
| 192 | array you loop over. This prefethces memory some 128 array elements later, |
192 | array you loop over. This prefetches memory some 128 array elements later, |
| 193 | in the hope that it will be ready when the CPU arrives at that location. |
193 | in the hope that it will be ready when the CPU arrives at that location. |
| 194 | |
194 | |
| 195 | int sum = 0; |
195 | int sum = 0; |
| 196 | |
196 | |
| 197 | for (i = 0; i < N; ++i) |
197 | for (i = 0; i < N; ++i) |
