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| <h3 class="section">19.9 Is Fast Code or Small Code preferred?</h3> |
| |
| <p><a name="index-Optimization-2319"></a> |
| If an application uses many floating point functions it is often the case |
| that the cost of the function calls themselves is not negligible. |
| Modern processors can often execute the operations themselves |
| very fast, but the function call disrupts the instruction pipeline. |
| |
| <p>For this reason the GNU C Library provides optimizations for many of the |
| frequently-used math functions. When GNU CC is used and the user |
| activates the optimizer, several new inline functions and macros are |
| defined. These new functions and macros have the same names as the |
| library functions and so are used instead of the latter. In the case of |
| inline functions the compiler will decide whether it is reasonable to |
| use them, and this decision is usually correct. |
| |
| <p>This means that no calls to the library functions may be necessary, and |
| can increase the speed of generated code significantly. The drawback is |
| that code size will increase, and the increase is not always negligible. |
| |
| <p>There are two kind of inline functions: Those that give the same result |
| as the library functions and others that might not set <code>errno</code> and |
| might have a reduced precision and/or argument range in comparison with |
| the library functions. The latter inline functions are only available |
| if the flag <code>-ffast-math</code> is given to GNU CC. |
| |
| <p>In cases where the inline functions and macros are not wanted the symbol |
| <code>__NO_MATH_INLINES</code> should be defined before any system header is |
| included. This will ensure that only library functions are used. Of |
| course, it can be determined for each file in the project whether |
| giving this option is preferable or not. |
| |
| <p>Not all hardware implements the entire IEEE 754<!-- /@w --> standard, and even |
| if it does there may be a substantial performance penalty for using some |
| of its features. For example, enabling traps on some processors forces |
| the FPU to run un-pipelined, which can more than double calculation time. |
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