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When an inline function is called, a copy of the C/C++ source code for the function is inserted at the point of the call. This is known as inline function expansion, commonly called function inlining or just inlining. Inline function expansion can speed up execution by eliminating function call overhead. This is particularly beneficial for very small functions that are called frequently. Function inlining involves a tradeoff between execution speed and code size, because the code is duplicated at each function call site. Large functions that are called in many places are poor candidates for inlining.
Excessive Inlining Can Degrade Performance: Excessive inlining can make the compiler dramatically slower and degrade the performance of generated code.
Function inlining is triggered by the following situations:
inline
keyword or the equivalent
__inline
keyword. Functions declared with the inline keyword may be inlined by the compiler if you set --opt_level=0 or greater. The inline keyword is a suggestion from the programmer to the compiler. Even if your optimization level is high, inlining is still optional for the compiler. The compiler decides whether to inline a function based on the length of the function, the number of times it is called, your --opt_for_speed setting, and any contents of the function that disqualify it from inlining (see Section 2.11.2). Functions can be inlined at --opt_level=0 or above if the function body is visible in the same module or if -pm is also used and the function is visible in one of the modules being compiled. Functions may be inlined at link time if the file containing the definition and the call site were both compiled with --opt_level=4. Functions defined as both static and inline are more likely to be inlined.
always_inline
attribute (Section 5.17.2) force a function to be inlined (where it is legal to do so) unless --opt_level=off. That is, the pragma FUNC_ALWAYS_INLINE forces function inlining even if the function is not declared as inline and the --opt_level=0 or --opt_level=1.Function Inlining Can Greatly Increase Code Size: Function inlining increases code size, especially inlining a function that is called in a number of places. Function inlining is optimal for functions that are called only from a small number of places and for small functions.
The semantics of the inline
keyword in C code follow the C99 standard. The semantics of the
inline
keyword in C++ code follow the C++ standard.
The inline
keyword is supported
in all C++ modes, in relaxed ANSI mode for all C standards, and in strict ANSI mode
for C99 and C11. It is disabled in strict ANSI mode for C89, because it is a
language extension that could conflict with a strictly conforming program. If you
want to define inline functions while in strict ANSI C89 mode, use the alternate
keyword __inline
.
Compiler options that affect inlining are: --opt_level, --auto_inline, --remove_hooks_when_inlining, --opt_for_speed, and --disable_inlining.