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 5.36 An Inline Function is As Fast As a Macro
 =============================================
 
 By declaring a function inline, you can direct GCC to make calls to
 that function faster.  One way GCC can achieve this is to integrate
 that function's code into the code for its callers.  This makes
 execution faster by eliminating the function-call overhead; in
 addition, if any of the actual argument values are constant, their
 known values may permit simplifications at compile time so that not all
 of the inline function's code needs to be included.  The effect on code
 size is less predictable; object code may be larger or smaller with
 function inlining, depending on the particular case.  You can also
 direct GCC to try to integrate all "simple enough" functions into their
 callers with the option `-finline-functions'.
 
  GCC implements three different semantics of declaring a function
 inline.  One is available with `-std=gnu89' or `-fgnu89-inline' or when
 `gnu_inline' attribute is present on all inline declarations, another
 when `-std=c99' or `-std=gnu99' (without `-fgnu89-inline'), and the
 third is used when compiling C++.
 
  To declare a function inline, use the `inline' keyword in its
 declaration, like this:
 
      static inline int
      inc (int *a)
      {
        (*a)++;
      }
 
  If you are writing a header file to be included in ISO C89 programs,
 write `__inline__' instead of `inline'.   Alternate Keywords.
 
  The three types of inlining behave similarly in two important cases:
 when the `inline' keyword is used on a `static' function, like the
 example above, and when a function is first declared without using the
 `inline' keyword and then is defined with `inline', like this:
 
      extern int inc (int *a);
      inline int
      inc (int *a)
      {
        (*a)++;
      }
 
  In both of these common cases, the program behaves the same as if you
 had not used the `inline' keyword, except for its speed.
 
  When a function is both inline and `static', if all calls to the
 function are integrated into the caller, and the function's address is
 never used, then the function's own assembler code is never referenced.
 In this case, GCC does not actually output assembler code for the
 function, unless you specify the option `-fkeep-inline-functions'.
 Some calls cannot be integrated for various reasons (in particular,
 calls that precede the function's definition cannot be integrated, and
 neither can recursive calls within the definition).  If there is a
 nonintegrated call, then the function is compiled to assembler code as
 usual.  The function must also be compiled as usual if the program
 refers to its address, because that can't be inlined.
 
  Note that certain usages in a function definition can make it
 unsuitable for inline substitution.  Among these usages are: use of
 varargs, use of alloca, use of variable sized data types (
 Variable Length), use of computed goto ( Labels as Values),
 use of nonlocal goto, and nested functions ( Nested Functions).
 Using `-Winline' will warn when a function marked `inline' could not be
 substituted, and will give the reason for the failure.
 
  As required by ISO C++, GCC considers member functions defined within
 the body of a class to be marked inline even if they are not explicitly
 declared with the `inline' keyword.  You can override this with
 `-fno-default-inline';  Options Controlling C++ Dialect C++
 Dialect Options.
 
  GCC does not inline any functions when not optimizing unless you
 specify the `always_inline' attribute for the function, like this:
 
      /* Prototype.  */
      inline void foo (const char) __attribute__((always_inline));
 
  The remainder of this section is specific to GNU C89 inlining.
 
  When an inline function is not `static', then the compiler must assume
 that there may be calls from other source files; since a global symbol
 can be defined only once in any program, the function must not be
 defined in the other source files, so the calls therein cannot be
 integrated.  Therefore, a non-`static' inline function is always
 compiled on its own in the usual fashion.
 
  If you specify both `inline' and `extern' in the function definition,
 then the definition is used only for inlining.  In no case is the
 function compiled on its own, not even if you refer to its address
 explicitly.  Such an address becomes an external reference, as if you
 had only declared the function, and had not defined it.
 
  This combination of `inline' and `extern' has almost the effect of a
 macro.  The way to use it is to put a function definition in a header
 file with these keywords, and put another copy of the definition
 (lacking `inline' and `extern') in a library file.  The definition in
 the header file will cause most calls to the function to be inlined.
 If any uses of the function remain, they will refer to the single copy
 in the library.
 

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