Benefits of inline functions in C

Question

What is the advantages disadvantages of using inline functions in C    I see that it only increases performance for the code that the compiler outputs  but with today s optimized compilers  fast CPUs  huge memory etc   not like in the 1980 lt  where memory was scarce and everything had to fit in 100KB of memory  what advantages do they really have today

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In archaic C and C    inline is like register  a suggestion  nothing more than a suggestion  to the compiler about a possible optimization   In modern C    inline tells the linker that  if multiple definitions  not declarations  are found in different translation units  they are all the same  and the linker can freely keep one and discard all the other ones   inline is mandatory if a function  no matter how complex or  linear   is defined in a header file  to allow multiple sources to include it without getting a  multiple definition  error by the linker   Member functions defined inside a class are  inline  by default  as are template functions  in contrast to global functions      fileA h inline void afunc     std  cout  lt  lt   this is afunc   lt  lt  std  endl       file1 cpp  include  fileA h  void acall     afunc         main cpp  include  fileA h  void acall     int main         afunc        acall         output this is afunc this is afunc   Note the inclusion of fileA h into two  cpp files  resulting in two instances of afunc    The linker will discard one of them  If no inline is specified  the linker will complain

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Why not make all functions inline by default  Because it s an engineering trade off  There are at least two types of  optimization   speeding up the program and reducing the size  memory footprint  of the program  Inlining generally speeds things up  It gets rid of the function call overhead  avoiding pushing then pulling parameters from the stack  However  it also makes the memory footprint of the program bigger  because every function call must now be replaced with the full code of the function  To make things even more complicated  remember that the CPU stores frequently used chunks of memory in a cache on the CPU for ultra-rapid access  If you make the program s memory image big enough  your program won t be able to use the cache efficiently  and in the worst case inlining could actually slow your program down  To some extent the compiler can calculate what the trade offs are  and may be able to make better decisions than you can  just looking at the source code

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Fell into the same trouble with inlining functions into so libraries  It seems that inlined functions are not compiled into the library  as a result the linker puts out a  undefined reference  error  if a executable wants to use the inlined function of the library   happened to me compiling Qt source with gcc 4 5

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It is not all about performance  Both C   and C are used for embedded programming  sitting on top of hardware  If you would  for example  write an interrupt handler  you need to make sure that the code can be executed at once  without additional registers and or memory pages being  being swapped  That is when inline comes in handy  Good compilers do some  inlining  themselves when speed is needed  but  inline  compels them

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Inline function is the optimization technique used by the compilers  One can simply prepend inline keyword to function prototype to make a function inline  Inline function instruct compiler to insert complete body of the function wherever that function got used in code   Advantages  -   It does not require function calling overhead  It also save overhead of variables push pop on the stack  while function calling  It also save overhead of return call from a function  It increases locality of reference by utilizing instruction cache  After in-lining compiler can also apply intra-procedural optimization if specified  This is the most important one  in this way compiler can now focus on dead code elimination  can give more stress on branch prediction  induction variable elimination etc     To check more about it one can follow this link http   tajendrasengar blogspot com 2010 03 what-is-inline-function-in-cc html

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Advantages  By inlining your code where it is needed  your program will spend less time in the function call and return parts  It is supposed to make your code go faster  even as it goes larger  see below   Inlining trivial accessors could be an example of effective inlining  By marking it as inline  you can put a function definition in a header file  i e  it can be included in multiple compilation unit  without the linker complaining   Disadvantages  It can make your code larger  i e  if you use inline for non-trivial functions   As such  it could provoke paging and defeat optimizations from the compiler  It slightly breaks your encapsulation because it exposes the internal of your object processing  but then  every  quot private quot  member would  too   This means you must not use inlining in a PImpl pattern  It slightly breaks your encapsulation 2  C   inlining is resolved at compile time  Which means that should you change the code of the inlined function  you would need to recompile all the code using it to be sure it will be updated  for the same reason  I avoid default values for function parameters  When used in a header  it makes your header file larger  and thus  will dilute interesting informations  like the list of a class methods  with code the user don t care about  this is the reason that I declare inlined functions inside a class  but will define it in an header after the class body  and never inside the class body    Inlining Magic  The compiler may or may not inline the functions you marked as inline  it may also decide to inline functions not marked as inline at compilation or linking time  Inline works like a copy paste controlled by the compiler  which is quite different from a pre-processor macro  The macro will be forcibly inlined  will pollute all the namespaces and code  won t be easily debuggable  and will be done even if the compiler would have ruled it as inefficient  Every method of a class defined inside the body of the class itself is considered as  quot inlined quot   even if the compiler can still decide to not inline it Virtual methods are not supposed to be inlinable  Still  sometimes  when the compiler can know for sure the type of the object  i e  the object was declared and constructed inside the same function body   even a virtual function will be inlined because the compiler knows exactly the type of the object  Template methods functions are not always inlined  their presence in an header will not make them automatically inline   The next step after  quot inline quot  is template metaprograming   I e  By  quot inlining quot  your code at compile time  sometimes  the compiler can deduce the final result of a function    So a complex algorithm can sometimes be reduced to a kind of return 42   statement  This is for me extreme inlining  It happens rarely in real life  it makes compilation time longer  will not bloat your code  and will make your code faster  But like the grail  don t try to apply it everywhere because most processing cannot be resolved this way    Still  this is cool anyway    -p

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inline allows you to place a function definition in a header file and  include that header file in multiple source files without violating the one definition rule

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Inlining is a suggestion to the compiler which it is free to ignore   It s ideal for small bits of code   If your function is inlined  it s basically inserted in the code where the function call is made to it  rather than actually calling a separate function   This can assist with speed as you don t have to do the actual call   It also assists CPUs with pipelining as they don t have to reload the pipeline with new instructions caused by a call   The only disadvantage is possible increased binary size but  as long as the functions are small  this won t matter too much   I tend to leave these sorts of decisions to the compilers nowadays  well  the smart ones anyway   The people who wrote them tend to have far more detailed knowledge of the underlying architectures

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Conclusion from another discussion here   Are there any drawbacks with inline functions   Apparently  There is nothing wrong with using inline functions   But it is worth noting the following points    Overuse of inlining can actually make programs slower  Depending on a function s size  inlining it can cause the code size to increase or decrease  Inlining a very small accessor function will usually decrease code size while inlining a very large function can dramatically increase code size  On modern processors smaller code usually runs faster due to better use of the instruction cache  - Google Guidelines The speed benefits of inline functions tend to diminish as the function grows in size  At some point the overhead of the function call becomes small compared to the execution of the function body  and the benefit is lost - Source There are few situations where an inline function may not work    For a function returning values  if a return statement exists  For a function not returning any values  if a loop  switch or goto statement exists   If a function is recursive  -Source  The   inline keyword causes a function to be inlined only if you specify the optimize option  If optimize is specified  whether or not   inline is honored depends on the setting of the inline optimizer option  By default  the inline option is in effect whenever the optimizer is run  If you specify optimize   you must also specify the noinline option if you want the   inline keyword to be ignored  -Source

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I d like to add that inline functions are crucial when you are building shared library  Without marking function inline  it will be exported into the library in the binary form  It will be also present in the symbols table  if exported  On the other side  inlined functions are not exported  neither to the library binaries nor to the symbols table   It may be critical when library is intended to be loaded at runtime  It may also hit binary-compatible-aware libraries  In such cases don t use inline

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Inline functions are faster because you don t need to push and pop things on off the stack like parameters and the return address  however  it does make your binary slightly larger    Does it make a significant difference   Not noticeably enough on modern hardware for most   But it can make a difference  which is enough for some people    Marking something inline does not give you a guarantee that it will be inline   It s just a suggestion to the compiler   Sometimes it s not possible such as when you have a virtual function  or when there is recursion involved   And sometimes the compiler just chooses not to use it    I could see a situation like this making a detectable difference   inline int aplusb pow2 int a  int b      return  a   b   a   b       for int a   0  a  lt  900000    a      for int b   0  b  lt  900000    b          aplusb pow2 a  b

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Generally speaking  these days with any modern compiler worrying about inlining anything is pretty much a waste of time  The compiler should actually optimize all of these considerations for you through its own analysis of the code and your specification of the optimization flags passed to the compiler  If you care about speed  tell the compiler to optimize for speed  If you care about space  tell the compiler to optimize for space  As another answer alluded to  a decent compiler will even inline automatically if it really makes sense   Also  as others have stated  using inline does not guarantee inline of anything  If you want to guarantee it  you will have to define a macro instead of an inline function to do it   When to inline and or define a macro to force inclusion  - Only when you have a demonstrated and necessary proven increase in speed for a critical section of code that is known to have an affect on the overall performance of the application

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During optimization many compilers will inline functions even if you didn t mark them  You generally only need to mark functions as inline if you know something the compiler doesn t  as it can usually make the correct decision itself

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Our computer science professor urged us to never use inline in a c   program  When asked why  he kindly explained to us that modern compilers should detect when to use inline automatically   So yes  the inline can be an optimization technique to be used wherever possible  but apparently this is something that is already done for you whenever it s possible to inline a function anyways

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