Is there a performance difference between i and i in C

Question

Is there a performance difference between i   and   i if the resulting value is not used

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I have been reading through most of the answers here and many of the comments, and I didn't see any reference to the one instance that I could think of where i++ is more efficient than ++i (and perhaps surprisingly --i was more efficient than i--). That is for C compilers for the DEC PDP-11!

The PDP-11 had assembly instructions for pre-decrement of a register and post-increment, but not the other way around. The instructions allowed any "general-purpose" register to be used as a stack pointer. So if you used something like *(i++) it could be compiled into a single assembly instruction, while *(++i) could not.

This is obviously a very esoteric example, but it does provide the exception where post-increment is more efficient(or I should say was, since there isn't much demand for PDP-11 C code these days).

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My C is a little rusty  so I apologize in advance   Speedwise  I can understand the results   But  I am confused as to how both files came out to the same MD5 hash   Maybe a for loop runs the same  but wouldn t the following 2 lines of code generate different assembly   myArray i       hello     vs  myArray   i     hello     The first one writes the value to the array  then increments i   The second increments i then writes to the array   I m no assembly expert  but I just don t see how the same executable would be generated by these 2 different lines of code   Just my two cents

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Here s an additional observation if you re worried about micro optimisation  Decrementing loops can  possibly  be more efficient than incrementing loops  depending on instruction set architecture e g  ARM   given   for  i   0  i  lt  100  i      On each loop you you will have one instruction each for    Adding 1 to i    Compare whether i is less than a 100  A conditional branch if i is less than a 100    Whereas a decrementing loop   for  i   100  i    0  i--    The loop will have an instruction for each of    Decrement i  setting the CPU register status flag  A conditional branch depending on CPU register status  Z  0     Of course this works only when decrementing to zero   Remembered from the ARM System Developer s Guide

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In C  the compiler can generally optimize them to be the same if the result is unused   However  in C   if using other types that provide their own    operators  the prefix version is likely to be faster than the postfix version   So  if you don t need the postfix semantics  it is better to use the prefix operator

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I always prefer pre-increment  however      I wanted to point out that even in the case of calling the operator   function  the compiler will be able to optimize away the temporary if the function gets inlined   Since the operator   is usually short and often implemented in the header  it is likely to get inlined   So  for practical purposes  there likely isn t much of a difference between the performance of the two forms   However  I always prefer pre-increment since it seems better to directly express what I m trying to say  rather than relying on the optimizer to figure it out   Also  giving the optmizer less to do likely means the compiler runs faster

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First of all  The difference between i   and   i is neglegible in C   To the details  1  The well known C   issue    i is faster In C      i is more efficient iff i is some kind of an object with an overloaded increment operator  Why  In   i  the object is first incremented  and can subsequently passed as a const reference to any other function  This is not possible if the expression is foo i    because now the increment needs to be done before foo   is called  but the old value needs to be passed to foo    Consequently  the compiler is forced to make a copy of i before it executes the increment operator on the original  The additional constructor destructor calls are the bad part  As noted above  this does not apply to fundamental types  2  The little known fact  i   may be faster If no constructor destructor needs to be called  which is always the case in C    i and i   should be equally fast  right  No  They are virtually equally fast  but there may be small differences  which most other answerers got the wrong way around  How can i   be faster  The point is data dependencies  If the value needs to be loaded from memory  two subsequent operations need to be done with it  incrementing it  and using it  With   i  the incrementation needs to be done before the value can be used  With i    the use does not depend on the increment  and the CPU may perform the use operation in parallel to the increment operation  The difference is at most one CPU cycle  so it is really neglegible  but it is there  And it is the other way round then many would expect

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Mark Even though the compiler is allowed to optimize away the  stack based  temporary copy of the variable and gcc  in recent versions  is doing so  doesn t mean all compilers will always do so   I just tested it with the compilers we use in our current project and 3 out of 4 do not optimize it   Never assume the compiler gets it right  especially if the possibly faster  but never slower code is as easy to read   If you don t have a really stupid implementation of one of the operators in your code   Alwas prefer   i over i

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I can think of a situation where postfix is slower than prefix increment   Imagine a processor with register A is used as accumulator and it s the only register used in many instructions  some small microcontrollers are actually like this    Now imagine the following program and their translation into a hypothetical assembly   Prefix increment   a     b   c     increment b LD    A    amp b  INC   A ST    A    amp b     add with c ADD   A    amp c     store in a ST    A    amp a    Postfix increment   a   b     c     load b LD    A    amp b     add with c ADD   A    amp c     store in a ST    A    amp a     increment b LD    A    amp b  INC   A ST    A    amp b    Note how the value of b was forced to be reloaded  With prefix increment  the compiler can just increment the value and go ahead with using it  possibly avoid reloading it since the desired value is already in the register after the increment  However  with postfix increment  the compiler has to deal with two values  one the old and one the incremented value which as I show above results in one more memory access   Of course  if the value of the increment is not used  such as a single i    statement  the compiler can  and does  simply generate an increment instruction regardless of postfix or prefix usage     As a side note  I d like to mention that an expression in which there is a b   cannot simply be converted to one with   b without any additional effort  for example by adding a - 1   So comparing the two if they are part of some expression is not really valid  Often  where you use b   inside an expression you cannot use   b  so even if   b were potentially more efficient  it would simply be wrong  Exception is of course if the expression is begging for it  for example a   b     1  which can be changed to a     b

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From Efficiency versus intent by Andrew Koenig       First  it is far from obvious that   i is more efficient than i    at least where integer variables are concerned    And       So the question one should be asking is not which of these two operations is faster  it is which of these two operations expresses more accurately what you are trying to accomplish   I submit that if you are not using the value of the expression  there is never a reason to use i   instead of   i  because there is never a reason to copy the value of a variable  increment the variable  and then throw the copy away    So  if the resulting value is not used  I would use   i  But not because it is more efficient  because it correctly states my intent

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A better answer is that   i will sometimes be faster but never slower   Everyone seems to be assuming that i is a regular built-in type such as int  In this case there will be no measurable difference   However if i is  complex type then you may well find a measurable difference  For i   you must make a copy of your class before incrementing it  Depending on what s involved in a copy it could indeed be slower since with   it you can just return the final value   Foo Foo  operator         Foo oldFoo    this     copy existing value - could be slow      yadda yadda  do increment   return oldFoo      Another difference is that with   i you have the option of returning a reference instead of a value  Again  depending on what s involved in making a copy of your object this could be slower   A real-world example of where this can occur would be the use of iterators  Copying an iterator is unlikely to be a bottle-neck in your application  but it s still good practice to get into the habit of using   i instead of i   where the outcome is not affected

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Executive summary   No   i   could potentially be slower than   i  since the old value of i might need to be saved for later use  but in practice all modern compilers will optimize this away   We can demonstrate this by looking at the code for this function  both with   i and i       cat i   c extern void g int i   void f         int i       for  i   0  i  lt  100  i            g i        The files are the same  except for   i and i       diff i   c   i c 6c6  lt      for  i   0  i  lt  100  i    ---  gt      for  i   0  i  lt  100    i    We ll compile them  and also get the generated assembler     gcc -c i   c   i c   gcc -S i   c   i c   And we can see that both the generated object and assembler files are the same     md5 i   s   i s MD5  i   s    90f620dda862cd0205cd5db1f2c8c06e MD5    i s    90f620dda862cd0205cd5db1f2c8c06e    md5   o MD5    i o    dd3ef1408d3a9e4287facccec53f7d22 MD5  i   o    dd3ef1408d3a9e4287facccec53f7d22

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Please don t let the question of  which one is faster  be the deciding factor of which to use   Chances are you re never going to care that much  and besides  programmer reading time is far more expensive than machine time   Use whichever makes most sense to the human reading the code

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Taking a leaf from Scott Meyers  More Effective c   Item 6  Distinguish between prefix and postfix forms of increment and decrement operations   The prefix version is always preferred over the postfix in regards to objects  especially in regards to iterators   The reason for this if you look at the call pattern of the operators      Prefix Integer amp  Integer  operator            this    1      return  this        Postfix const Integer Integer  operator   int        Integer oldValue    this          this       return oldValue      Looking at this example it is easy to see how the prefix operator will always be more efficient than the postfix  Because of the need for a temporary object in the use of the postfix   This is why when you see examples using iterators they always use the prefix version   But as you point out for int s there is effectively no difference because of compiler optimisation that can take place

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Short answer    There is never any difference between i   and   i in terms of speed  A good compiler should not generate different code in the two cases   Long answer   What every other answer fails to mention is that the difference between   i versus i   only makes sense within the expression it is found    In the case of for i 0  i lt n  i     the i   is alone in its own expression  there is a sequence point before the i   and there is one after it  Thus the only machine code generated is  increase i by 1  and it is well-defined how this is sequenced in relation to the rest of the program  So if you would change it to prefix     it wouldn t matter in the slightest  you would still just get the machine code  increase i by 1    The differences between   i and i   only matters in expressions such as array i      x  versus array   i    x   Some may argue and say that the postfix will be slower in such operations because the register where i resides have to be reloaded later  But then note that the compiler is free to order your instructions in any way it pleases  as long as it doesn t  break the behavior of the abstract machine  as the C standard calls it   So while you may assume that array i      x  gets translated to machine code as    Store value of i in register A  Store address of array in register B  Add A and B  store results in A  At this new address represented by A  store the value of x  Store value of i in register A      inefficient because extra instruction here  we already did this once  Increment register A  Store register A in i    the compiler might as well produce the code more efficiently  such as    Store value of i in register A  Store address of array in register B  Add A and B  store results in B  Increment register A  Store register A in i         rest of the code    Just because you as a C programmer is trained to think that the postfix    happens at the end  the machine code doesn t have to be ordered in that way   So there is no difference between prefix and postfix    in C  Now what you as a C programmer should be vary of  is people who inconsistently use prefix in some cases and postfix in other cases  without any rationale why  This suggests that they are uncertain about how C works or that they have incorrect knowledge of the language  This is always a bad sign  it does in turn suggest that they are making other questionable decisions in their program  based on superstition or  religious dogmas      Prefix    is always faster  is indeed one such false dogma that is common among would-be C programmers

[c] Is there a performance difference between i++ and ++i in C?

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