Reset C int array to zero the fastest way

Question

Assuming that we have a T myarray 100  with T   int  unsigned int  long long int or unsigned long long int  what is the fastest way to reset all its content to zero  not only for initialization but to reset the content several times in my program   Maybe with memset   Same question for a dynamic array like T  myarray   new T 100

User · Answer

You can use memset  but only because our selection of types is restricted to integral types   In general case in C it makes sense to implement a macro   define ZERO ANY T  a  n  do      T  a     a       size t n     n       for    n   gt  0  --n     a          a     T    0       while  0    This will give you C  -like functionality that will let you to  reset to zeros  an array of objects of any type without having to resort to hacks like memset  Basically  this is a C analog of C   function template  except that you have to specify the type argument explicitly   On top of that you can build a  template  for non-decayed arrays   define ARRAY SIZE a   sizeof  a    sizeof   a    define ZERO ANY A T  a  ZERO ANY T   a   ARRAY SIZE a     In your example it would be applied as  int a 100    ZERO ANY int  a  100      or ZERO ANY A int  a     It is also worth noting that specifically for objects of scalar types one can implement a type-independent macro   define ZERO a  n  do      size t i    0  n     n       for    i   lt  n     i          a  i     0     while  0    and    define ZERO A a  ZERO  a   ARRAY SIZE a     turning the above example into   int a 100     ZERO a  100       or  ZERO A a

User · Answer

zero myarray   is all you need in C     Just add this to a header   template lt typename T  size t SIZE gt  inline void zero T  amp arr  SIZE        memset arr  0  SIZE sizeof T

User · Answer

memset  from  lt string h gt   is probably the fastest standard way  since it s usually a routine written directly in assembly and optimized by hand   memset myarray  0  sizeof myarray       for automatically-allocated arrays memset myarray  0  N sizeof  myarray       for heap-allocated arrays  where N is the number of elements     By the way  in C   the idiomatic way would be to use std  fill  from  lt algorithm gt     std  fill myarray  myarray N  0     which may be optimized automatically into a memset  I m quite sure that it will work as fast as memset for ints  while it may perform slightly worse for smaller types if the optimizer isn t smart enough  Still  when in doubt  profile

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Here s the function I use   template lt typename T gt  static void setValue T arr    size t length  const T amp  val        std  fill arr  arr   length  val      template lt typename T  size t N gt  static void setValue T   amp arr  N   const T amp  val        std  fill arr  arr   N  val       You can call it like this     fixed arrays int a 10   setValue a  0      dynamic arrays int  d   new int length   setValue d  length  0     Above is more C  11 way than using memset  Also you get compile time error if you use dynamic array with specifying the size

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This question  although rather old  needs some benchmarks  as it asks for not the most idiomatic way  or the way that can be written in the fewest number of lines  but the fastest way  And it is silly to answer that question without some actual testing  So I compared four solutions  memset vs  std  fill vs  ZERO of AnT s answer vs a solution I made using AVX intrinsics    Note that this solution is not generic  it only works on data of 32 or 64 bits  Please comment if this code is doing something incorrect    include lt immintrin h gt   define intrin ZERO a n    size t x   0   const size t inc   32   sizeof   a     size of 256 bit register over size of variable    for   x  lt  n-inc x  inc        mm256 storeu ps  float     a  x   mm256 setzero ps      if 4    sizeof   a          switch n-x        case 3            a  x    0 x         case 2            mm storeu ps  float     a  x   mm setzero ps    break       case 1            a  x    0           break       case 0           break              else if 8    sizeof   a      switch n-x        case 7            a  x    0 x         case 6            a  x    0 x         case 5            a  x    0 x         case 4            mm storeu ps  float     a  x   mm setzero ps    break       case 3            a  x    0 x         case 2             long long    a   x    0 break       case 1            a  x    0           break       case 0           break              I will not claim that this is the fastest method  since I am not a low level optimization expert  Rather it is an example of a correct architecture dependent implementation that is faster than memset   Now  onto the results  I calculated performance for size 100 int and long long arrays  both statically and dynamically allocated  but with the exception of msvc  which did a dead code elimination on static arrays  the results were extremely comparable  so I will show only dynamic array performance  Time markings are ms for 1 million iterations  using time h s low precision clock function   clang 3 8  Using the clang-cl frontend  optimization flags   OX  arch AVX  Oi  Ot   int  memset       99 fill         97 ZERO         98 intrin ZERO  90  long long  memset       285 fill         286 ZERO         285 intrin ZERO  188   gcc 5 1 0  optimization flags  -O3 -march native -mtune native -mavx    int  memset       268 fill         268 ZERO         268 intrin ZERO  91 long long  memset       402 fill         399 ZERO         400 intrin ZERO  185   msvc 2015  optimization flags   OX  arch AVX  Oi  Ot    int memset       196 fill         613 ZERO         221 intrin ZERO  95 long long  memset       273 fill         559 ZERO         376 intrin ZERO  188   There is a lot interesting going on here  llvm killing gcc  MSVC s typical spotty optimizations  it does an impressive dead code elimination on static arrays and then has awful performance for fill   Although my implementation is significantly faster  this may only be because it recognizes that bit clearing has much less overhead than any other setting operation    Clang s implementation merits more looking at  as it is significantly faster  Some additional testing shows that its memset is in fact specialized for zero--non zero memsets for 400 byte array are much slower   220ms  and are comparable to gcc s  However  the nonzero memsetting with an 800 byte array makes no speed difference  which is probably why in that case  their memset has worse performance than my implementation--the specialization is only for small arrays  and the cuttoff is right around 800 bytes  Also note that gcc  fill  and  ZERO  are not optimizing to memset  looking at generated code   gcc is simply generating code with identical performance characteristics   Conclusion  memset is not really optimized for this task as well as people would pretend it is  otherwise gcc and msvc and llvm s memset would have the same performance   If performance matters then memset should not be a final solution  especially for these awkward medium sized arrays  because it is not specialized for bit clearing  and it is not hand optimized any better than the compiler can do on its own

User · Answer

From memset     memset myarray  0  sizeof myarray      You can use sizeof myarray  if the size of myarray is known at compile-time  Otherwise  if you are using a dynamically-sized array  such as obtained via malloc or new  you will need to keep track of the length

User · Answer

For static declaration I think you could use   T myarray 100     0     For dynamic declaration I suggest the same way  memset

[c++] Reset C int array to zero : the fastest way?

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