Optimize “for” loop

Question

std::vector someVector;    
for (unsigned int i = 0; i < someVector.size(); i++)
{
   // do something
}

Does the value of someV

Answer 1

I have checked with GCC explorer:

Code entered:

#include

int sum(const std::vector & someVector) {
  int s = 0;
  for (int i = 0; i < someVector.size(); i++) {
    s += someVector[i];
  }
  return s;
}

int main() {
  std::vector someVector;
  return sum(someVector);
};

Assembly generated for sum():

  movq  (%rdi), %rcx
  movq  8(%rdi), %rdx
  xorl  %eax, %eax
  cmpq  %rcx, %rdx
  je    .LBB0_3
  subq  %rcx, %rdx
  sarq  $2, %rdx
  xorl  %eax, %eax
  xorl  %esi, %esi
.LBB0_2:                                # =>This Inner Loop Header: Depth=1
  addl  (%rcx,%rsi,4), %eax
  incq  %rsi
  cmpq  %rdx, %rsi
  jb    .LBB0_2
.LBB0_3:                                # %._crit_edge
  ret

i.e. the size is kept in %rdx -- there is no call to size() each time.

As others have pointed out already, results may depend on

• your compiler,
• optimization settings and
• what you actually do in the loop (click the gcc explorer link above to try yourself).

Without calculating anything, the whole loop gets optimized away.