Which variable should be optimize in following C code

If your compiler a开发者_如何转开发ctually optimizes access time of only two registers variables per function, which two variable in the following program are the best one to be made into register variables?

void main(void)
{
  int i,j,k,m;
  do
  {
    printf("enter value");
    scanf(“%d”,&i);
    m=0;
    for(k=0;k<100;k++)
      m=k+m;
  }
  while(i>0);
}

Please ignore if any mistake is there...

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Trick question? In a smart compiler, none of the variables are registerized. i has its address taken, so it can't be in a register all the time. j, k and m should be optimized away.

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Certainly not j, since it is never used. Not i either, as you are using the address-of operator to write to it, which means it needs to be read back from memory after it's been written by the scanf. That only leaves k and m.

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Good compiler will optimize this part of code:

m=0;
for(k=0;k<100;k++)
  m=k+m;

And replaced it with m = 4950; :) The better one will optimize m = 4950; and put nothing in place). j also will be optimized. And i can't be register because in scanf its address is needed. So final answer is "NO ONE".

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I'd guess the compiler would pick k and m

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Since the result of the computation is never used the compiler can optimize out almost all of your code. The only things that must remain are equivalent to

int main(void) {
  int i;
  do {
    printf("enter value");
    scanf(“%d”,&i);
  } while(i>0);
  return 0;
}

As others have already said the only remaining variable i can't be of register storage class since its address is taken.