Question

I was always interested in assembler, however so far I didn’t have a true chance to confront it in a best way. Now, when I do have some time, I began coding some small programs using assembler in a c++, but that’s just small ones, i.e. define x, store it somewhere and so on, so forth. I wanted to implement foor loop in assembler, but I couldn’t make it, so I would like to ask if anyone here has ever done with it, would be nice to share here. Example of some function would be

for(i=0;i<10;i++) { std::cout<< "A"; }

Anyone has some idea how to implement this in a assembler?

edit2: ISA x86

Answer 1

Here’s the unoptimized output¹ of GCC for this code:

void some_function(void);

int main()
{
  for (int i = 0; i < 137; ++i) { some_function(); }
}

    movl    $0, 12(%esp)            // i = 0; i is stored at %esp + 12
    jmp .L2
.L3:
    call    some_function           // some_function()
    addl    $1, 12(%esp)            // ++i
.L2:
    cmpl    $136, 12(%esp)          // compare i to 136 ...
    jle .L3                         //   ... and repeat loop less-or-equal

    movl    $0, %eax                // return 0
    leave                           //  --"--

With optimization -O3, the addition+comparison is turned into subtraction:

    pushl   %ebx          // save %ebx
    movl    $137, %ebx    // set %ebx to 137

    // some unrelated parts

.L2:
    call    some_function // some_function()
    subl    $1, %ebx      // subtract 1 from %ebx
    jne .L2               // if not equal to 0, repeat loop

¹_{The generated assembly can be examined by invoking GCC with the -S flag.}