while loop in assembly language

Question

There is such code in C++:

#include <iostream>

int main(){
  int a = 4;
  while(a--){
    std::cout << (a + 1) << '\n';
  }
  return 0;
}

and corresponding code of main function in assembly code produced by g++:

.globl main
    .type   main, @function
main:
.LFB957:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $32, %esp
    movl    $4, 28(%esp)    # int a = 4;
    jmp .L2
.L3:
    movl    28(%esp), %eax     # std::cout << (a + 1) << '\n';
    addl    $1, %eax
    movl    %eax, 4(%esp)
    movl    $_ZSt4cout, (%esp)
    call    _ZNSolsEi
    movl    $10, 4(%esp)
    movl    %eax, (%esp)
    call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_c
.L2:
    cmpl    $0, 28(%esp)
    setne   %al
    subl    $1, 28(%esp)    # a = a - 1
    testb   %al, %al
    jne .L3
    movl    $0, %eax
    leave
    ret
    .cfi_endproc
.LFE957:
    .size   main, .-main

What are used instructions setne and testb in following fragment for?

 .L2:
        cmpl    $0, 28(%esp)
        setne   %al
        subl    $1, 28(%esp)    # a = a - 1
        testb   %al, %al
        jne .L3

Couldn't it be just so to check in while loop whether a is not zero and jump?

It implements the post-decrement operator. Test first, modify afterwards. The final value of a is -1, not 0. Presumably the reason that 1 needs to be added again for display. — Hans Passant
– Hans Passant, Commented Sep 21, 2011 at 7:49

James Kanze · Accepted Answer · 2011-09-21 07:48:11Z

5

The while condition is formally the equivalent of:

while ( a -- != 0 )

(Omitting the comparison is a legal obfuscation.)

The compiler is generating code to compare a with 0, save the results in register al, then decrement a, and then test the saved results.

answered Sep 21, 2011 at 7:48

James Kanze

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2 Comments

user824425 Over a year ago

An addendum: yes, the compiler could have optimized this better, but this code is probably much easier to generate.

James Kanze Over a year ago

@Tinctorius Just looking at the code, I assumed no optimization. (With optimization, I would expect the variable a to disappear completely, with it's value being held in a register. And there are definitely more clever ways of generating the code: something like sub a, 1; jc comes to mind.)

Flexo - Save the data dump · Accepted Answer · 2011-09-21 22:11:43Z

1

Because a-- means

tmpval=a;
a=a-1;
return tmpval;

so compiler needs to save the previous value of a. In this program, the body part of while will be executed when a = 0 (after a--, so it will print 1).

edited Sep 21, 2011 at 22:11

Flexo - Save the data dump♦

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answered Sep 21, 2011 at 7:54

user685684

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Comments

GazTheDestroyer · Accepted Answer · 2011-09-21 07:47:02Z

0

It's been a long while since I did assembler, but I would assume it's some optimisation to keep the pipelines busy / optimise register use.

answered Sep 21, 2011 at 7:47

GazTheDestroyer

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