in reply to Help me to understand increment / decrement operator behavior in perl

$a places scalar $a on the stack (the variable $a, not its value 10).

$a-- places a scalar containing the original value of $a on the stack, so $a == $a-- compares $a with the original value of $a. They will never be equal.

--$a places scalar $a on the stack, so $a == --$a compares $a and $a. They will always be equal.

C behaves differently since it places values on the stack or in the registers. You can replicate the behaviour using pointers in C or references in C++. 

#include <stdio.h>
#include <stdlib.h>

int& postdec(int& a) {
   int* rv_ptr = (int*)malloc(sizeof(int));
   *rv_ptr = a;
   --a;
   return *rv_ptr;
}

int& predec(int& a) {
   --a;
   return a;
}

bool& eq(int& a, int& b) {
   bool* rv_ptr = (bool*)malloc(sizeof(bool));
   *rv_ptr = a == b;
   return *rv_ptr;
}

int main() {
   int a = 10;
   if (eq(a, postdec(a))) {
      printf("T1\n");
   }

   a = 10;
   if (eq(a, predec(a))) {
      printf("T2\n");
   }
}

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Update: Removed overspecification identified by BrowserUk.

Replies are listed 'Best First'.
Re^2: Help me to understand increment / decrement operator behavior in perl
by BrowserUk (Patriarch) on Mar 04, 2015 at 17:06 UTC
    C behaves differently since it places values on the stack.

    Um. Actually, C doesn't place anything on the stack. It loads the values into registers.

    This is the MS compiler:

    ; Listing generated by Microsoft (R) Optimizing Compiler Version 15.00
+.21022.08 

include listing.inc

INCLUDELIB LIBCMT
INCLUDELIB OLDNAMES

_DATA    SEGMENT
$SG2413    DB    'T1', 0aH, 00H
$SG2415    DB    'T2', 0aH, 00H
_DATA    ENDS
PUBLIC    main
EXTRN    printf:PROC
pdata    SEGMENT
$pdata$main DD    imagerel $LN5
    DD    imagerel $LN5+87
    DD    imagerel $unwind$main
pdata    ENDS
xdata    SEGMENT
$unwind$main DD    010401H
    DD    06204H
; Function compile flags: /Odtp
; File c:\test\c\junk.c
xdata    ENDS
_TEXT    SEGMENT
a$ = 32
main    PROC

; 3    : void main( void ) {

$LN5:
    sub    rsp, 56                    ; 00000038H

; 4    :     int a = 10;

    mov    DWORD PTR a$[rsp], 10

; 5    :     if( a == a-- ) printf( "T1\n" );

    mov    edx, DWORD PTR a$[rsp]
    mov    ecx, DWORD PTR a$[rsp]
    mov    eax, DWORD PTR a$[rsp]
    sub    eax, 1
    mov    DWORD PTR a$[rsp], eax
    cmp    edx, ecx
    jne    SHORT $LN2@main
    lea    rcx, OFFSET FLAT:$SG2413
    call    printf
$LN2@main:

; 6    :     if( a == --a ) printf( "T2\n" );

    mov    eax, DWORD PTR a$[rsp]
    sub    eax, 1
    mov    DWORD PTR a$[rsp], eax
    mov    eax, DWORD PTR a$[rsp]
    cmp    DWORD PTR a$[rsp], eax
    jne    SHORT $LN1@main
    lea    rcx, OFFSET FLAT:$SG2415
    call    printf
$LN1@main:

; 7    : }

    xor    eax, eax
    add    rsp, 56                    ; 00000038H
    ret    0
main    ENDP
_TEXT    ENDS
END

    [download]

    The interesting thing about that is that for the post decrement, it loads the value of a into two registers (edx & ecx), then loads it a third time in to eax, where it decrements it and then writes it back. Only then does it compare hte first two values and branch accordingly.

    And for the predecrement, it loads one register, decrements it and then saves it back before loading the modified value back into the same register and the comparing it with the stored value and branching accordingly.

    That's before the optimiser takes a look. After the optimiser, all those shenanigins are optimised away and all that remains is the printfs:

    ; Listing generated by Microsoft (R) Optimizing Compiler Version 15.00
+.21022.08 

include listing.inc

INCLUDELIB LIBCMT
INCLUDELIB OLDNAMES

_DATA    SEGMENT
$SG2478    DB    'T1', 0aH, 00H
$SG2480    DB    'T2', 0aH, 00H
_DATA    ENDS
PUBLIC    main
EXTRN    printf:PROC
pdata    SEGMENT
$pdata$main DD    imagerel $LN5
    DD    imagerel $LN5+35
    DD    imagerel $unwind$main
pdata    ENDS
xdata    SEGMENT
$unwind$main DD    010401H
    DD    04204H
; Function compile flags: /Ogtpy
; File c:\test\c\junk.c
xdata    ENDS
_TEXT    SEGMENT
main    PROC

; 3    : void main( void ) {

$LN5:
    sub    rsp, 40                    ; 00000028H

; 4    :     int a = 10;
; 5    :     if( a == a-- ) printf( "T1\n" );

    lea    rcx, OFFSET FLAT:$SG2478
    call    printf

; 6    :     if( a == --a ) printf( "T2\n" );

    lea    rcx, OFFSET FLAT:$SG2480
    call    printf

; 7    : }

    xor    eax, eax
    add    rsp, 40                    ; 00000028H
    ret    0
main    ENDP
_TEXT    ENDS
END

    [download]
    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
    In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
[reply]
[d/l]
[select]
      Whether the stack or a register is used depends on a number of factors. Whether the stack or a register is used is not important here. Fixed.
[reply]
        Whether the stack or a register is used depends on a number of factors.

        Can you show me one example of C pushing a integer any numeric argument onto the stack in order to compare it to another integer numeric argument?

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I'm with torvalds on this
        In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked
[reply]

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