Re^2: Help me to understand increment / decrement operator behavior in perl

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

Comment on Re^2: Help me to understand increment / decrement operator behavior in perl Select or Download Code

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Re^3: Help me to understand increment / decrement operator behavior in perl by ikegami (Patriarch) on Mar 04, 2015 at 17:11 UTC
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]
Re^4: Help me to understand increment / decrement operator behavior in perl by BrowserUk (Patriarch) on Mar 04, 2015 at 17:28 UTC
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]
Re^5: Help me to understand increment / decrement operator behavior in perl by ikegami (Patriarch) on Mar 04, 2015 at 17:50 UTC
How about machine without registers? How about values retrieved early in the sub and compared late in the sub?	[reply]
Re^6: Help me to understand increment / decrement operator behavior in perl by BrowserUk (Patriarch) on Mar 05, 2015 at 05:41 UTC
Re^5: Help me to understand increment / decrement operator behavior in perl by oiskuu (Hermit) on Mar 05, 2015 at 07:33 UTC
Try compiling with `-O0`, that usually forces auto variables to stack (may be useful when debugging). Intels icc with `-Os` (size optimized) may load small constants like so: `pushq $18; popq %rdi`.	[reply] [d/l] [select]
Re^6: Help me to understand increment / decrement operator behavior in perl by BrowserUk (Patriarch) on Mar 05, 2015 at 08:29 UTC
Re^7: Help me to understand increment / decrement operator behavior in perl by oiskuu (Hermit) on Mar 05, 2015 at 10:29 UTC
Some notes below your chosen depth have not been shown here


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