Corrupt behavior of a primitive C program on a MacBook Pro, Windows XP, VS 2008 Express Edition

I know this is hard to believe, but I am 100% serious about this.

When I compile the code below with Visual Studio 2008 Express Edition in Release mode on my MacBook Pro (Core 2 Duo P8600) with Windows XP Professional 32 bit SP3 running natively, run the executable, the printf is hit sporadically as soon as I touch the touchpad (no joke) - which should definitely never happen.

Can anybody reproduce the same problem on his MacBook Pro (or any other laptop)? Can anybody see in the assembly listing what might be the problem?

My guess is that the touchpad driver somehow manages to manipulate a register that is responsible for the floating point comparison. With integers, the problem does not occur.

Any idea what is going on here would be very welcome.

#include <stdio.h>

int main()
{
    while (true)
    {
        float x = 1.0f;

        for (int i = 0; i < 50; i++)
        {
            if (0.0f < x)
                x = 0.0f;
        }

        if (x == 1.0f)
            printf("bad: %.2f\n", x);
    }

    return 0;
}

Here is the assembly listing for the code above, produced by Visual Studio 2008 Express Edition:

; Listing generated by Microsoft (R) Optimizing Compiler Version 15.00.30729.01 

    TITLE   c:\Dokumente und Einstellungen\azad\Desktop\WeirdProblem\main.cpp
    .686P
    .XMM
    include listing.inc
    .model  flat

INCLUDELIB MSVCRT
INCLUDELIB OLDNAMES

PUBLIC  ??_C@_0L@LNNNMCPH@bad?3?5?$CF?42f?6?$AA@    ; `string'
PUBLIC  __real@00000000
PUBLIC  __real@0000000000000000
PUBLIC  __real@3f800000
PUBLIC  _main
EXTRN   __imp__printf:PROC
EXTRN   __fltused:DWORD
;   COMDAT ??_C@_0L@LNNNMCPH@bad?3?5?$CF?42f?6?$AA@
; File c:\dokumente und einstellungen\azad\desktop\weirdproblem\main.cpp
CONST   SEGMENT
??_C@_0L@LNNNMCPH@bad?3?5?$CF?42f?6?$AA@ DB 'bad: %.2f', 0aH, 00H ; `string'
CONST   ENDS
;   COMDAT __real@00000000
CONST   SEGMENT
__real@00000000 DD 000000000r           ; 0
CONST   ENDS
;   COMDAT __real@0000000000000000
CONST   SEGMENT
__real@0000000000000000 DQ 00000000000000000r   ; 0
CONST   ENDS
;   COMDAT __real@3f800000
CONST   SEGMENT
__real@3f800000 DD 03f800000r           ; 1
; Function compile flags: /Ogtpy
CONST   ENDS
;   COMDAT _main
_TEXT   SEGMENT
_x$3834 = -4                        ; size = 4
_main   PROC                        ; COMDAT

; 4    : {

  开发者_如何转开发  push    ebp
    mov ebp, esp
    and esp, -64                ; ffffffc0H
    fld1
    sub esp, 60                 ; 0000003cH
    fldz
    push    esi
    fldz
    mov esi, DWORD PTR __imp__printf
    jmp SHORT $LN7@main
$LN43@main:

; 10   :        {
; 11   :            if (0.0f < x)
; 12   :                x = 0.0f;
; 13   :        }
; 14   : 
; 15   :        if (x == 1.0f)

    fstp    ST(0)
    fxch    ST(2)
$LN7@main:
    fxch    ST(2)
    mov ecx, 10                 ; 0000000aH
    fst DWORD PTR _x$3834[esp+64]
    fld DWORD PTR _x$3834[esp+64]
$LN5@main:
    fcom    ST(2)
    fnstsw  ax
    test    ah, 65                  ; 00000041H
    jne SHORT $LN4@main
    fstp    ST(0)
    fxch    ST(2)
    fst DWORD PTR _x$3834[esp+64]
    fld DWORD PTR _x$3834[esp+64]
    fxch    ST(1)
    fxch    ST(3)
    fxch    ST(1)
$LN4@main:
    fcom    ST(2)
    fnstsw  ax
    test    ah, 65                  ; 00000041H
    jne SHORT $LN14@main
    fstp    ST(0)
    fxch    ST(2)
    fst DWORD PTR _x$3834[esp+64]
    fld DWORD PTR _x$3834[esp+64]
    fxch    ST(1)
    fxch    ST(3)
    fxch    ST(1)
$LN14@main:
    fcom    ST(2)
    fnstsw  ax
    test    ah, 65                  ; 00000041H
    jne SHORT $LN15@main
    fstp    ST(0)
    fxch    ST(2)
    fst DWORD PTR _x$3834[esp+64]
    fld DWORD PTR _x$3834[esp+64]
    fxch    ST(1)
    fxch    ST(3)
    fxch    ST(1)
$LN15@main:
    fcom    ST(2)
    fnstsw  ax
    test    ah, 65                  ; 00000041H
    jne SHORT $LN16@main
    fstp    ST(0)
    fxch    ST(2)
    fst DWORD PTR _x$3834[esp+64]
    fld DWORD PTR _x$3834[esp+64]
    fxch    ST(1)
    fxch    ST(3)
    fxch    ST(1)
$LN16@main:
    fcom    ST(2)
    fnstsw  ax
    test    ah, 65                  ; 00000041H
    jne SHORT $LN17@main
    fstp    ST(0)
    fxch    ST(2)
    fst DWORD PTR _x$3834[esp+64]
    fld DWORD PTR _x$3834[esp+64]
    fxch    ST(1)
    fxch    ST(3)
    fxch    ST(1)
$LN17@main:

; 5    :    while (true)
; 6    :    {
; 7    :        float x = 1.0f;
; 8    : 
; 9    :        for (int i = 0; i < 50; i++)

    sub ecx, 1
    jne $LN5@main

; 10   :        {
; 11   :            if (0.0f < x)
; 12   :                x = 0.0f;
; 13   :        }
; 14   : 
; 15   :        if (x == 1.0f)

    fld ST(1)
    fucomp  ST(1)
    fnstsw  ax
    test    ah, 68                  ; 00000044H
    jp  $LN43@main
    fstp    ST(2)

; 16   :            printf("bad: %.2f\n", x);

    sub esp, 8
    fstp    ST(2)
    fstp    ST(1)
    fstp    QWORD PTR [esp]
    push    OFFSET ??_C@_0L@LNNNMCPH@bad?3?5?$CF?42f?6?$AA@
    call    esi

; 17   :    }

    fld1
    fldz
    add esp, 12                 ; 0000000cH
    fldz
    jmp $LN7@main
_main   ENDP
_TEXT   ENDS
END

---

I don't see how the touchpad driver would be able to produce this kind of behaviour.

My feeling is that this is a hardware issue of some kind. Have you overclocked your MacBook, by any chance? The CPU can start doing all sorts of strange things once you overclock it (Eric Raymond has some war stories to tell of this). If you're not overclocking, maybe your CPU is just getting too hot? Might be a good idea to check the cooling vents. Or maybe it's just a flaky CPU.

If it is the CPU, why does it happen only when you touch the touchpad? Pure speculation, but maybe the power drain from the touchpad lowers the CPU voltage just enough to cause it to do silly things...

---

May have something to do with FPU and optimizations. Does it happen if you define x as volatile float or compile with /O0? If it does, maybe buggy driver changes the state of the FPU.