In release mode, code behavior is not as expected

Question

The following code generates different results under debug mode and release mode (using Visual Studio 2008):

int _tmain(int argc, _TCHAR* argv[])
{

    for( int         


        

Answer 1

This is interesting, at least from a historical perspective. I can reproduce the problem with VC 2008 (15.00.30729.01) and VC 2010 (16.00.40219.01) (targeting either 32-bit x86 or 64-bit x64). The problem doesn't occur with any of the compilers I have tried starting with VC 2012 (17.00.61030).

The command I used to compile: cl /Ox vc15-bug.cpp /FAsc

Since VC 2008 (and 2010) is rather old and the fix has been in for several years now, I don't think you can expect any action from Microsoft except to use a newer compiler (though maybe someone can suggest a workaround).

The problem is that the test to determine if the value should be forced to 255 is done based on the loop count rather than the actual result of the i * 16 expression. And the compiler simply gets the count wrong for when it should start forcing the value to 255. I have no idea why that happens - it's just the effect that I see:

; 6    :    for( int i = 0; i < 17; i++ ) 

  00001 33 f6        xor     esi, esi
$LL4@main:
  00003 8b c6        mov     eax, esi
  00005 c1 e0 04     shl     eax, 4

; 7    :    { 
; 8    :        int result = i * 16;
; 9    : 
; 10   :        if( result > 255 )

  // the value `esi` is compared with in the following line should be 15!
  00008 83 fe 0e     cmp     esi, 14            ; 0000000eH
  0000b 7e 05        jle     SHORT $LN1@main

; 11   :        {
; 12   :            result = 255;

  0000d b8 ff 00 00 00   mov     eax, 255       ; 000000ffH
$LN1@main:

; 13   :        }

Update: All versions of VC I have installed earlier than VC 2008 have the same bug, except VC6 - compiling the program crashes the VC6 compiler:

vc15-bug.cpp(10) : fatal error C1001: INTERNAL COMPILER ERROR

So this is a bug that lasted in MSVC in one form or another for more than 10 years!