## FIX: /G3 Optimization Generates Bad Code for Struct Pointers

Q116137

    1.00
    WINDOWS
    kbprg kbfixlist kbbuglist

    ---------------------------------------------------------------------
    The information in this article applies to:

     - The Microsoft C/C++ Compiler (CL.EXE) included with:
       Microsoft Visual C++ for Windows, version 1.0
    ---------------------------------------------------------------------

    SYMPTOMS
    ========

    The /G3 option, used to generate 386 instructions, may generate
    incorrect instructions when attempting to access fields within a
    structure.

    RESOLUTION
    ==========

    There are two workarounds to this problem:

     - Compile using the /G2 option.

       - or -

     - Disable optimization locally using the optimize pragma:

             #pragma optimize("",off)

             void mycode(void)
             {
             }

             #pragma optimize("",on)

    STATUS
    ======

    Microsoft has confirmed this to be a problem with the C/C++ compiler,
    version 8.0. The problem was corrected with C/C++, version 8.0c,
    supplied with Microsoft Visual C++ for Windows, version 1.5. We are
    researching this problem and will post new information in the
    Microsoft Knowledge Base as soon as it becomes available.

    MORE INFORMATION
    ================

    The sample code below can be used to illustrate this problem. If
    you use the /Fc option to generate mixed-source and assembly code,
    the line commented below generates the following code:

       ;|***    t2 = (lpDisp->right - lpDisp->left);
       ; Line 22
         *** 000027  c4 5e 0c         les bx,DWORD PTR [bp+12]          ;lpDisp
         *** 00002a  66 26 8b 47 0c   mov eax,DWORD PTR es:[bx+12]
         *** 00002f  66 26 2b 47 04   sub eax,DWORD PTR es:[bx+4]
         *** 000034  89 46 f8         mov WORD PTR [bp-8],ax        ;t2
         *** 000037  89 56 fa         mov WORD PTR [bp-6],dx

    NOTE: The first three lines of assembly code correctly perform the
    arithmetic operations, but the last two lines do not use the extended
    registers "EAX" and "EDX", but "AX" and "DX". Compiling using the /G2
    switch instead of the /G3 switch generates correct register use.

    Sample Code
    -----------

    /* Compile options needed: /c /G3
    */

       #include <windows.h>

       struct displayinfo
       {
        long leftlim, left, rightlim, right;
       };

       void HorizScrollProc(HWND hWnd, WORD wParam, long lParam,
                            struct displayinfo far *lpDisp)
       {
        long t1,t2;

        switch (wParam)
        {
         case SB_LINEUP:
           t1 = (lpDisp->left + lpDisp->right) / 2;
           t2 = (lpDisp->right - lpDisp->left);        /* bad code here */
           if (lpDisp->leftlim + t2 / 8 > lpDisp->left)
           {
            lpDisp->left = lpDisp->leftlim;
            lpDisp->right = lpDisp->leftlim + t2;
           }
           else
           {
            lpDisp->left -= t2 / 8;
            lpDisp->right -= t2 / 8;
           }
           break ;

         default:
           return;
           break;
        }
       }

    Additional reference words: 1.00 8.00
    KBCategory: kbprg kbfixlist kbbuglist
    KBSubcategory: CLIss

Keywords : kb16bitonly kbCompiler
Issue type :
Technology :

Last Reviewed: September 23, 1997 © 2001 Microsoft Corporation. All rights reserved. Terms of Use.