DOCUMENT:Q42761 25-JUL-2001 [visualc] TITLE :PRB: Use /Op to Avoid Loss of Precision from Optimization PRODUCT :Microsoft C Compiler PROD/VER:winnt: OPER/SYS: KEYWORDS:kb16bitonly kbprb ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - The C/C++ Compiler (CL.EXE), included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 6.0, 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++, versions 1.0, 1.5 ------------------------------------------------------------------------------- SYMPTOMS ======== The program below fails when compiled with default optimizations with C 6.x and with any optimizations except /Op under the later versions of the compiler. The program prints the following with optimizations enabled: 23.31 * 100 = 2330 However, if compiled with /Od, the following is printed: 23.31 * 100 = 2331 The problem seems to be in the conversion of the double to int; the rounding apparently is done incorrectly. CAUSE ===== When the code is optimized, the result of the division is kept on the floating-point stack rather than being stored into memory and reloaded onto the stack. This causes precision problems, because the numbers on the stack are stored in extended (10- byte) precision, while doubles in memory are stored using 8 bytes. NOTE: The constant 23.31 cannot be represented exactly in the binary floating-point scheme used by Microsoft C. The fact that it is represented slightly small and then multiplied by 100 (magnifying the error) contributes to this problem. RESOLUTION ========== To avoid this problem, use the /Op option when compiling. The /Op switch forces in-memory storage of intermediate results. /Op can be combined with other optimizations (for example, /Oxp). This problem does not occur in Visual C++ 32-bit Edition. MORE INFORMATION ================ Sample Code ----------- /* Compile options needed: none */ #include void main( ) { int a; double f; f = 23.31; f = f * 100; a = (int) f; printf( "23.31 * 100 = %d\n", a ); } Additional query words: 1.00 1.50 6.00 6.00a 6.00ax 7.00 8.00 8.00c ====================================================================== Keywords : kb16bitonly kbprb Technology : kbVCsearch kbAudDeveloper kbCVCComp Version : winnt: Issue type : kbprb ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.