Microsoft KB Archive/136799

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BUG: Wrong Structure Member Modified with /Ox or /Oeg

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Q136799

--- The information in this article applies to: - The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52 --- SYMPTOMS ======== When the /Ox or /Oeg optimizations are used with the code listed in this article, the wrong structure member is modified when a huge pointer is incremented. RESOLUTION ========== Remove the /Ox or /Oeg optimizations from the command line or use the optimize pragma to disable them from the function that is generating the incorrect code as in this example: #pragma optimize("eg",off) IncrementFunction{...} #pragma optimize("eg",on) Another option is to modify the code slightly or change its location. For example, in the following sample if the code from the IncrementFunction is moved to the calling function, it works correctly. STATUS ====== Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available. MORE INFORMATION ================ Sample Code to Reproduce Problem /* Compile options needed: /AL and /Ox or /Oeg */ #include #include typedef struct { long x; long y; } StructA, huge * pStructA; typedef struct { short points; pStructA pPoints; } StructB, huge * pStructB; #define NumStructs 2 /* number of StructB structures */ #define NumPoints 5 /* number of points per StructB */ void IncrementFunction( pStructB pStructBs, short numpoints) { short i=0; for (i = 0; i < NumStructs; i++) { if (pStructBs[i].pPoints > pStructBs->pPoints) // The points member is incorrectly modified on the next line. pStructBs[i].pPoints += numpoints; } } void main(void) { pStructB StructBs = halloc(NumStructs, sizeof(StructB)); pStructA datapoints = halloc((NumStructs+1)*NumPoints, sizeof(StructA)); short i=0; for (i = 0; i < (NumStructs+1)*NumPoints; i++) { datapoints[i].x = datapoints[i].y = (long)i; } for (i = 0; i < NumStructs; i++) { StructBs[i].points = 10; StructBs[i].pPoints = &datapoints[i*NumPoints]; } IncrementFunction(StructBs, NumPoints); for (i = 0; i < NumStructs; i++) { if (StructBs[i].points != 10) printf("Error: Value should be 10 and it is %d\n", StructBs[i].points ); } } Additional query words: 8.00c

Keywords : kb16bitonly kbCodeGen

Issue type : kbbug

Technology : kbVCsearch kbAudDeveloper kbCVCComp