Microsoft KB Archive/119871

= FIX: Optimization Bug Using Shift Operators and outp =

Article ID: 119871

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition

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This article was previously published under Q119871



SYMPTOMS
Optimizing the shift operators may generate incorrect code when used in conjunction with outp. In the sample code in the "MORE INFORMATION" section, below, no code is generated for the line "addr >>= 8;".



RESOLUTION
Disable optimization, or perform the shift operation directly in the argument list of the function outp.



STATUS
Microsoft has confirmed this to be a problem in the Microsoft products listed above. This problem was corrected in Visual C++, 32-bit Edition, version 2.0.



MORE INFORMATION
When compiled with /Ox, the following sample does not generate code for the line "addr >>= 8;"

NOTE: When the line of code marked "3" replaces the lines marked "1" and "2", "mov al,ah" is no longer incorrectly optimized out of the assembler listing produced by /Fc.

Sample Code
// Compile options needed: /c /Ox /Fc

unsigned long test(char * baseport, unsigned long addr) {      unsigned long   offset;

offset = ((addr >> 3) & 0x00003fff);

addr >>= 8;                    //Line 1 outp(baseport + 4, addr);      //Line 2

// Replacing the above two lines of code with the line below // fixes the problem.

//      outp(baseport + 4, addr >> 8);  //Line 3--okay with /Ox.

return offset; }

Additional query words: 1.00 8.00

Keywords: kbbug kbfix KB119871

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