FIX: Optimization Bug Using Shift Operators and outp()

The information in this article applies to:

• The C/C++ Compiler (CL.EXE), included with

      - Microsoft Visual C++, 32-bit Edition, version 1.0
  

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;
   }