PRB: Generating P-Code Disables Native Code Optimization

The information in this article applies to:

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

      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SYMPTOMS

In Microsoft C/C++ version 7.0 and Visual C++ version 1.0, generating p- code with the /Oq compiler option or with the following pragma

   #pragma optimize( "q", on )

   #pragma optimize( "q", off )

CAUSE

This is a limitation of the p-code compiler. Any native code functions in a module compiled for p-code will get a /f level of optimization.

RESOLUTION

Break the module into two modules, one that uses native code and another that uses p-code.

MORE INFORMATION

The following sample code can be used to demonstrate the behavior. When compiled with /Olz, the code generated for the for loop places a 1 into j and returns. With /Oqlz, this optimization does not occur.

Sample Code

/* Compile options needed: /Oqlz /Fc
   Compile options needed: /Olz /Fc

   Compare the resulting listings generated by /Fc
*/

int main( )

   int i, j;

   for( i=0; i<10; i++ )    /* This loop should be optimized away. */
   {
      j = 1;
      j = 1;
      j = 1;
      j = 1;
      j = 1;
   }

   return j;