Q40756: Bad Code for Expression Parameter of outp() with -Oi

Article: Q40756
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# G881205-7390 | mspl13_c
Last Modified: 15-JAN-1990

When compiling the source line below, the Microsoft C Compiler Version
5.10 generates incorrect code in some cases for the intrinsic outp and
outpw functions when the -Oi option is used and the second parameter
is an expression. Workarounds are listed below.

The following is the C source line and the generated code:

        outp(port + INT_ENABLE_OFF, (i == SIO_CNT) ? 3 : 1);

            cmp    WORD PTR [bp-6], 8                     ;i
            je    $L20003                                 ;error !
            mov   ax, 3
            jmp   SHORT $L20004
$L20003:
            mov   ax, 1
$L20004:
            push  ax
            mov   ax,   WORD PTR [bp-4]                   ;port
            inc   ax
            push  ax
            call  FAR PTR _outp

This assembly code would be equal to a source line such as the
following:

   outp(port + IN_ENABLE_OFF, (i == SIO_CNT) ? 1 : 3);

This is the exact reverse of the original source line. Therefore, the
generated assembler code should read as follows:

            cmp    WORD PTR [bp-6], 8                     ;i
            jne    $L20003                                ;
            mov   ax, 3
            jmp   SHORT $L20004
$L20003:
            mov   ax, 1
$L20004:
            push  ax
            mov   ax,   WORD PTR [bp-4]                   ;port
            inc   ax
            push  ax
            call  FAR PTR _outp

There are two workarounds:

1. Use a temporary variable -- for example:

       x = (i == SIO_CNT) ? 1 : 3;
       outp(port + stuff, x);

2. Don't use -Oi.

3. Use -Oi, but use the "#pragma function(outp outpw)" statement to
   have the non-intrinsic version of the function used.

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