Q65308: Default Optimization Generates Bad Code in Switch Construct

Article: Q65308
Product(s): See article
Version(s): 6.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist6.00 | mspl13_c
Last Modified: 24-OCT-1990

The following sample code is compiled incorrectly when the default
optimizations are used. When compiled with /Ot, /On, /Oc, or the
default options (/Ocnt), the program fails to recognize certain keys
as "default" (function keys and the top row of number keys 1, 2, 3,
etc.). When compiled without the offending optimizations, all keys are
recognized.

The workaround is to use the optimize pragma, as follows:

   #pragma optimize("tn","off")

Or, use the quick compile option /qc.

Sample Code
-----------

#include <stdio.h>
#include <bios.h>
#include <stdlib.h>

#define ESC   0x1b
#define HT    0x09
#define S_TAB 0x0f
#define RT    0x4d
#define BS    0x08
#define CR    0x0d

void main(void)
{
   unsigned c;

   printf("\n");
   while (1)
   {
      c = _bios_keybrd(_KEYBRD_READ);

      if (! (c&0xff))
         c >>= 8;
      else
         c &= 0xff;
      switch (c)
      {
         case ESC:
            printf("ESCAPE\n");
            exit(0);
         case HT:
            printf("HT\n");
            break;
         case S_TAB:
            printf("SHIFTED TAB\n");
            break;
         case RT:
            printf("RIGHT ARROW\n");
            break;
         case BS:
            printf("BACK SPACE\n");
            break;
         case CR:
            printf("CARRIAGE RETURN\n");
            break;
         default:
            printf("DEFAULT\n");
            break;
      }
   }
}
Microsoft has confirmed this to be a problem in C version 6.00. We are
researching this problem and will post new information here as it
becomes available.