none/tests/x86/shift_ndep.c

   1 #include "tests/asm.h"
   2 #include <stdio.h>
   3
   4 /* Test whether a shift by zero properly preserves the CC_NDEP thunk. */
   5
   6 /* Check whether the carry flag is properly preserved by a variable
   7    shift when the shift amount happens to be zero. */
   8 int shift_ndep( void )
   9 {
  10   char shift_amt = 0;
  11   int x = -2;
  12   /* First we set the carry flag. Then we increment %x, which sets
  13      CC_OP to X86G_CC_OP_INCL and stores the carry (1) in
  14      CC_NDEP. Then we left shift %x by a variable amount that happens
  15      to be zero, which should leave both %x and all the flags
  16      unchanged. Then we add-with-carry 0 to %x, which (assuming the
  17      carry is still set as it should be) increments %x again. Thus the
  18      expected final value for x is -2 + 1 + 1 = 0.
  19
  20      If instead the shift clears CC_NDEP (as it would legally do if
  21      the shift amount were non-zero), this will be interpreted as
  22      clearing the carry bit, so the adc will be a no-op and the final
  23      value of %x will instead be -1.
  24   */
  25   asm (
  26        "stc"                    "\n\t"
  27        "inc %[x]"               "\n\t"
  28        "shl %[shift_amt], %[x]" "\n\t"
  29        "adc $0, %[x]"           "\n\t"
  30        : [x] "+r" (x) : [shift_amt] "c" (shift_amt));
  31   return x;
  32 }
  33
  34 int main ( void )
  35 {
  36   int r = shift_ndep();
  37   if (r == 0)
  38     printf("Passed (%d).\n", r);
  39   else
  40     printf("Failed (%d).\n", r);
  41   return 0;
  42 }