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      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 interpeted 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 }
     43