[Bug target/36503] x86 can use x >> -y for x >> 32-y

["bonzini at gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #5 from bonzini at gnu dot org  2009-11-07 09:21 -------
bt with a memory object and a register index will _not_ truncate the argument:

static int btm(int *a, int b) {
  asm ("btl %2, %1; movl $0, %0; setc %b0" : "=r" (b) : "m" (*a), "0" (b));
  return b;
}

static int btr(int a, int b) {
  asm ("btl %2, %1; movl $0, %0; setc %b0" : "=r" (b) : "r" (a), "0" (b));
  return b;
}

#define btmi(a, b) ({ int __x; \
  asm ("btl %2, %1; movl $0, %0; setc %b0" : "=r" (__x) : "m" (*a), "i" (b)); \
  __x; \
})

int main () {
  int y[3] = { 0x5, 0, 0x5 };
  printf ("bt %%reg, %%reg: %d %d %d\n", btr(5, 0), btr(5, 32), btr(5, 64));
  printf ("bt $bit, (mem): %d %d %d\n", btmi(y, 0), btmi(y, 32), btmi(y, 64)); 
  printf ("bt %%reg, (mem): %d %d %d\n", btm(y, 0), btm(y, 32), btm(y, 64));
}


>> bt %reg, %reg: 1 1 1
>> bt $val, (mem): 1 1 1
>> bt %reg, (mem): 1 0 1


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36503