[Bug middle-end/53986] New: missing vrp on bit-mask test

[vries at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53986

             Bug #: 53986
           Summary: missing vrp on bit-mask test
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: vr...@gcc.gnu.org


Consider the following test-case:
...
/* { dg-do link } */
/* { dg-options "-O2" } */

/* Based on f3 from vrp63.c, but with switch instead of if-chain.  */

extern void link_error (void);

void
f3 (int s)
{
  if (s >> 3 == -2)
    /* s in range [ -16, -9].  */
    ;
  else
    {
      /* s in range ~[-16, -9], so none of the case labels can be taken.  */
      switch (s)
    {
    case -16:
    case -12:
    case -9:
      link_error ();
      break;
    default:
      break;
    }
    }
}

int
main ()
{
  return 0;
}
...

the switchconv dump shows that the switch is converted to a bit-test:
...
;; Function f3 (f3, funcdef_no=0, decl_uid=1710, cgraph_uid=0)

beginning to process the following SWITCH statement (vrp72.c:18) : ------- 
switch (s_1(D)) <default: <L8>, case -16: <L2>, case -12: <L2>, case -9: <L2>>

  expanding as bit test is preferable
Switch converted
--------------------------------
f3 (int s)
{
  _Bool D.1736;
  long unsigned int D.1735;
  long unsigned int D.1734;
  long unsigned int csui.1;
  _Bool D.1732;
  int D.1730;
  unsigned int D.1731;
  int D.1720;

<bb 2>:
  D.1720_2 = s_1(D) >> 3;
  if (D.1720_2 == -2)
    goto <bb 5> (<L8>);
  else
    goto <bb 3>;

<bb 3>:
  D.1730_6 = s_1(D) + 16;
  D.1731_7 = (unsigned int) D.1730_6;
  D.1732_8 = D.1731_7 > 7;
  if (D.1732_8 != 0)
    goto <bb 6> (<L9>);
  else
    goto <bb 7>;

<bb 7>:
  D.1734_10 = (long unsigned int) D.1731_7;
  csui.1_9 = 1 << D.1734_10;
  D.1735_11 = csui.1_9 & 145;
  D.1736_12 = D.1735_11 != 0;
  if (D.1736_12 != 0)
    goto <bb 4> (<L2>);
  else
    goto <bb 8>;

<bb 8>:

<L9>:
  goto <bb 5> (<L8>);

<L2>:
  link_error ();

<L8>:
  return;

}
...


vrp doesn't manage to remove the path to function link_error.

Test-case vrp63 uses 'if (s == -16 || s == -12 || s == -9)' instead of a
switch. In that case, the path to link_error is removed.

Btw, if the switch is not converted to a bit-test the path to link_error is
also not removed by vrp, because it doesn't handle anti-ranges for switches.