[Bug tree-optimization/15826] don't use "if" to extract a single bit bit-field.

[steven at gcc dot gnu dot org]

------- Comment #6 from steven at gcc dot gnu dot org  2007-04-05 22:01 -------
The tree dump for the original test case now looks like this for me:

;; Function foo (foo)

foo (p)
{
<bb 2>:
  return (unsigned int) ((BIT_FIELD_REF <*p, 8, 0> & 1) != 0);

}



;; Function bar (bar)

bar (p)
{
<bb 2>:
  return (unsigned int) p->bit;

}



The resulting assembler output is the same, but I imagine VRP should be able to
fold away the "& 1" test.  I don't know if the BIT_FIELD_REF itself should be
optimized away, but I guess so.  Consider the following test case:

struct s
{
  unsigned int bit:1;
};

unsigned int
foo (struct s *p)
{
  if (p->bit)
    return (unsigned int) (p->bit);
  else
    return 0;
}


This gets "optimized" at the tree level to this ugly code:
;; Function foo (foo)

foo (p)
{
  unsigned int D.1979;

<bb 2>:
  if ((BIT_FIELD_REF <*p, 8, 0> & 1) != 0) goto <L0>; else goto <L4>;

<L4>:;
  D.1979 = 0;
  goto <bb 5> (<L2>);

<L0>:;
  D.1979 = (unsigned int) p->bit;

<L2>:;
  return D.1979;

}

In summary, I don't think we can close this bug just yet.


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2005-12-10 04:15:46         |2007-04-05 22:01:25
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15826