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

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|gcc-bugs at gcc dot gnu.org |

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-01-24 00:03:45 UTC ---
(In reply to comment #6)
Only comment #6 remains to be fixed.  
FRE does not recognize p->bit as "BIT_FIELD_REF <*p, 8, 0> & 1".

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

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=15826
Bug 15826 depends on bug 15459, which changed state.

Bug 15459 Summary: [meta-bug] there should be a tree combiner like the rtl one
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=15459

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

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

[rguenth at gcc dot gnu.org]

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

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Which boils down to the premature fold-const.c:optimize_bit_field_compare
which creates this BIT_FIELD_REF (fold_truth_andor_1 does similar stupid
stuff).

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

[law at redhat dot com]

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

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |law at redhat dot com
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=81601
         Resolution|---                         |FIXED

--- Comment #19 from Jeffrey A. Law <law at redhat dot com> ---
So the core issue here, using an if, conditional moves and the like to do
single bit field extraction/testing is resolved.

There is still an issue of canonicalizing the two representations which in turn
opens up the possibility of finding more common subexpressions when both forms
might be used.  That is actually being tracked via pr81601.

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

[rguenth at gcc dot gnu dot org]

------- Comment #7 from rguenth at gcc dot gnu dot org  2008-04-30 19:21 -------
On the trunk I have after phiopt1:

;; Function andrew (andrew)

Removing basic block 3
Merging blocks 2 and 4
andrew (struct s * p)
{
  _Bool D.1212;
  int i;
  unsigned int D.1183;
  <unnamed-unsigned:1> D.1182;

<bb 2>:
  D.1182_3 = p_2(D)->bit;
  (void) 0;
  D.1212_9 = D.1182_3 != 0;
  i_10 = (int) D.1212_9;
  D.1183_6 = (unsigned int) i_10;
  return D.1183_6;

}

;; Function foo (foo)

Removing basic block 3
Merging blocks 2 and 4
foo (struct s * p)
{
  _Bool D.1199;
  unsigned int D.1177;
  <unnamed-unsigned:1> D.1176;

<bb 2>:
  D.1176_3 = p_2(D)->bit;
  (void) 0;
  D.1199_8 = D.1176_3 != 0;
  D.1177_9 = (unsigned int) D.1199_8;
  return D.1177_9;

}

which the next forwprop pass optimizes to

foo (struct s * p)
{
  unsigned int D.1177;
  <unnamed-unsigned:1> D.1176;

<bb 2>:
  D.1176_3 = p_2(D)->bit;
  D.1177_9 = D.1176_3 != 0;
  return D.1177_9;

}

andrew (struct s * p)
{
  unsigned int D.1183;
  <unnamed-unsigned:1> D.1182;

<bb 2>:
  D.1182_3 = p_2(D)->bit;
  D.1183_6 = D.1182_3 != 0;
  return D.1183_6;

}


-- 


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

[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

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

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2006-03-02 14:24 -------
This is interesting, we now get BIT_FIELD_REF.


-- 


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

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

[pinskia at gcc dot gnu dot org]

-- 
Bug 15826 depends on bug 15618, which changed state.

Bug 15618 Summary: Missed bool optimization
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15618

           What    |Old Value                   |New Value
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

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

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

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-06-16 06:20 -------
Now that we have a single return statement, let me regenerate t50.tailc.

struct s {
  unsigned int bit : 1;
};

unsigned int
foo (struct s *p)
{
  if (p->bit)
    return 1;
  else
    return 0;
}

unsigned int
bar (struct s *p)
{
  return (unsigned int) (p->bit);
}

unsigned int
andrew (struct s *p)
{
  int i;
  if (p->bit)
    i = 1;
  else
    i = 0;
  return i;
}

I get:

;; Function foo (foo)

foo (p)
{
  _Bool T.7;
  unsigned int T.2;
  int T.1;
  unsigned int T.0;

<bb 0>:
  T.0_3 = p_2->bit;
  T.7_8 = T.0_3 != 0;
  T.2_1 = (unsigned int)T.7_8;
  return T.2_1;

}



;; Function bar (bar)

bar (p)
{
  unsigned int T.3;

<bb 0>:
  T.3_2 = p_1->bit;
  return T.3_2;

}



;; Function andrew (andrew)

andrew (p)
{
  _Bool T.16;
  int i;
  unsigned int T.6;
  int T.5;
  unsigned int T.4;

<bb 0>:
  T.4_3 = p_2->bit;
  T.16_9 = T.4_3 != 0;
  i_1 = (int)T.16_9;
  T.6_5 = (unsigned int)i_1;
  return T.6_5;

}

So basically, the problems boils down to the tree combiner.


-- 


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

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

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-06-07 18:05 -------
I should note now that PR 15268 is fixed, the only thing that is needed is for one-wide-bitfield != 0 
needs to be converted to (_Bool)one-wide-bitfield.

-- 


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

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

[pinskia at gcc dot gnu dot org]

-- 
Bug 15826 depends on bug 15268, which changed state.

Bug 15268 Summary: [tree-ssa] return statements are not combined on the tree level
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15268

           What    |Old Value                   |New Value
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

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

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

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-06-04 19:26 -------
Here is another example:
struct s {
  unsigned int bit : 1;
};

unsigned int
foo (struct s *p)
{
  int i;
  if (p->bit)
    i = 1;
  else
    i = 0;
  return i;
}

I will note that the example in comment #1 needs to have the return values merged together before the 
real optimization happens (see PR 15268 and a patch which I posted but have not gotten around to 
answering/tying  RTH's question/seguestion yet).

But basically it can be optimization by putting a cast to bool first so a != 0 becomes (bool)a and PHI-
opt already optimizated out the branches.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |15268


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

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

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-06-04 19:07 -------
Confirmed, this is related to bug 15618 which is for bools.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |15618
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-06-04 19:07:43
               date|                            |


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