[Bug middle-end/18029] New: a twiddleing of a

[Bug middle-end/18029] New: a twiddleing of a

[kazu at cs dot umass dot edu]

Consider:

struct B {
  unsigned bit : 1;
};

void
plus (struct B *b)
{
  b->bit = b->bit + 1;
}

void
xor (struct B *b)
{
  b->bit = b->bit ^ 1;
}

./cc1 -O2 -fomit-frame-pointer -mregparm=3 generates

plus:
	xorb	$1, (%eax)
	ret

xor:
	movb	(%eax), %cl
	andl	$1, %ecx
	xorl	$1, %ecx
	movzbl	%cl, %ecx
	movl	(%eax), %edx
	andl	$-2, %edx
	orl	%ecx, %edx
	movl	%edx, (%eax)
	ret

Note that the two functions do the same thing.  Obviously, we should optimize
xor just like plus.

-- 
           Summary: a twiddleing of a
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029

[Bug middle-end/18029] an xor of a single bit bitfield is inefficient

[kazu at cs dot umass dot edu]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|a twiddleing of a           |an xor of a single bit
                   |                            |bitfield is inefficient


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029

[Bug middle-end/18029] an xor of a single bit bitfield is inefficient

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-16 03:05 -------
Confirmed on PPC also.  (note for PPC to get about the same code we have to use the following 
definition for the struct:
struct B {
  unsigned unused: 31;
  unsigned bit : 1;
};

).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-10-16 03:05:12
               date|                            |
            Version|unknown                     |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029

[Bug middle-end/18029] an xor of a single bit bitfield is inefficient

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-10-16 15:49 -------
A patch here:
http://gcc.gnu.org/ml/gcc-patches/2004-10/msg01333.html


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029

[Bug middle-end/18029] an xor of a single bit bitfield is inefficient

[giovannibajo at libero dot it]

------- Additional Comments From giovannibajo at libero dot it  2004-10-17 11:26 -------
Approved for 4.1.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |17652
              nThis|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029

[Bug middle-end/18029] an xor of a single bit bitfield is inefficient

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029

[Bug middle-end/18029] an xor of a single bit bitfield is inefficient

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-03-02 16:05 -------
Subject: Bug 18029

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	kazu@gcc.gnu.org	2005-03-02 16:05:00

Modified files:
	gcc            : ChangeLog expr.c 

Log message:
	PR middle-end/18029 and PR middle-end/18030
	* expr.c (optimize_bitfield_assignment_op): Add a special case
	to handle BIT_IOR_EXPR and BIT_XOR_EXPR.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.7632&r2=2.7633
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/expr.c.diff?cvsroot=gcc&r1=1.778&r2=1.779



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029

[Bug middle-end/18029] an xor of a single bit bitfield is inefficient

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2005-03-02 16:07 -------
Just checked in a patch.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18029