[Bug c/16632] New: A couple fold-const.c optimizations are non-functional

[Bug c/16632] New: A couple fold-const.c optimizations are non-functional

[trt at acm dot org]

In fold-const.c there are a couple of bitmask/comparision optimizations:
      /* If we have (A & C) == D where D & ~C != 0, convert this into 0.
and
      /* If we have (A | C) == D where C & ~D != 0, convert this into 0.

Sample C code:
  void subr (int i)
  {
    if ((i|3)  == 1) return; /* never taken */
    if ((i&4)  == 2) return; /* never taken */
  }


Unfortunately fold-const.c neglects to fold ~C (~D) and so
the detection of these cases is non-functional.
I'm not sure how to demonstrate this outside the debugger,
since later pieces of gcc seem to clean this up.

For example ~D is computed on approx line 8391 with

                            build1 (BIT_NOT_EXPR, TREE_TYPE (arg1), arg1)));

This should be fold(build1 ...), or fold_not_const(arg1, TREE_TYPE(arg1)).
Similarly for ~C on approx line 8373.

-- 
           Summary: A couple fold-const.c optimizations are non-functional
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: trt at acm dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632

[Bug tree-optimization/16632] A couple fold-const.c optimizations are non-functional

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-07-19 18:35 -------
Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|minor                       |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|c                           |tree-optimization
     Ever Confirmed|                            |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2004-07-19 18:35:45
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632

[Bug tree-optimization/16632] A couple fold-const.c optimizations are non-functional

[trt at acm dot org]

------- Additional Comments From trt at acm dot org  2004-07-20 16:50 -------
Silly me, the demo code should have returned a value:
  int subr(int i)
  {
     if ((i|3)  == 1) return 1; /* never taken */
     if ((i&4)  == 2) return 1; /* never taken */
     return 0;
  }
This should simplify to "return 0;" but the assembler code retains the 'or'
and 'and' even at high optimization levels.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632

[Bug tree-optimization/16632] A couple fold-const.c optimizations are non-functional

[trt at acm dot org]

------- Additional Comments From trt at acm dot org  2004-08-24 14:41 -------
Could someone please review and apply this patch?
In my opinion, the problem and patch are quite straightforward.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632

[Bug tree-optimization/16632] A couple fold-const.c optimizations are non-functional

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-03 14:43 -------
Patch here: <http://gcc.gnu.org/ml/gcc-patches/2004-10/msg00194.html>.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632

[Bug tree-optimization/16632] A couple fold-const.c optimizations are non-functional

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2004-10-03 15:33 -------
Subject: Bug 16632

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	kazu@gcc.gnu.org	2004-10-03 15:33:21

Modified files:
	gcc            : ChangeLog 
	gcc/testsuite  : ChangeLog 

Log message:
	PR tree-optimization/16632
	* fold-const.c (fold) [EQ_EXPR]: When seeing if D & ~C != 0 to
	fold (A & C) == D into 0, fold ~C.  Similarly, for the case
	where | is used instead of &.
	
	PR tree-optimization/16632
	* testsuite/gcc.dg/tree-ssa/20041002-1.c: New.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.5730&r2=2.5731
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.4374&r2=1.4375



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632

[Bug tree-optimization/16632] A couple fold-const.c optimizations are non-functional

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-10-03 15:34 -------
Just checked in the fix.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632

[Bug tree-optimization/16632] A couple fold-const.c optimizations are non-functional

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16632