[Bug optimization/14846] New: [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[Bug optimization/14846] New: [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[kazu at cs dot umass dot edu]

I wonder if the shift in (a >> 3) & 1 == 0 does any good for tree-ssa.

Consider:

void bar (void);

void
foo (unsigned int a)
{
  if ((a & 8) != 0)
    bar ();
}

I get:

  T.0_2 = a_1 >> 3;
  T.1_3 = (int)T.0_2;
  T.2_4 = T.1_3 & 1;
  T.3_5 = (_Bool)T.2_4;
  if (T.3_5) goto <L0>; else goto <L1>;

Even if Andrew Pinski removes the _Bool and int cast,

  T.0_2 = a_1 >> 3;
  T.2_4 = T.0_2 & 1;
  if (T.2_4 != 0) goto <L0>; else goto <L1>;

Without the shift,

  T.2_4 = a_1 & 1;
  if (T.2_4 != 0) goto <L0>; else goto <L1>;

Without the shift, it's a lot easier to combine two consecutive bit tests like

  if (a & 1) goto there;
  if (a & 4) goto there;

into

  if (a & 5) goto there;

Removing the shift should also make it easier to solve PR 14752
because we don't have to look as many statements to do the optimization.

If we need the shift for some machine-specific reasons,
we should introduce it very late in tree-ssa optimizations or at expand time.

-- 
           Summary: [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0
                    until very late in tree-ssa optimizations
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: optimization
        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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[kazu at cs dot umass dot edu]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |roger at eyesopen dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|14847                       |
              nThis|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[kazu at cs dot umass dot edu]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |14847
              nThis|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-04-04 16:56 -------
Confirmed, rewritting fold will help.  Aka we should have an early fold and a late fold.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |memory-hog, pessimizes-code
   Last reconfirmed|0000-00-00 00:00:00         |2004-04-04 16:56:51
               date|                            |
   Target Milestone|---                         |tree-ssa


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug tree-optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-05-15 11:45 -------
As of today, the last tree-ssa form of foo() looks like:

foo (a)
{
  _Bool T.3;
  int T.2;
  int T.1;
  unsigned int T.0;

<bb 0>:
  T.0_2 = a_1 >> 3;
  T.1_3 = (int)T.0_2;
  T.2_4 = T.1_3 & 1;
  if (T.2_4 != 0) goto <L0>; else goto <L1>;

<L0>:;
  bar () [tail call];

<L1>:;
  return;

}

Note that the cast to _Bool is gone.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug tree-optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|3.5.0                       |---


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug tree-optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-04-21 00:39 -------
Subject: Bug 14846

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	kazu@gcc.gnu.org	2005-04-21 00:39:25

Modified files:
	gcc            : ChangeLog fold-const.c 

Log message:
	PR tree-optimization/14846
	* fold-const.c (fold_single_bit_test_into_sign_test): New,
	split out from ...
	(fold_single_bit_test): ... here.
	(fold_binary): Call fold_single_bit_test_into_sign_test
	instead of fold_single_bit_test.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.8381&r2=2.8382
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/fold-const.c.diff?cvsroot=gcc&r1=1.566&r2=1.567



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846

[Bug tree-optimization/14846] [tree-ssa] don't use a shift in A & CST_POWER_OF_2 == 0 until very late in tree-ssa optimizations

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2005-04-21 00:40 -------
Just checked in a patch.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.1.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14846