[Bug regression/16456] New: PowerPC - redundant subtract involving pointer types

[Bug regression/16456] New: PowerPC - redundant subtract involving pointer types

[gcc-bugzilla at gcc dot gnu dot org]

Description:
A non-optimal code sequence is illustrated.  Two subtract operations are
done where only one is necessary.  Duplicate using gcc 3.5 and command
line:

gcc -O3 -m64 -c test.c

Testcase:
char * ptr1;
char * ptr2;
int x;
long int y;

void foo ()
{
    x = ptr1 - ptr2;
    y = ptr1 - ptr2;
}

Assembly:
      ld 9,.LC0@toc(2)
      ld 11,.LC1@toc(2)
      ld 0,0(9)
      ld 10,0(11)
      ld 8,.LC3@toc(2)
      subf 11,10,0
      ld 9,.LC2@toc(2)
      subf 0,10,0  -- no need to do this subtract, just use the previous
result.
      std 11,0(8)
      stw 0,0(9)
      blr



-- 
           Summary: PowerPC - redundant subtract involving pointer types
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P1
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steinmtz at us dot ibm dot com
                CC: gcc-bugs at gcc dot gnu dot org,steinmtz at us dot ibm
                    dot com
 GCC build triplet: powerpc64-linux
  GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16456

[Bug other/16456] PowerPC - redundant subtract involving pointer types

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|regression                  |other
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16456

[Bug rtl-optimization/16456] PowerPC - redundant subtract involving pointer types

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-07-09 19:15 -------
Confirmed, the problem is they are different modes.
(set (reg:SI 124)
        (minus:SI (subreg:SI (reg:DI 119 [ ptr1.1 ]) 4)
            (subreg:SI (reg:DI 120 [ ptr2.4 ]) 4)))

(set (reg:DI 126)
        (minus:DI (reg:DI 119 [ ptr1.1 ])
            (reg:DI 120 [ ptr2.4 ])))

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|other                       |rtl-optimization
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-07-09 19:15:24
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16456

[Bug rtl-optimization/16456] PowerPC - redundant subtract involving pointer types

[nathan at gcc dot gnu dot org]

------- Additional Comments From nathan at gcc dot gnu dot org  2004-11-03 11:20 -------
What is happening is that convert_to_integer, which is used to convert the
subtraction result to the type of 'x', reassociates the cast from
  (int)((long)ptr1 - (long)ptr2)
to
  (int)ptr1 - (int)ptr2
As it thinks that's a win.  Unfortunately this stops CSE noticing that
the later subtraction is actually the same.

This is another example of premature tree optimization, and it should be addressed
in the reworking of fold itself.  I don't think this should be attempted at stage 3

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16456