[Bug other/17108] New: Missed opportunity for strength reduction

[Bug other/17108] New: Missed opportunity for strength reduction

[gcc-bugzilla at gcc dot gnu dot org]

Description:
A non-optimal code sequence is illustraded.    Duplicate using gcc 3.5 and
command line:

gcc -O3 -m64 -c test.c


Testcase:
void foo(float *data, float d) {
   long i;

   for (i = 0; i < 8; i++)
      data[i] = d;
}

Assembly:
Code generated by gcc 3.5:

.foo:
      li 0,8
      li 9,0
      mtctr 0
.L2:
      sldi 0,9,2
      addi 9,9,1
      stfsx 1,3,0
      bdnz .L2
      blr

We can eliminate the multiply(shift) as well as the increment of "i" and
use
a store with update form to simplify the loop to this:

.foo:
      ai 3,3,-4
      li 0,8
      mtctr 0
.L2:
      stfsu 1,3,4
      bdnz .L2
      blr




-- 
           Summary: Missed opportunity for strength reduction
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P1
         Component: other
        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=17108

[Bug other/17108] Missed opportunity for strength reduction

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-08-19 15:25 -------
Confirmed, I thought I saw another bug asking for the store with update being used more but I cannot 
find it right now, it might have just been to the gcc@ mailing list though.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2004-08-19 15:25:53
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17108

[Bug other/17108] Missed opportunity for strength reduction

[nathan at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |nathan at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17108

[Bug other/17108] Missed opportunity for strength reduction

[nathan at gcc dot gnu dot org]

------- Additional Comments From nathan at gcc dot gnu dot org  2004-11-03 11:25 -------
The situation has improved, as the biv has now been spotted, and the loop
turned into,
.foo:
        li 0,8
        mtctr 0
        .align 4
.L2:
        stfs 1,0(3)
        addi 3,3,4
        bdnz .L2
        blr

Unfortunately this has placed the increment after the load, and so
cannot be merged into it.  If the loop optimizer knew the target had
pre-inc, it might be able to swap them round by biasing the pointer
before the loop. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|nathan at gcc dot gnu dot   |unassigned at gcc dot gnu
                   |org                         |dot org
             Status|ASSIGNED                    |NEW


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17108

[Bug target/17108] Missed opportunity for strength reduction

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-11-03 15:33 -------
Corresponding code which gives what is wantted here is:
void foo(float *data, float d) {
   long i;

   data--;
   for (i = 0; i < 8; i++)
     {
      data[1] = d;
      data++;
     }
}

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2004-08-19 15:25:53         |2004-11-03 15:33:17
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17108