[Bug optimization/14886] New: strength reduction on floating point

[Bug optimization/14886] New: strength reduction on floating point

[amodra at bigpond dot net dot au]

/* I found an interesting xlc strength reduction optimization recently,
   that had xlc producing fp code that ran over twice as fast as gcc
   code on a powerpc benchmark.  Some improvement on the benchmark code
   was due to xlc using floating multiply-add more aggressively, but the
   main improvement was converting code as in f1 to as in f2.  */

float bar;

void f1 (void)
{
  int i;
  for (i = 0; i < 500; i++)
    __asm__ __volatile__ ("# %0" : : "f" (i * bar));
}

void f2 (void)
{
  register long i;
  register float f, bar2 = bar;
  for (i = 500, f = 0.0; --i >= 0;)
    {
      __asm__ __volatile__ ("# %0" : : "f" (f));
      f += bar2;
    }
}

/* On ppc32, the f1 loop generates
.L9:
        xoris 0,9,0x8000
        stw 11,8(1)
        stw 0,12(1)
        lfd 0,8(1)
        fsub 0,0,13
        frsp 0,0
        fmuls 0,0,12
#APP
        # 0
#NO_APP
        addi 9,9,1
        bdnz .L9

the f2 loop is
.L19:
#APP
        # 0
#NO_APP
        fadds 0,0,13
        bdnz .L19
*/

-- 
           Summary: strength reduction on floating point
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: amodra at bigpond dot net dot au
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: powerpc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14886

[Bug optimization/14886] strength reduction on floating point

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-04-08 04:05 -------
Confirmed, the main reason why f1 is faster than f2 is that you no longer have to go through the stack 
and store on the stack.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |pessimizes-code
   Last reconfirmed|0000-00-00 00:00:00         |2004-04-08 04:05:24
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14886

[Bug rtl-optimization/14886] strength reduction on floating point

[anton at samba dot org]

------- Additional Comments From anton at samba dot org  2004-07-04 06:00 -------
Retested on 3.5 cvs (20040703) and the probelm is still there:

.L2:
        xoris 0,9,0x8000
        stw 11,8(1)
        stw 0,12(1)
        lfd 0,8(1)
        fsub 0,0,13
        frsp 0,0
        fmuls 0,0,12
#APP
        # 0
#NO_APP
        addi 9,9,1
        bdnz .L2

vs:

.L8:
#APP
        # 0
#NO_APP
        fadds 0,0,13
        bdnz .L8

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14886

[Bug rtl-optimization/14886] strength reduction on floating point

[anton at samba dot org]

------- Additional Comments From anton at samba dot org  2004-07-04 06:21 -------
f1 when compiled 64bit is worse, no use of count register (bug 16356), redundant
sign extension etc:

.L2:
        rldicl. 0,11,0,53
        sradi 9,11,53
        addi 9,9,1
        cmpldi 7,9,2
        beq- 0,.L3
        xor 0,11,0
        blt- 7,.L3
        ori 11,0,2048
.L3:
        lfs 13,0(10)
        std 11,-16(1)
        lfd 12,-16(1)
        fcfid 12,12
        frsp 0,12
        fmuls 0,0,13
#APP
        # 0
#NO_APP
        addi 0,11,1
        extsw 11,0
        cmpwi 7,11,499
        ble+ 7,.L2


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14886

[Bug tree-optimization/14886] strength reduction on floating point

[anton at samba dot org]

------- Additional Comments From anton at samba dot org  2005-03-16 20:54 -------
FYI this is still present in 4.0.0 20050313

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14886