[Bug target/18154] New: Inefficient max/min code for PowerPC

[Bug target/18154] New: Inefficient max/min code for PowerPC

[dje at gcc dot gnu dot org]

int min(int a, int b) {
  if (a < b)
    return a;
  else
    return b;
}

should produce

        rlwinm  r2,r3,1,31,31
        subfc   r0,r4,r3
        rlwinm  r0,r4,1,31,31
        subfe   r0,r2,r0
        andc    r2,r4,r0
        and     r0,r3,r0
        or      r3,r0,r2
        blr

but instead produces a branch sequence

         mr r0,r3
         mr r3,r4
         cmpw cr7,r0,r4
         bgelr- cr7
         mr r3,r0
         blr

with unnecessary dependencies.

-- 
           Summary: Inefficient max/min code for PowerPC
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dje at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: *-*-*
  GCC host triplet: powerpc-*-*
GCC target triplet: *-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18154

[Bug target/18154] Inefficient max/min code for PowerPC

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-26 04:25 -------
Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-10-26 04:25:17
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18154

[Bug target/18154] Inefficient max/min code for PowerPC

[dje at gcc dot gnu dot org]

------- Additional Comments From dje at gcc dot gnu dot org  2004-10-26 20:06 -------
XLC chooses the straight-line code sequence versus compare and branch based on 
a cost model.  This should not be a uniform change in behavior for PowerPC.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18154

[Bug target/18154] Inefficient max/min code for PowerPC

[dje at gcc dot gnu dot org]

------- Additional Comments From dje at gcc dot gnu dot org  2004-10-26 21:25 -------
Also, do not enable when optimizing for size.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18154

[Bug target/18154] Inefficient max/min code for PowerPC

[geoffk at gcc dot gnu dot org]

------- Additional Comments From geoffk at gcc dot gnu dot org  2004-10-27 23:45 -------
I'm not sure that subfc/subfe is going to be cheaper than a compare and a branch, even if the branch is 
mispredicted half the time.  Do you have timing results?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18154

[Bug target/18154] Inefficient max/min code for PowerPC

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-27 23:52 -------
I should note when I was doing SPEC work, using subfc/subfe did not help SPEC at all (I tried to change 
the source and also rs6000.md).

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18154

[Bug target/18154] Inefficient max/min code for PowerPC

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-24 14:31 -------
On the mainline, we now produce:
        cmpw cr7,r3,r4
        blelr- cr7
        mr r3,r4
        blr



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18154