[Bug target/25128] New: [m68k] Suboptimal comparisons against 65536

[Bug target/25128] New: [m68k] Suboptimal comparisons against 65536

[kazu at gcc dot gnu dot org]

Consider:

unsigned int bar (void);

void
foo (void)
{
  unsigned int a = bar ();
  if (65536 > a)
    bar ();
}

./cc1 -quiet -O2 -fomit-frame-pointer generates

foo:
        move.l %a2,-(%sp)
        lea bar,%a2
        jbsr (%a2)
        cmp.l #65535,%d0
        jbhi .L10
        jbsr (%a2)
.L10:
        move.l (%sp)+,%a2
        rts

Note that

        cmp.l #65535,%d0
        jbhi .L10

can be replaced with

        swap %d0
        tst.w %d0
        jbne .L10

A similar trick can be applied to signed comparisons as well.


-- 
           Summary: [m68k] Suboptimal comparisons against 65536
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at gcc dot gnu dot org
GCC target triplet: m68k-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25128

[Bug target/25128] [m68k] Suboptimal comparisons against 65536

[kazu at gcc dot gnu dot org]

-- 

kazu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25128

[Bug target/25128] [m68k] Suboptimal comparisons against 65536

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2005-11-28 05:04 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-11-28 05:04:23
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25128

[Bug target/25128] [m68k] Suboptimal comparisons against 65536

[gunnar at greyhound-data dot com]

------- Comment #2 from gunnar at greyhound-data dot com  2008-06-10 16:02 -------

> Note that
> 
>         cmp.l #65535,%d0
>         jbhi .L10
> 
> can be replaced with
> 
>         swap %d0
>         tst.w %d0
>         jbne .L10
> 
> A similar trick can be applied to signed comparisons as well.

But this "trick" will run slower on the higher 68k CPUs.
On 68040 or 68060 or SuperScalar Coldfire its better to generate less
instructions that do not have dependancies.

I think "cmp.l #65535,%d0" is the code that should be generated by "O2" as its
faster on many 68K models.
The shorter two instruction trick might be an option for compile optiont "Os"


Kind regards

Gunnar von Boehn


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25128