[Bug fortran/30409] New: [fortran] missed optimization with pure function arguments

[Bug fortran/30409] New: [fortran] missed optimization with pure function arguments

[kargl at gcc dot gnu dot org]

The attached code demonstrates a missed optimization opportunity that
can have a severe impact on code.  Here are the timings for the 2 loops
on 2 GHz pentium4-m processor.

laptop:kargl[208] gfc4x -o z -O2 a.f90
laptop:kargl[209] ./z
 time 1:   5.0741002E-02
 time 2:    31.28215    

For a pure function the arguments are not changed during execution.  Thus,
in the second loop, the 1/y can be hoisted out of the nested loops.  Currently,
gfortran allocates/deallocates a temp array with every iteration on the 
outer loop, and it does the 1/y array operation on every iteration.


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           Summary: [fortran] missed optimization with pure function
                    arguments
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kargl at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30409

[Bug fortran/30409] [fortran] missed optimization with pure function arguments

[kargl at gcc dot gnu dot org]

------- Comment #1 from kargl at gcc dot gnu dot org  2007-01-08 21:32 -------
Created an attachment (id=12871)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12871&action=view)
missed optimization


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30409

[Bug fortran/30409] [fortran] missed optimization with pure function arguments

[kargl at gcc dot gnu dot org]

------- Comment #2 from kargl at gcc dot gnu dot org  2007-01-08 21:36 -------
Sorry about the long URL, but the code comes from this comp.lang.fortran
thread.

http://groups-beta.google.com/group/comp.lang.fortran/browse_thread/thread/9f9bf1c116dc4b69/712366ef4318e84d#712366ef4318e84d


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30409

[Bug fortran/30409] [fortran] missed optimization with pure function arguments

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2007-01-09 11:11 -------
In the middle-end this somewhat is related to PR26387.  Of course this is a
place
where frontend optimization is probably easier to do.

Confirmed.


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rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |26387
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2007-01-09 11:11:16
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30409

[Bug fortran/30409] [fortran] missed optimization with pure function arguments

[kargl at gcc dot gnu dot org]

------- Comment #4 from kargl at gcc dot gnu dot org  2007-01-09 16:08 -------
Note,  above the first FORALL statement one needs to add
the following 2 lines of code

xmin = 0.
xmax = 1.

As a side note, both Pathscale and Intel in the c.l.f thread have
acknowledged that their compilers also miss this optimization.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30409