[Bug tree-optimization/36258] New: remove useless edge split in copy_loop_headers

[Bug tree-optimization/36258] New: remove useless edge split in copy_loop_headers

[zhouyi04 at ios dot cn]

The function copy_loop_headers after duplicate the region for a loop, 
237       /* Ensure that the latch and the preheader is simple (we know that
they
238          are not now, since there was the loop exit condition.  */
239       split_edge (loop_preheader_edge (loop));
240       split_edge (loop_latch_edge (loop));
241     }

In fact, the statements in line 239 and 240 are useless because there will
be no code motion optimizing (for example loop invariant movement) related to
that splitting in this pass, and the edges created by above statements are
removed by cfg cleanup in execute_todo that follows. 

I have comment out the statements of line 239 and 240, and bootstraped and
tested on Linux i686 i686 i686 GNU/Linux.

I also compared the output asm files on about 2000 C files under gcc.c-torture,
there is no difference when compiled using the compilers before commenting out
above statements and after commenting out the statements.

Thanks alot
Zhouyi Zhou


-- 
           Summary: remove useless edge split  in copy_loop_headers
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: zhouyi04 at ios dot cn
 GCC build triplet: Linux i686 i686 i686 GNU/Linux
  GCC host triplet: Linux i686 i686 i686 GNU/Linux
GCC target triplet: Linux i686 i686 i686 GNU/Linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36258

[Bug tree-optimization/36258] remove useless edge split in copy_loop_headers

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2008-05-18 16:59 -------
Zdenek probably remembers why this was done.  In any case, posting a patch
to gcc-patches would be appropriate.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org, rakdver at gcc dot gnu
                   |                            |dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36258

[Bug tree-optimization/36258] remove useless edge split in copy_loop_headers

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36258

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
The comment indicates that this is to keep the constraints of the loop state
which are LOOPS_HAVE_PREHEADERS|LOOPS_HAVE_SIMPLE_LATCHES, which would be
important at least for the pass that's now inside the loop pipeline.