[Bug rtl-optimization/18395] New: [meta-bug] combine needs to be templatized like a peepholer

["nathan at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

This is a catch all bug report to document failures in combine.

There are at least two problems with combine.

1) It only combines chains of instructions that have no other uses of the
intermediate results.

2) It functions by generating promising patterns and then seeing if they exist.
 This is wasteful, in that it generates many patterns  that don't exist on the
particular target, and it is blind in that it doesn't spot any special cases the
target might have.

Really combine, which is a generic peepholer, needs some kind of templatizing on
the target machine.   What's needed is something like pre-reg-alloc peepholes. 
That of course would be a large amount of work.

-- 
           Summary: [meta-bug] combine needs to be templatized like a
                    peepholer
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, meta-bug
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: nathan at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18395