Re: [patch] combine ICE fix

[Jeff Law <law at redhat dot com>]

On 12/03/13 12:25, Kenneth Zadeck wrote:
> On 12/03/2013 01:52 PM, Mike Stump wrote:
> > On Dec 2, 2013, at 10:26 PM, Jeff Law <law@redhat.com> wrote:
> > > On 11/27/13 17:13, Cesar Philippidis wrote:
> > > > I looked into adding support for incremental DF scanning from df*.[ch]
> > > > in combine but there are a couple of problems. First of all, combine
> > > > does its own DF analysis. It does so because its usage falls under this
> > > > category (df-core.c):
> > > >
> > > >     c) If the pass modifies insns several times, this incremental
> > > >        updating may be expensive.
> > > >
> > > > Furthermore, combine's DF relies on the DF scanning to be deferred, so
> > > > the DF_REF_DEF_COUNT values would be off. Eg, calls to SET_INSN_DELETED
> > > > take place before it updates the notes for those insns. Also, combine
> > > > has a tendency to undo its changes occasionally.
> > > I think at this stage of the release cycle, converting combine to
> > > incremental DF is probably a no-go.  However, we should keep it in
> > > mind for the future -- while hairy I'd really like to see that happen
> > > in the long term.
> > I think Kenny has some thoughts in this area.  I'll cc him to ensure
> > he sees it.
> it is the tendency to undo things (i would use the word "frequently"
> rather than) occasionally that kept me from doing this years ago.
Shove a bunch of things together, simplify, then try to recognize the 
result.  If that fails, undo everything.

In theory, this could be replaced by making a copy of the original, 
doing the combination/simplification, then recognition.  If successful, 
then update DF and remove the original I3, if not successful, drop the 
copy.  That avoids the undo nonsense.

jeff