On 12/03/2013 01:52 PM, Mike Stump wrote:
On Dec 2, 2013, at 10:26 PM, Jeff Law <l...@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.

kenny

Reply via email to