Re: [PATCH V3 1/7] df: Add DF_LIVE_SUBREG problem

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Nov 13, 2023 at 11:39 PM Vladimir Makarov <vmakarov@redhat.com> wrote:
>
>
> On 11/12/23 07:08, Lehua Ding wrote:
> > This patch adds a live_subreg problem to extend the original live_reg to
> > track the liveness of subreg. We will only try to trace speudo registers
> > who's mode size is a multiple of nature size and eventually a small portion
> > of the inside will appear to use subreg. With live_reg problem, live_subreg
> > prbolem will have the following output. full_in/out mean the entire pesudo
> > live in/out, partial_in/out mean the subregs of the pesudo are live in/out,
> > and range_in/out indicates which part of the pesudo is live. all_in/out is
> > the union of full_in/out and partial_in/out:
> >
> I am not a maintainer or reviewer of data-flow analysis framework and
> can not approve this patch except changes in regs.h.  Richard Sandiford
> or Jeff Law as global reviewers probably can do this.
>
> As for regs.h changes, they are ok for me after fixing general issues I
> mentioned in my previous email (two spaces after sentence ends in the
> comments).
>
> I think all this code is a major compiler time and memory consumer in
> all set of the patches.  DF analysis is slow by itself even when only
> effective data structures as bitmaps are used but you are introducing
> even slower data structure as maps (I believe better performance data
> structure can be used instead).  In the very first version of LRA I used
> DFA but it made LRA so slow that I had to introduce own data structures
> which are faster in case of massive RTL changes in LRA.  The same
> problem exists for using generic C++ standard library data as vectors
> and maps for critical code.  It is hard to get a needed performance when
> the exact implementation can vary or be not what you need, e.g. vector
> initial capacity, growth etc.  But again the performance issues can be
> addressed later.

I think the important bit should be the subreg live analysis should be
opt-in and when not enabled shouldn't have a bad effect on memory
usage and compile-time.  At -O0 and -O1 RA consumes a major
amount of compile-time.

Richard.

>