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

[Vladimir Makarov <vmakarov@redhat.com>]

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.