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speeding up liveness analysis ideas.

Not only on Brad's testcase we spend very important amount of time in the
liveness analysis.  This is relatively sad score and I think we should try
to improve it.  It is also interesting that the times appears to be considerably
worse in the current code than they was in 3.0 times and I don't recall any
important change to the code possibly responsible for that.

Here are several assorted ideas what I think can help

1) Use df.c module to find the references and uses - we currently spend
quite a lot of time in mark_set_1 and for_each_rtx modules.  Discovering all
uses/sets at once before relaxation process can be good idea...
2) Reverse topological order the blocks for relaxation progress
   - this should help any iterative dataflow..
3) Avoid the global liveness rebuild in optimize_for_mode_switching.

This is dificult because it do use commit_edge_insertions that basically
does arbitary modifications of the CFG and makes it dificult to track down
all modified blocks to locally re-examine.

I think I can get this done at lower level.  If I create new BB flag BB_MODIFIED
and clear it at the begginig of pass and make emit_insn/remove_insn familly to
set this flag, I can get the information for lowlevel.
Then I can easilly prepare the bitmap of blocks to update.  The if converison
and cfg_cleanup can benefit from this infrastructure.

Other alternative is to do it at higher level in the CFG manipulation routines,
so there can be made difference between local and global transformation
so we don't need to rerun the relaxation progress (or know whether the
pass can do global changes from it's overall design).  But I am not quite
sure how many places would need modification then.

Also why the local update is cheaper - if we rerun relaxation and we do only
local updates it should use exactly one iteration.  Is the local update
basically designed to catch the bugs?

What do you think?  Does that make sense?


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