[patch] Reduce memory overhead for large functions

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Hello,

This patch tried to use non-clearing memory allocation where possible.
This is especially important for very large functions, when arrays of
size in the order of n_basic_blocks or num_ssa_names are allocated to
hold sparse data sets. For such cases the overhead of memset becomes
measurable (and even dominant for the time spent in a pass in some
cases, such as the one I recently fixed in ifcvt.c).

This cuts off ~20% of the compile time for the test case of PR54146 at
-O1. Not bad for a patch that basically only removes a bunch of
memsets.

I got another 5% for the changes in tree-ssa-loop-manip.c. A loop over
an array with num_ssa_names there is expensive and unnecessary, and it
helps to stuff all bitmaps together on a single obstack if you intend
to blow them all away at the end (this could be done in a number of
other places in the compiler). Clearing livein at the end of
add_exit_phis_var also reduces peak memory with ~250MB at that point
in the passes pipeline (only to blow up from ~1.5GB peak memory in the
GIMPLE optimizers to ~3.6 GB in expand, and to ~8.6GB in IRA, but hey,
who's counting? :-)

Actually, the worst cases are not fixed with this patch. That'd be IRA
(which consumes ~5GB on the test case, out of 8GB total), and
tree-PRE.

The IRA case looks like it may be hard to fix: Allocating multiple
arrays of size O(max_regno) for every loop in init_loop_tree_node.

The tree-PRE case is one where the avail arrays are allocated and
cleared for every PRE candidate. This looks like a place where a
pointer_map should be used instead. I'll tackle that later, when I've
addressed more pressing problems in the compilation of the PR54146
test case.

This patch was bootstrapped&tested on powerpc64-unknown-linux-gnu. OK for trunk?

Kudos to the compile farm people, without them I couldn't even hope to
get any of this work done!

Ciao!
Steven

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