Great increase in memory usage by tree-SSA optimizers

[Andrew Haley <aph at redhat dot com>]

Recently (I can't tell when this changed, exactly, but it's within the
last few weeks) I've been unable to compile a big Java program because
my computer runs out of memory.  gcj version 4.1 compiles this program
correctly, although it uses about a gigabyte of RAM.  gcj version 4.2
can't do it even with more than 2 gigabytes.

The explosion in memory usage seems to be in PRE.

This program is the testcase for
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27754.

I'd be grateful if someone who understands the tree-SSA optimizers
could have a look and find out why there has been this severe
regression in compile-time memory usage.

This requires a gcc later than revision 114042, which includes my
patch for PR 27754.

Thanks,
Andrew.