[LLVMdev] Compression Stabilization

[Reid Spencer]

In preparation for Release 1.4, I've consolidated the bytecode
compression to only use bzip2. zlib is not supported any more. We chose
bzip2 because it offers about 10% better compression on bytecode files. 

Its unlikely, but this change *could* affect you if you ever built the
CFE or your own LLVM project on a machine that had zlib but didn't have
bzip2. If you have bzip2 on your machine, you'll be fine even with this
change.  If you didn't have bzip2, the bytecode files you previously
generated will be compressed with zlib but the current Compressor
implementation now does not support zlib. So, if you're in this
situation, and you update today, you need to completely rebuild your CFE
runtime libraries (libgcc.a, libstdc++.a etc) and re-install them. You
should do this after you've rebuilt LLVM.

The reason this is important for 1.4 stability is so that bytecode can
be portable between machines. Previously, If one machine had zlib and
another only had bzip2, they could not exchange bytecode files because
of the differing compression schemes used. My recent patches fix this
by: a) adding bzip2 to LLVM so its always available, b) only supporting
bzip2 compression.

Reid.
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