Add a new wart, I mean feature, on to gdb-remote protocol: compression.


Add a new wart, I mean feature, on to gdb-remote protocol: compression.
For some communication channels, sending large packets can be very
slow. In those cases, it may be faster to compress the contents of
the packet on the target device and decompress it on the debug host
system. For instance, communicating with a device using something
like Bluetooth may be an environment where this tradeoff is a good one.

This patch adds a new field to the response to the "qSupported" packet
(which returns a "qXfer:features:" response) -- SupportedCompressions
and DefaultCompressionMinSize. These tell you what the remote
stub can support.

lldb, if it wants to enable compression and can handle one of those
algorithms, it can send a QEnableCompression packet specifying the
algorithm and optionally the minimum packet size to use compression
on. lldb may have better knowledge about the best tradeoff for
a given communication channel.

I added support to debugserver an lldb to use the zlib APIs
(if -DHAVE_LIBZ=1 is in CFLAGS and -lz is in LDFLAGS) and the
libcompression APIs on Mac OS X 10.11 and later
(if -DHAVE_LIBCOMPRESSION=1). libz "zlib-deflate" compression.
libcompression can support deflate, lz4, lzma, and a proprietary
lzfse algorithm. libcompression has been hand-tuned for Apple
hardware so it should be preferred if available.

debugserver currently only adds the SupportedCompressions when
it is being run on an Apple watch (TARGET_OS_WATCH). Comment
that #if out from RNBRemote.cpp if you want to enable it to
see how it works. I haven't tested this on a native system
configuration but surely it will be slower to compress & decompress
the packets in a same-system debug session.

I haven't had a chance to add support for this to
GDBRemoteCommunciationServer.cpp yet.



jmolendaJun 18 2015, 2:46 PM
rL240065: [Driver] Compute MacOS/iOS version from triple if/when we actually need it. NFC.