INSTALLING.md: instructions on getting gdb pretty-printers to work

docs/INSTALLING.md

@@ -175,6 +175,30 @@ TODO: GDB should be able to determine its data directory.  maybe it should be in
 from inside the source dir?
 --->
 
+#### getting gdb to pretty-print STL types
+
+GDB ships with python scripts that tell it how to pretty-print STL containers, but they only work against the stdlib in the corresponding version of GCC. Unfortunately, we compile against an old version of gcc's C++ stdlib. Here's how to get GDB to use the *old* pretty-print scripts for GCC 4.8.2. (Note: Have only tested this with gdb 7.8.1-ubuntu4 and gcc-4.8.2.)
+
+If you've built GCC, the GDB scripts will be in `~/pyston_deps/gcc-4.8.2-install/share/gcc-4.8.2/python`. Unfortunately they're for Python 2, and GDB 7.8.1 uses Python 3. So we'll use `2to3` to update them:
+
+```
+cd ~/pyston_deps/gcc-4.8.2-install/share/gcc-4.8.2/python/libstdcxx/v6
+2to3 -w printers.py
+```
+
+Don't worry, `2to3` makes a backup of the file it changes if you need to go back. Now, edit your `~/.gdbinit` file to contain the following:
+
+```
+python
+import sys
+sys.path.insert(0, '/YOUR/HOME/DIR/pyston_deps/gcc-4.8.2-install/share/gcc-4.8.2/python')
+from libstdcxx.v6.printers import register_libstdcxx_printers
+register_libstdcxx_printers(None)
+print('--- Registered cxx printers for gcc 4.8.2! ---')
+```
+
+And there you go!
+
 ### gold
 
 gold is highly recommended as a faster linker, and Pyston contains build-system support for automatically using gold if available.  gold may already be installed on your system; you can check by typing `which gold`.  If it's not installed already: