by [Reinhard Max], August 2003

Starting at version 3.3 gcc supports profile based optimizations,
which are said to be especially beneficial for things like interpreters.
In my tests, Tcl gained about 3..5% in speed when compiled with profile
based optimization, depending on the CPU architecture.

Compiling Tcl (or any other project) with GCC's profile based optimization
goes in three steps:

   1. Use -fprofile-arcs as an additional compiler flag to compile the code with profiling support.
   2. Run the result at some "typical workload" to gain statistics on which branches are taken how often.
   3. Compile the code again using -fbranch-probabilities as an additional compiler flag, which will use the statistics written before to better optimise the code.

Normally one would feed these additional flags into Tcl's configure script in the CFLAGS environment variable, but Tcl's build system doesn't handle this properly, and so the best practice is to hack the Makefile after running configure, and add the options mentioned above to the CFLAGS_OPTIMIZE variable.

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