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bpo-24318: Rewrite the README PGO section. (GH-6863)

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* bpo-24318: Rewrite the README PGO section.

Merged from a phone on an airplane. :)
(cherry picked from commit 93f9a8a5af)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
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Miss Islington (bot) 2018-05-16 09:34:57 -07:00 • committed by GitHub
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@ -93,25 +93,26 @@ Profile Guided Optimization
^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^
PGO takes advantage of recent versions of the GCC or Clang compilers. If used, PGO takes advantage of recent versions of the GCC or Clang compilers. If used,
either via ``configure --enable-optimizations`` above or by manually running either via ``configure --enable-optimizations`` or by manually running
``make profile-opt`` regardless of configure flags it will do several steps. ``make profile-opt`` regardless of configure flags, the optimized build
process will perform the following steps:
First, the entire Python directory is cleaned of temporary files that may have The entire Python directory is cleaned of temporary files that may have
resulted in a previous compilation. resulted from a previous compilation.
Then, an instrumented version of the interpreter is built, using suitable An instrumented version of the interpreter is built, using suitable compiler
compiler flags for each flavour. Note that this is just an intermediary step. flags for each flavour. Note that this is just an intermediary step. The
The binary resulting from this step is not good for real life workloads as binary resulting from this step is not good for real life workloads as it has
it has profiling instructions embedded inside. profiling instructions embedded inside.
After this instrumented version of the interpreter is built, the Makefile will After the instrumented interpreter is built, the Makefile will run a training
automatically run a training workload. This is necessary in order to profile workload. This is necessary in order to profile the interpreter execution.
the interpreter execution. Note also that any output, both stdout and stderr, Note also that any output, both stdout and stderr, that may appear at this step
that may appear at this step is suppressed. is suppressed.
Finally, the last step is to rebuild the interpreter, using the information The final step is to build the actual interpreter, using the information
collected in the previous one. The end result will be a Python binary that is collected from the instrumented one. The end result will be a Python binary
optimized and suitable for distribution or production installation. that is optimized; suitable for distribution or production installation.
Link Time Optimization Link Time Optimization