.. _pgo:

===========================
Profile Guided Optimization
===========================

:abbr:`PGO (Profile Guided Optimization)` is the process of adding
probes to a compiled binary, running said binary, then using the
run-time information to *recompile* the binary to (hopefully) make it
faster.

How PGO Builds Work
===================

The supported interface for invoking a PGO build is to evaluate the
*build* target of client.mk with *MOZ_PGO* defined. e.g.::

    $ make -f client.mk MOZ_PGO=1

This is equivalent to::

    $ make -f client.mk profiledbuild

Which is roughly equivalent to:

#. Perform a build with *MOZ_PROFILE_GENERATE=1* and *MOZ_PGO_INSTRUMENTED=1*
#. Package with *MOZ_PGO_INSTRUMENTED=1*
#. Performing a run of the instrumented binaries
#. $ make maybe_clobber_profiledbuild
#. Perform a build with *MOZ_PROFILE_USE=1*

Differences between toolchains
==============================

There are some implementation differences depending on the compiler
toolchain being used.

The *maybe_clobber_profiledbuild* step gets its name because of a
difference. On Windows, this step merely moves some *.pgc* files around.
Using GCC or Clang, it is equivalent to a *make clean*.