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.. _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*.
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