michael@0: .. _pgo:
michael@0: 
michael@0: ===========================
michael@0: Profile Guided Optimization
michael@0: ===========================
michael@0: 
michael@0: :abbr:`PGO (Profile Guided Optimization)` is the process of adding
michael@0: probes to a compiled binary, running said binary, then using the
michael@0: run-time information to *recompile* the binary to (hopefully) make it
michael@0: faster.
michael@0: 
michael@0: How PGO Builds Work
michael@0: ===================
michael@0: 
michael@0: The supported interface for invoking a PGO build is to evaluate the
michael@0: *build* target of client.mk with *MOZ_PGO* defined. e.g.::
michael@0: 
michael@0:     $ make -f client.mk MOZ_PGO=1
michael@0: 
michael@0: This is equivalent to::
michael@0: 
michael@0:     $ make -f client.mk profiledbuild
michael@0: 
michael@0: Which is roughly equivalent to:
michael@0: 
michael@0: #. Perform a build with *MOZ_PROFILE_GENERATE=1* and *MOZ_PGO_INSTRUMENTED=1*
michael@0: #. Package with *MOZ_PGO_INSTRUMENTED=1*
michael@0: #. Performing a run of the instrumented binaries
michael@0: #. $ make maybe_clobber_profiledbuild
michael@0: #. Perform a build with *MOZ_PROFILE_USE=1*
michael@0: 
michael@0: Differences between toolchains
michael@0: ==============================
michael@0: 
michael@0: There are some implementation differences depending on the compiler
michael@0: toolchain being used.
michael@0: 
michael@0: The *maybe_clobber_profiledbuild* step gets its name because of a
michael@0: difference. On Windows, this step merely moves some *.pgc* files around.
michael@0: Using GCC or Clang, it is equivalent to a *make clean*.