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The mechanism by which PGO builds are kicked off is kinda wonky. The MOZ_PGO environment variable is recognized by configure and setting it will result in MOZ_PGO being defined in substs. In addition, the build backend (previously client.mk, now Makefile.in) also recognizes MOZ_PGO (from mozconfig or environment) and takes appropriate action. In-tree mozconfigs set MOZ_PGO via mk_add_options. mk_add_options is intended as a mechanism to inject state into client.mk and the make-based build system. In addition, there is code in mozharness (unchanged by this commit) that sets MOZ_PGO if appropriate. A PGO build configuration is different from a non-PGO build configuration. Therefore a make-centric environment variable to control PGO is not ideal. Instead, this should be defined as a configure-time flag and the build invocation should key off that. This commit normalizes in-tree mozconfigs to set MOZ_PGO via ac_add_options and updates the PGO documentation to recommend this method. MozReview-Commit-ID: k6AZyJuXjs --HG-- extra : rebase_source : b1a6348611eba08dd67ec938cca5586fbe8e6910 extra : source : 25c7ebc7c44dd253f421b6de3d0337635d0c99d0
41 lines
1.1 KiB
ReStructuredText
41 lines
1.1 KiB
ReStructuredText
.. _pgo:
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===========================
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Profile Guided Optimization
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===========================
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:abbr:`PGO (Profile Guided Optimization)` is the process of adding
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probes to a compiled binary, running said binary, then using the
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run-time information to *recompile* the binary to (hopefully) make it
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faster.
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How PGO Builds Work
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===================
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The supported interface for invoking a PGO build is to add ``MOZ_PGO=1`` to
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configure flags and then build. e.g. in your mozconfig::
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ac_add_options MOZ_PGO=1
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Then::
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$ ./mach build
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This is roughly equivalent to::
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#. Perform a build with *MOZ_PROFILE_GENERATE=1* and *MOZ_PGO_INSTRUMENTED=1*
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#. Package with *MOZ_PGO_INSTRUMENTED=1*
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#. Performing a run of the instrumented binaries
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#. $ make maybe_clobber_profiledbuild
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#. Perform a build with *MOZ_PROFILE_USE=1*
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Differences between toolchains
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==============================
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There are some implementation differences depending on the compiler
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toolchain being used.
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The *maybe_clobber_profiledbuild* step gets its name because of a
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difference. On Windows, this step merely moves some *.pgc* files around.
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Using GCC or Clang, it is equivalent to a *make clean*.
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