[BOLT][Docs] Add Sphinx documentation

[Amir]

Summary

Add stub Sphinx documentation, with configuration copy-pasted from lld and
index page converted from bolt/README.md.

bolt/docs/CMakeLists.txt

 endif()
 
 if (LLVM_ENABLE_SPHINX)
   include(AddSphinxTarget)
   if (SPHINX_FOUND)
     if (${SPHINX_OUTPUT_HTML})
       add_sphinx_target(html bolt)
     endif()
-    if (${SPHINX_OUTPUT_MAN})
-      add_sphinx_target(man bolt)
-    endif()
   endif()
 endif()
 

bolt/docs/README.txt

+BOLT Documentation
+====================
+
+The BOLT documentation is written using the Sphinx documentation generator. It
+is currently tested with Sphinx 1.1.3.
+
+To build the documents into html configure BOLT with the following cmake options:
+
+  * -DLLVM_ENABLE_SPHINX=ON
+  * -DBOLT_INCLUDE_DOCS=ON
+
+After configuring BOLT with these options the make rule `docs-bolt-html` should
+be available.

bolt/docs/conf.py

+# -*- coding: utf-8 -*-
+#
+# BOLT documentation build configuration file.
+#
+# This file is execfile()d with the current directory set to its containing dir.
+#
+# Note that not all possible configuration values are present in this
+# autogenerated file.
+#
+# All configuration values have a default; values that are commented out
+# serve to show the default.
+
+import sys, os
+from datetime import date
+
+# If extensions (or modules to document with autodoc) are in another directory,
+# add these directories to sys.path here. If the directory is relative to the
+# documentation root, use os.path.abspath to make it absolute, like shown here.
+#sys.path.insert(0, os.path.abspath('.'))
+
+# -- General configuration -----------------------------------------------------
+
+# If your documentation needs a minimal Sphinx version, state it here.
+#needs_sphinx = '1.0'
+
+# Add any Sphinx extension module names here, as strings. They can be extensions
+# coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
+extensions = ['sphinx.ext.intersphinx', 'sphinx.ext.todo']
+
+# Add any paths that contain templates here, relative to this directory.
+templates_path = ['_templates']
+
+# The suffix of source filenames.
+source_suffix = '.rst'
+
+# The encoding of source files.
+#source_encoding = 'utf-8-sig'
+
+# The master toctree document.
+master_doc = 'index'
+
+# General information about the project.
+project = u'BOLT'
+copyright = u'2015-%d, BOLT team' % date.today().year
+
+# The language for content autogenerated by Sphinx. Refer to documentation
+# for a list of supported languages.
+#language = None
+
+# There are two options for replacing |today|: either, you set today to some
+# non-false value, then it is used:
+#today = ''
+# Else, today_fmt is used as the format for a strftime call.
+today_fmt = '%Y-%m-%d'
+
+# List of patterns, relative to source directory, that match files and
+# directories to ignore when looking for source files.
+exclude_patterns = ['_build']
+
+# The reST default role (used for this markup: `text`) to use for all documents.
+#default_role = None
+
+# If true, '()' will be appended to :func: etc. cross-reference text.
+#add_function_parentheses = True
+
+# If true, the current module name will be prepended to all description
+# unit titles (such as .. function::).
+#add_module_names = True
+
+# If true, sectionauthor and moduleauthor directives will be shown in the
+# output. They are ignored by default.
+show_authors = True
+
+# The name of the Pygments (syntax highlighting) style to use.
+pygments_style = 'friendly'
+
+# A list of ignored prefixes for module index sorting.
+#modindex_common_prefix = []
+
+
+# -- Options for HTML output ---------------------------------------------------
+
+# The theme to use for HTML and HTML Help pages.  See the documentation for
+# a list of builtin themes.
+html_theme = 'haiku'
+
+# Theme options are theme-specific and customize the look and feel of a theme
+# further.  For a list of options available for each theme, see the
+# documentation.
+#html_theme_options = {}
+
+# Add any paths that contain custom themes here, relative to this directory.
+html_theme_path = ["."]
+
+# The name for this set of Sphinx documents.  If None, it defaults to
+# "<project> v<release> documentation".
+#html_title = None
+
+# A shorter title for the navigation bar.  Default is the same as html_title.
+#html_short_title = None
+
+# The name of an image file (relative to this directory) to place at the top
+# of the sidebar.
+#html_logo = None
+
+# If given, this must be the name of an image file (path relative to the
+# configuration directory) that is the favicon of the docs. Modern browsers use
+# this as icon for tabs, windows and bookmarks. It should be a Windows-style
+# icon file (.ico), which is 16x16 or 32x32 pixels large. Default: None.  The
+# image file will be copied to the _static directory of the output HTML, but
+# only if the file does not already exist there.
+html_favicon = '_static/favicon.ico'
+
+# Add any paths that contain custom static files (such as style sheets) here,
+# relative to this directory. They are copied after the builtin static files,
+# so a file named "default.css" will overwrite the builtin "default.css".
+html_static_path = ['_static']
+
+# If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
+# using the given strftime format.
+html_last_updated_fmt = '%Y-%m-%d'
+
+# If true, SmartyPants will be used to convert quotes and dashes to
+# typographically correct entities.
+#html_use_smartypants = True
+
+# Custom sidebar templates, maps document names to template names.
+html_sidebars = {'index': ['indexsidebar.html']}
+
+# Additional templates that should be rendered to pages, maps page names to
+# template names.
+# html_additional_pages = {'index': 'index.html'}
+
+# If false, no module index is generated.
+#html_domain_indices = True
+
+# If false, no index is generated.
+#html_use_index = True
+
+# If true, the index is split into individual pages for each letter.
+#html_split_index = False
+
+# If true, links to the reST sources are added to the pages.
+html_show_sourcelink = True
+
+# If true, "Created using Sphinx" is shown in the HTML footer. Default is True.
+#html_show_sphinx = True
+
+# If true, "(C) Copyright ..." is shown in the HTML footer. Default is True.
+#html_show_copyright = True
+
+# If true, an OpenSearch description file will be output, and all pages will
+# contain a <link> tag referring to it.  The value of this option must be the
+# base URL from which the finished HTML is served.
+#html_use_opensearch = ''
+
+# This is the file name suffix for HTML files (e.g. ".xhtml").
+#html_file_suffix = None
+
+# Output file base name for HTML help builder.
+htmlhelp_basename = 'boltdoc'
+
+
+# -- Options for LaTeX output --------------------------------------------------
+
+latex_elements = {
+# The paper size ('letterpaper' or 'a4paper').
+#'papersize': 'letterpaper',
+
+# The font size ('10pt', '11pt' or '12pt').
+#'pointsize': '10pt',
+
+# Additional stuff for the LaTeX preamble.
+#'preamble': '',
+}
+
+# Grouping the document tree into LaTeX files. List of tuples
+# (source start file, target name, title, author, documentclass [howto/manual]).
+latex_documents = [
+  ('contents', 'bolt.tex', u'BOLT Documentation',
+   u'LLVM project', 'manual'),
+]
+
+# The name of an image file (relative to this directory) to place at the top of
+# the title page.
+#latex_logo = None
+
+# For "manual" documents, if this is true, then toplevel headings are parts,
+# not chapters.
+#latex_use_parts = False
+
+# If true, show page references after internal links.
+#latex_show_pagerefs = False
+
+# If true, show URL addresses after external links.
+#latex_show_urls = False
+
+# Documents to append as an appendix to all manuals.
+#latex_appendices = []
+
+# If false, no module index is generated.
+#latex_domain_indices = True
+
+
+# -- Options for manual page output --------------------------------------------
+
+# One entry per manual page. List of tuples
+# (source start file, name, description, authors, manual section).
+man_pages = [
+    ('contents', 'bolt', u'BOLT Documentation',
+     [u'LLVM project'], 1)
+]
+
+# If true, show URL addresses after external links.
+#man_show_urls = False
+
+
+# -- Options for Texinfo output ------------------------------------------------
+
+# Grouping the document tree into Texinfo files. List of tuples
+# (source start file, target name, title, author,
+#  dir menu entry, description, category)
+texinfo_documents = [
+  ('contents', 'BOLT', u'BOLT Documentation',
+   u'LLVM project', 'BOLT', 'Binary Optimization and Layout Tool',
+   'Miscellaneous'),
+]
+
+# Documents to append as an appendix to all manuals.
+#texinfo_appendices = []
+
+# If false, no module index is generated.
+#texinfo_domain_indices = True
+
+# How to display URL addresses: 'footnote', 'no', or 'inline'.
+#texinfo_show_urls = 'footnote'
+
+
+# FIXME: Define intersphinx configuration.
+intersphinx_mapping = {}
+
+
+# -- Options for extensions ----------------------------------------------------
+
+# Enable this if you want TODOs to show up in the generated documentation.
+todo_include_todos = True

bolt/docs/index.rst

+BOLT
+====
+
+BOLT is a post-link optimizer developed to speed up large applications.
+It achieves the improvements by optimizing application’s code layout
+based on execution profile gathered by sampling profiler, such as Linux
+``perf`` tool. An overview of the ideas implemented in BOLT along with a
+discussion of its potential and current results is available in `CGO’19
+paper <https://research.fb.com/publications/bolt-a-practical-binary-optimizer-for-data-centers-and-beyond/>`__.
+
+Input Binary Requirements
+-------------------------
+
+BOLT operates on X86-64 and AArch64 ELF binaries. At the minimum, the
+binaries should have an unstripped symbol table, and, to get maximum
+performance gains, they should be linked with relocations
+(``--emit-relocs`` or ``-q`` linker flag).
+
+BOLT disassembles functions and reconstructs the control flow graph
+(CFG) before it runs optimizations. Since this is a nontrivial task,
+especially when indirect branches are present, we rely on certain
+heuristics to accomplish it. These heuristics have been tested on a code
+generated with Clang and GCC compilers. The main requirement for C/C++
+code is not to rely on code layout properties, such as function pointer
+deltas. Assembly code can be processed too. Requirements for it include
+a clear separation of code and data, with data objects being placed into
+data sections/segments. If indirect jumps are used for intra-function
+control transfer (e.g., jump tables), the code patterns should be
+matching those generated by Clang/GCC.
+
+NOTE: BOLT is currently incompatible with the
+``-freorder-blocks-and-partition`` compiler option. Since GCC8 enables
+this option by default, you have to explicitly disable it by adding
+``-fno-reorder-blocks-and-partition`` flag if you are compiling with
+GCC8 or above.
+
+NOTE2: DWARF v5 is the new debugging format generated by the latest LLVM
+and GCC compilers. It offers several benefits over the previous DWARF
+v4. Currently, the support for v5 is a work in progress for BOLT. While
+you will be able to optimize binaries produced by the latest compilers,
+until the support is complete, you will not be able to update the debug
+info with ``-update-debug-sections``. To temporarily work around the
+issue, we recommend compiling binaries with ``-gdwarf-4`` option that
+forces DWARF v4 output.
+
+PIE and .so support has been added recently. Please report bugs if you
+encounter any issues.
+
+Installation
+------------
+
+Docker Image
+~~~~~~~~~~~~
+
+You can build and use the docker image containing BOLT using our `docker
+file <utils/docker/Dockerfile>`__. Alternatively, you can build BOLT
+manually using the steps below.
+
+Manual Build
+~~~~~~~~~~~~
+
+BOLT heavily uses LLVM libraries, and by design, it is built as one of
+LLVM tools. The build process is not much different from a regular LLVM
+build. The following instructions are assuming that you are running
+under Linux.
+
+Start with cloning LLVM repo:
+
+::
+
+    > git clone https://github.com/llvm/llvm-project.git
+    > mkdir build
+    > cd build
+    > cmake -G Ninja ../llvm-project/llvm -DLLVM_TARGETS_TO_BUILD="X86;AArch64" -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="bolt"
+    > ninja bolt
+
+``llvm-bolt`` will be available under ``bin/``. Add this directory to
+your path to ensure the rest of the commands in this tutorial work.
+
+Optimizing BOLT’s Performance
+-----------------------------
+
+BOLT runs many internal passes in parallel. If you foresee heavy usage
+of BOLT, you can improve the processing time by linking against one of
+memory allocation libraries with good support for concurrency. E.g. to
+use jemalloc:
+
+::
+
+    > sudo yum install jemalloc-devel
+    > LD_PRELOAD=/usr/lib64/libjemalloc.so llvm-bolt ....
+
+Or if you rather use tcmalloc:
+
+::
+
+    > sudo yum install gperftools-devel
+    > LD_PRELOAD=/usr/lib64/libtcmalloc_minimal.so llvm-bolt ....
+
+Usage
+-----
+
+For a complete practical guide of using BOLT see `Optimizing Clang with
+BOLT <docs/OptimizingClang.md>`__.
+
+Step 0
+~~~~~~
+
+In order to allow BOLT to re-arrange functions (in addition to
+re-arranging code within functions) in your program, it needs a little
+help from the linker. Add ``--emit-relocs`` to the final link step of
+your application. You can verify the presence of relocations by checking
+for ``.rela.text`` section in the binary. BOLT will also report if it
+detects relocations while processing the binary.
+
+Step 1: Collect Profile
+~~~~~~~~~~~~~~~~~~~~~~~
+
+This step is different for different kinds of executables. If you can
+invoke your program to run on a representative input from a command
+line, then check **For Applications** section below. If your program
+typically runs as a server/service, then skip to **For Services**
+section.
+
+The version of ``perf`` command used for the following steps has to
+support ``-F brstack`` option. We recommend using ``perf`` version 4.5
+or later.
+
+For Applications
+^^^^^^^^^^^^^^^^
+
+This assumes you can run your program from a command line with a typical
+input. In this case, simply prepend the command line invocation with
+``perf``:
+
+::
+
+    $ perf record -e cycles:u -j any,u -o perf.data -- <executable> <args> ...
+
+For Services
+^^^^^^^^^^^^
+
+Once you get the service deployed and warmed-up, it is time to collect
+perf data with LBR (branch information). The exact perf command to use
+will depend on the service. E.g., to collect the data for all processes
+running on the server for the next 3 minutes use:
+
+::
+
+    $ perf record -e cycles:u -j any,u -a -o perf.data -- sleep 180
+
+Depending on the application, you may need more samples to be included
+with your profile. It’s hard to tell upfront what would be a sweet spot
+for your application. We recommend the profile to cover 1B instructions
+as reported by BOLT ``-dyno-stats`` option. If you need to increase the
+number of samples in the profile, you can either run the ``sleep``
+command for longer and use ``-F<N>`` option with ``perf`` to increase
+sampling frequency.
+
+Note that for profile collection we recommend using cycle events and not
+``BR_INST_RETIRED.*``. Empirically we found it to produce better
+results.
+
+If the collection of a profile with branches is not available, e.g.,
+when you run on a VM or on hardware that does not support it, then you
+can use only sample events, such as cycles. In this case, the quality of
+the profile information would not be as good, and performance gains with
+BOLT are expected to be lower.
+
+With instrumentation
+^^^^^^^^^^^^^^^^^^^^
+
+If perf record is not available to you, you may collect profile by first
+instrumenting the binary with BOLT and then running it.
+
+::
+
+    llvm-bolt <executable> -instrument -o <instrumented-executable>
+
+After you run instrumented-executable with the desired workload, its
+BOLT profile should be ready for you in ``/tmp/prof.fdata`` and you can
+skip **Step 2**.
+
+Run BOLT with the ``-help`` option and check the category “BOLT
+instrumentation options” for a quick reference on instrumentation knobs.
+
+Step 2: Convert Profile to BOLT Format
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+NOTE: you can skip this step and feed ``perf.data`` directly to BOLT
+using experimental ``-p perf.data`` option.
+
+For this step, you will need ``perf.data`` file collected from the
+previous step and a copy of the binary that was running. The binary has
+to be either unstripped, or should have a symbol table intact (i.e.,
+running ``strip -g`` is okay).
+
+Make sure ``perf`` is in your ``PATH``, and execute ``perf2bolt``:
+
+::
+
+    $ perf2bolt -p perf.data -o perf.fdata <executable>
+
+This command will aggregate branch data from ``perf.data`` and store it
+in a format that is both more compact and more resilient to binary
+modifications.
+
+If the profile was collected without LBRs, you will need to add ``-nl``
+flag to the command line above.
+
+Step 3: Optimize with BOLT
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Once you have ``perf.fdata`` ready, you can use it for optimizations
+with BOLT. Assuming your environment is setup to include the right path,
+execute ``llvm-bolt``:
+
+::
+
+    $ llvm-bolt <executable> -o <executable>.bolt -data=perf.fdata -reorder-blocks=ext-tsp -reorder-functions=hfsort -split-functions -split-all-cold -split-eh -dyno-stats
+
+If you do need an updated debug info, then add
+``-update-debug-sections`` option to the command above. The processing
+time will be slightly longer.
+
+For a full list of options see ``-help``/``-help-hidden`` output.
+
+The input binary for this step does not have to 100% match the binary
+used for profile collection in **Step 1**. This could happen when you
+are doing active development, and the source code constantly changes,
+yet you want to benefit from profile-guided optimizations. However,
+since the binary is not precisely the same, the profile information
+could become invalid or stale, and BOLT will report the number of
+functions with a stale profile. The higher the number, the less
+performance improvement should be expected. Thus, it is crucial to
+update ``.fdata`` for release branches.
+
+Multiple Profiles
+-----------------
+
+Suppose your application can run in different modes, and you can
+generate multiple profiles for each one of them. To generate a single
+binary that can benefit all modes (assuming the profiles don’t
+contradict each other) you can use ``merge-fdata`` tool:
+
+::
+
+    $ merge-fdata *.fdata > combined.fdata
+
+Use ``combined.fdata`` for **Step 3** above to generate a universally
+optimized binary.
+
+License
+-------
+
+BOLT is licensed under the `Apache License v2.0 with LLVM
+Exceptions <./LICENSE.TXT>`__.