Expand use and support for LTO and FDO builds

Adds LTO to the CI build for Linux, which bring it as close as possible
to the planned formal release, which will additionally use FDO.

Adds some helper scripts to work with FDO files.

Improves the build notes for how to create and use FDO files.

.github/scripts/build-autofdo.sh

@@ -0,0 +1,64 @@
+#!/bin/bash
+
+# Copyright (c) 2020 Kevin R Croft <krcroft@gmail.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# Builds and installs the AutoFDO package from
+# https://github.com/google/autofdo
+#
+# AutoFDO is used to convert sample profiling data (collected using perf
+# or ocperf.py from a Linux kernel built with with last branch
+# record (LBR) tracing support running on an Intel CPU with LBR
+# support) to GCC-specific AutoFDO records or LLVM's raw profile
+# records.
+#
+# Pre-requisites: autoconf automake git libelf-dev libssl-dev pkg-config
+# If building for LLVM, clang and llvm-dev are needed.
+#
+# Usage: build-autofdo.sh [LLVM version]
+# Examples: ./build-autofdo.sh
+#           ./build-autofdo.sh 10
+#
+# Where the optional [LLVM version] allows building with LLVM support
+# for the provided LLVM version.
+
+set -euo pipefail
+
+rootdir="$(pwd)"
+prefix="$rootdir/afdo"
+
+# Clone the repo
+if [[ ! -d autofdo ]]; then
+	git clone --depth 1 --recursive https://github.com/google/autofdo.git
+fi
+
+# Enter and sync the repo (if it already exists)
+pushd autofdo
+git pull
+
+# Initialize auto-tools
+aclocal -I .
+autoheader
+autoconf
+automake --add-missing -c
+
+# Configure with the specified LLVM version if provided
+if [[ -n "${1:-}" ]]; then
+	ver="$1"
+	withllvm="--with-llvm=$(command -v llvm-config-"$ver")"
+fi
+
+# Configure
+flags="-Os -DNDEBUG -pipe"
+./configure CFLAGS="$flags" CXXFLAGS="$flags" --prefix="$prefix" "${withllvm:-}"
+
+# Build and install
+# Note: make cannot be run in parallel because the sub-projects'
+#       need to be configured serially with respect to eachother
+make
+make install
+popd
+
+# Strip the binaries
+cd "$prefix/bin"
+strip ./*

.github/scripts/fetch-and-merge-afdo.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# Copyright (c) 2019-2020 Kevin R Croft <krcroft@gmail.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# A helper script that fetches, converts, and merges kernel sample
+# (.prof) files (collected during prior DOSBox testing) into a single
+# GCC-compatible AutoFDO record that can be used to optimize builds.
+
+# Depedencies:
+#   - zstd
+#   - autofdo
+
+set -euo pipefail
+
+# Tarball containing profile records
+PROFILES="https://gitlab.com/luxtorpeda/dosbox-tests/-/raw/master/archives/profiles.tar.zst"
+BINARY="tests/dosbox"
+
+# Move to our repo root
+cd "$(git rev-parse --show-toplevel)"
+
+# Fetch and unpack the profiles
+wget "${PROFILES}" -O - | zstd -d | tar -x
+
+# Convert and merge the profiles
+find . -name '*.prof' -print0 \
+   | xargs -0 -P "$(nproc)" -I {} \
+     create_gcov --binary="${BINARY}" --profile="{}" -gcov="{}".afdo -gcov_version=1
+profile_merger -gcov_version=1 -output_file=current.afdo tests/*/*.afdo

.github/scripts/fetch-and-merge-profraw.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# Copyright (c) 2020 Kevin R Croft <krcroft@gmail.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# A helper script that fetches, converts, and merges kernel sample
+# (.prof) files (collected during prior DOSBox testing) into a single
+# LLVM-compatible Raw Profile record that can be used to optimize builds.
+
+# Depedencies:
+#   - zstd
+#   - autofdo
+
+set -euo pipefail
+
+# The tarball containing one or more profile records
+PROFILES="https://gitlab.com/luxtorpeda/dosbox-tests/-/raw/master/archives/profiles.tar.zst"
+BINARY="tests/dosbox"
+
+# Move to our repo root
+cd "$(git rev-parse --show-toplevel)"
+
+# Fetch and unpack the profiles
+wget "${PROFILES}" -O - | zstd -d | tar -x
+
+# Convert and merge the profiles
+find . -name '*.prof' -print0 \
+   | xargs -0 -P "$(nproc)" -I {} \
+     create_llvm_prof --binary="${BINARY}" --profile="{}" --out="{}".profraw
+llvm-profdata-9 merge -sample -output=current.profraw tests/*/*.profraw