Enable clang build timing analysis

I didn't enable this by default for clang due to making the build time
10% worse or so. Unfortunate, but tbh devs for whom 10% of build time is
not *that* bad should probably simply enable this.

Change-Id: I8d1e5b6f3f76c649a4e2f115f534f7f97cee46e6

.gitignore

@@ -154,3 +154,6 @@ result-*
 
 # Mac OS
 .DS_Store
+
+# ClangBuildAnalyzer output, see maintainers/buildtime_report.sh
+buildtime.bin

doc/manual/rl-next-dev/build-timing-analysis.md

@@ -0,0 +1,15 @@
+---
+synopsis: Clang build timing analysis
+cls: 587
+---
+
+We now have Clang build profiling available, which generates Chrome
+tracing files for each compilation unit. To enable it, run `meson configure
+build -Dprofile-build=enabled` then rerun the compilation.
+
+If you want to make the build go faster, do a clang build with meson, then run
+`maintainers/buildtime_report.sh build`, then contemplate how to improve the
+build time.
+
+You can also look at individual object files' traces in
+<https://ui.perfetto.dev>.

flake.nix

@@ -152,6 +152,7 @@
 
           build-release-notes =
             final.buildPackages.callPackage ./maintainers/build-release-notes.nix { };
+          clangbuildanalyzer = final.buildPackages.callPackage ./misc/clangbuildanalyzer.nix { };
           boehmgc-nix = (final.boehmgc.override {
             enableLargeConfig = true;
           }).overrideAttrs (o: {
@@ -360,6 +361,8 @@
                   # FIXME(Qyriad): remove once the migration to Meson is complete.
                   pkgs.buildPackages.meson
                   pkgs.buildPackages.ninja
+
+                  pkgs.buildPackages.clangbuildanalyzer
                 ];
 
               src = null;

maintainers/buildtime_report.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+# Generates a report of build time based on a meson build using -ftime-trace in
+# Clang.
+if [ $# -lt 1 ]; then
+    echo "usage: $0 BUILD-DIR [filename]" >&2
+    exit 1
+fi
+
+scriptdir=$(cd "$(dirname -- "$0")" || exit ; pwd -P)
+filename=${2:-$scriptdir/../buildtime.bin}
+
+if [ "$(meson introspect "$1" --buildoptions | jq -r '.[]  | select(.name == "profile-build") | .value')" != enabled ]; then
+    echo 'This build was not done with profile-build enabled, so cannot generate a report' >&2
+    # shellcheck disable=SC2016
+    echo 'Run `meson configure build -Dprofile-build=enabled` then rebuild, first' >&2
+    exit 1
+fi
+
+ClangBuildAnalyzer --all "$1" "$filename" && ClangBuildAnalyzer --analyze "$filename"

meson.build

@@ -307,6 +307,14 @@ if cxx.get_linker_id() in ['ld.bfd', 'ld.gold']
   add_project_link_arguments('-Wl,--no-copy-dt-needed-entries', language : 'cpp')
 endif
 
+# Generate Chromium tracing files for each compiled file, which enables
+# maintainers/buildtime_report.sh BUILD-DIR to simply work in clang builds.
+#
+# They can also be manually viewed at https://ui.perfetto.dev
+if get_option('profile-build').require(meson.get_compiler('cpp').get_id() == 'clang').enabled()
+  add_project_arguments('-ftime-trace', language: 'cpp')
+endif
+
 subdir('src')
 if enable_tests
   subdir('tests/unit')

meson.options

@@ -31,6 +31,10 @@ option('tests-brief', type : 'boolean', value : false,
   description : 'set to true for shorter tests output',
 )
 
+option('profile-build', type : 'feature', value: 'disabled',
+  description : 'whether to enable -ftime-trace in clang builds, allowing for speeding up the build.'
+)
+
 option('store-dir', type : 'string', value : '/nix/store',
   description : 'path of the Nix store',
 )

misc/clangbuildanalyzer.nix

@@ -0,0 +1,27 @@
+# Upstreaming here, can be deleted once it's upstreamed:
+# https://github.com/NixOS/nixpkgs/pull/297102
+{ stdenv, lib, cmake, fetchFromGitHub }:
+stdenv.mkDerivation (finalAttrs: {
+  pname = "clangbuildanalyzer";
+  version = "1.5.0";
+
+  src = fetchFromGitHub {
+    owner = "aras-p";
+    repo = "ClangBuildAnalyzer";
+    rev = "v${finalAttrs.version}";
+    sha256 = "sha256-kmgdk634zM0W0OoRoP/RzepArSipa5bNqdVgdZO9gxo=";
+  };
+
+  nativeBuildInputs = [
+    cmake
+  ];
+
+  meta = {
+    description = "Tool for analyzing Clang's -ftrace-time files";
+    homepage = "https://github.com/aras-p/ClangBuildAnalyzer";
+    maintainers = with lib.maintainers; [ lf- ];
+    license = lib.licenses.unlicense;
+    platforms = lib.platforms.unix;
+    mainProgram = "ClangBuildAnalyzer";
+  };
+})