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Meta: Add the start of a gn build based on the LLVM gn build
"based on" in this context means "largely copied from"
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170
Meta/gn/build/BUILD.gn
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170
Meta/gn/build/BUILD.gn
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import("//Meta/gn/build/buildflags.gni")
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import("//Meta/gn/build/mac_sdk.gni")
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import("//Meta/gn/build/sysroot.gni")
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import("//Meta/gn/build/toolchain/compiler.gni")
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declare_args() {
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# If set, puts relative paths in debug info.
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# Makes the build output independent of the build directory, but makes
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# most debuggers harder to use. See "Getting to local determinism" and
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# "Getting debuggers to work well with locally deterministic builds" in
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# http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
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# for more information.
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use_relative_paths_in_debug_info = false
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# The version of host gcc. Ignored if is_clang is true.
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gcc_version = 12
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}
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config("compiler_defaults") {
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defines = []
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asmflags = []
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cflags = []
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cflags_cc = []
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ldflags = []
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if (symbol_level == 2) {
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cflags += [ "-g" ]
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# For full debug-info -g builds, --gdb-index makes links ~15% slower, and
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# gdb symbol reading time 1500% faster (lld links in 4.4 instead of 3.9s,
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# and gdb loads and runs it in 2s instead of in 30s). It's likely that
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# people doing symbol_level=2 want to run a debugger (since
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# symbol_level=2 isn't the default). So this seems like the right
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# tradeoff.
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if (current_os != "mac" && use_lld) {
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cflags += [ "-ggnu-pubnames" ] # llvm PR34820
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ldflags += [ "-Wl,--gdb-index" ]
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# Use debug fission. In this mode, detailed debug information is
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# written to a .dwo file next to each .o file instead of into the .o
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# file directly. The linker then only links the .o files, which contain
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# a pointer to each .dwo file. The debugger then reads debug info out
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# of all the .dwo files instead of from the binary.
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#
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# (The dwp tool can link all the debug info together into a single
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# "debug info binary", but that's not done as part of the build.)
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#
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# This requires `-Wl,--gdb-index` (above) to work well.
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#
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# With lld, this reduces link time:
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# - in release + symbol_level=2 builds: From 2.3s to 1.3s
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# - in debug builds: From 5.2s to 4.6s
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#
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# Time needed for gdb startup and setting a breakpoint is comparable,
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# the time from from `r` to hititng a breakpoint on main goes from 4s
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# to 2s.
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#
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# (macOS's linker always keeps debug info out of its output executables
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# and debuggers there also know to load debug info from the .o files.
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# macOS also has a debug info linker like dwp, it's called dsymutil.
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# This happens by default, so there's no need to pass a flag there.)
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cflags += [ "-gsplit-dwarf" ]
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ldflags += [ "-gsplit-dwarf" ] # Needed for ThinLTO builds.
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}
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} else if (symbol_level == 1) {
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cflags += [ "-g1" ]
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# For linetable-only -g1 builds, --gdb-index makes links ~8% slower, but
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# links are 4x faster than -g builds so it's a fairly small absolute cost.
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# On the other hand, gdb startup is well below 1s with and without the
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# index, and people using -g1 likely don't use a debugger. So don't use
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# the flag here.
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# Linetables always go in the .o file, even with -gsplit-dwarf, so there's
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# no point in passing -gsplit-dwarf here.
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}
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if (is_optimized) {
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cflags += [ "-O3" ]
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}
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cflags += [ "-fdiagnostics-color" ]
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if (use_lld) {
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ldflags += [ "-Wl,--color-diagnostics" ]
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}
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cflags_cc += [
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"-std=c++20",
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"-fvisibility-inlines-hidden",
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]
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# Warning setup.
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cflags += [
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"-Wall",
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"-Wextra",
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]
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cflags += [ "-Wno-unused-parameter" ]
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if (is_clang) {
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cflags += [
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"-Wdelete-non-virtual-dtor",
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"-Wstring-conversion",
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"-Wno-user-defined-literals",
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"-fconstexpr-steps=16777216",
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"-Wno-unused-private-field",
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]
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} else {
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cflags += [
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# Disable gcc's potentially uninitialized use analysis as it presents
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# lots of false positives.
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"-Wno-maybe-uninitialized",
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# Disable -Wredundant-move on GCC>=9. GCC wants to remove std::move
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# in code like "A foo(ConvertibleToA a) { return std::move(a); }",
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# but this code does not compile (or uses the copy constructor
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# instead) on clang<=3.8. Clang also has a -Wredundant-move, but it
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# only fires when the types match exactly, so we can keep it here.
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"-Wno-redundant-move",
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]
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}
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if (use_lld) {
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ldflags += [ "-fuse-ld=lld" ]
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}
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# Deterministic build setup, see
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# http://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html
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if (is_clang) {
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cflags += [
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"-no-canonical-prefixes",
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"-Werror=date-time",
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]
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if (use_relative_paths_in_debug_info) {
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cflags += [ "-fdebug-compilation-dir=." ]
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}
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}
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if (sysroot != "") {
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if (current_os != "mac" && current_os != "android") {
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cflags += [ "--sysroot=" + rebase_path(sysroot, root_build_dir) ]
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}
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}
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if (current_os == "mac" && sysroot != "") {
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sdk_path = mac_sdk_path
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cflags += [
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"-isysroot",
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rebase_path(sdk_path, root_build_dir),
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]
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ldflags += [
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"-isysroot",
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rebase_path(sdk_path, root_build_dir),
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]
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}
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if (sysroot != "" && current_os != "win" && is_clang) {
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cflags += [ "-Wpoison-system-directories" ]
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}
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cflags_objcc = cflags_cc
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}
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config("no_exceptions") {
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cflags_cc = [ "-fno-exceptions" ]
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cflags_objcc = cflags_cc
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}
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config("zdefs") {
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if (current_os != "mac" && current_os != "android") {
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ldflags = [ "-Wl,-z,defs" ]
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}
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}
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config("pic") {
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if (current_os != "mac") {
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cflags_cc = [ "-fPIC" ]
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}
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}
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