Revert^2 "Enable full LTO optimization by default" (7549990a) · Commits · e / os / android_build_soong

We set the default optimization mode to --lto-O0 for LTO enabled projects, in order to save build time. This is missing some performance optimizations, esp. related to vectorization. Now that we suggest eng build for developers, we can enable full optimization by default. When we introduced --lto-O0, we achieved a 4.2% saving in system-processes-memory-direct. Enabling full LTO optimization will trade some of the memory / binary size savings for better code performance. For system-processes-memory-direct, it is 2.0% increase compared to --lto-O0, or a net 2.2% saving compared to baseline. Change-Id: I747939ac4d6e4a66e3ef776f4c36eebc7bf34c86

cc/lto.go

+5 −6

Original line number	Diff line number	Diff line
		@@ -54,6 +54,9 @@ type LTOProperties struct {

		// Use -fwhole-program-vtables cflag.
		Whole_program_vtables *bool

		// Use --lto-O0 flag.
		Lto_O0 *bool
		}

		type lto struct {
		@@ -110,12 +113,8 @@ func (lto *lto) flags(ctx ModuleContext, flags Flags) Flags {
		ltoCFlags := []string{"-flto=thin", "-fsplit-lto-unit"}
		var ltoLdFlags []string

		// The module did not explicitly turn on LTO. Only leverage LTO's
		// better dead code elimination and CFG simplification, but do
		// not perform costly optimizations for a balance between compile
		// time, binary size and performance.
		// Apply the same for Eng builds as well.
		if !lto.ThinLTO() \|\| ctx.Config().Eng() {
		// Do not perform costly LTO optimizations for Eng builds.
		if Bool(lto.Properties.Lto_O0) \|\| ctx.Config().Eng() {
		ltoLdFlags = append(ltoLdFlags, "-Wl,--lto-O0")
		}