Add LTO support to soong

Enabling the lto property for a module builds that module and all static
dependencies with LTO.

LTO (link-time optimization) allows the compiler to optimize and
generate code for the entire module at link time, rather than
per-compilation unit. LTO is required for Clang CFI and other
whole-program optimization techniques. LTO also allows cross-compilation
unit optimizations that should result in faster and smaller code, at the
expense of additional compilation time.

Test: make -j12 libc with lto: true for libc

Change-Id: Ib8baefedf60e02701d44673a7c473e0845730101

cc/lto.go

@@ -0,0 +1,112 @@
+// Copyright 2017 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cc
+
+import (
+	"github.com/google/blueprint"
+
+	"android/soong/android"
+)
+
+// LTO (link-time optimization) allows the compiler to optimize and generate
+// code for the entire module at link time, rather than per-compilation
+// unit. LTO is required for Clang CFI and other whole-program optimization
+// techniques. LTO also allows cross-compilation unit optimizations that should
+// result in faster and smaller code, at the expense of additional compilation
+// time.
+//
+// To properly build a module with LTO, the module and all recursive static
+// dependencies should be compiled with -flto which directs the compiler to emit
+// bitcode rather than native object files. These bitcode files are then passed
+// by the linker to the LLVM plugin for compilation at link time. Static
+// dependencies not built as bitcode will still function correctly but cannot be
+// optimized at link time and may not be compatible with features that require
+// LTO, such as CFI.
+//
+// This file adds support to soong to automatically propogate LTO options to a
+// new variant of all static dependencies for each module with LTO enabled.
+
+type LTOProperties struct {
+	// Lto must violate capitialization style for acronyms so that it can be
+	// referred to in blueprint files as "lto"
+	Lto    *bool `android:"arch_variant"`
+	LTODep bool  `blueprint:"mutated"`
+}
+
+type lto struct {
+	Properties LTOProperties
+}
+
+func (lto *lto) props() []interface{} {
+	return []interface{}{&lto.Properties}
+}
+
+func (lto *lto) begin(ctx BaseModuleContext) {
+}
+
+func (lto *lto) deps(ctx BaseModuleContext, deps Deps) Deps {
+	return deps
+}
+
+func (lto *lto) flags(ctx BaseModuleContext, flags Flags) Flags {
+	if Bool(lto.Properties.Lto) {
+		flags.CFlags = append(flags.CFlags, "-flto")
+		flags.LdFlags = append(flags.LdFlags, "-flto")
+		flags.ArFlags = append(flags.ArFlags, " --plugin ${config.LLVMGoldPlugin}")
+	}
+	return flags
+}
+
+// Can be called with a null receiver
+func (lto *lto) LTO() bool {
+	if lto == nil {
+		return false
+	}
+
+	return Bool(lto.Properties.Lto)
+}
+
+// Propagate lto requirements down from binaries
+func ltoDepsMutator(mctx android.TopDownMutatorContext) {
+	if c, ok := mctx.Module().(*Module); ok && c.lto.LTO() {
+		mctx.VisitDepsDepthFirst(func(m blueprint.Module) {
+			tag := mctx.OtherModuleDependencyTag(m)
+			switch tag {
+			case staticDepTag, staticExportDepTag, lateStaticDepTag, wholeStaticDepTag, objDepTag, reuseObjTag:
+				if cc, ok := m.(*Module); ok && cc.lto != nil {
+					cc.lto.Properties.LTODep = true
+				}
+			}
+		})
+	}
+}
+
+// Create lto variants for modules that need them
+func ltoMutator(mctx android.BottomUpMutatorContext) {
+	if c, ok := mctx.Module().(*Module); ok && c.lto != nil {
+		if c.lto.LTO() {
+			mctx.SetDependencyVariation("lto")
+		} else if c.lto.Properties.LTODep {
+			modules := mctx.CreateVariations("", "lto")
+			modules[0].(*Module).lto.Properties.Lto = boolPtr(false)
+			modules[1].(*Module).lto.Properties.Lto = boolPtr(true)
+			modules[0].(*Module).lto.Properties.LTODep = false
+			modules[1].(*Module).lto.Properties.LTODep = false
+			modules[1].(*Module).Properties.PreventInstall = true
+			modules[1].(*Module).Properties.HideFromMake = true
+		}
+		c.lto.Properties.LTODep = false
+	}
+}