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// 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{}{<o.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")
if ctx.Device() {
// Work around bug in Clang that doesn't pass correct emulated
// TLS option to target
flags.LdFlags = append(flags.LdFlags, "-Wl,-plugin-opt,-emulated-tls")
}
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
}
}
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