f5b9b95f01
In LTO mode the backend constructs a target exclusively from command line flags, without opportunity to accept code gen arguments from Clang. Clang does not currently pass the -emulated-tls parameter for Android, even though it defaults to emulated TLS for Android in non-LTO builds. Until this is fixed upstream, we can explicitly specify that the LTO output for Android should use emulated TLS. Test: build libandroid with LTO, verified that it uses __emutls_get_address rather than __tls_get_addr. Change-Id: I18acac41aac885fc6635fbd55f96ba7c845eb5e7
117 lines
3.9 KiB
Go
117 lines
3.9 KiB
Go
// Copyright 2017 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package cc
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import (
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"github.com/google/blueprint"
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"android/soong/android"
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)
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// LTO (link-time optimization) allows the compiler to optimize and generate
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// code for the entire module at link time, rather than per-compilation
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// unit. LTO is required for Clang CFI and other whole-program optimization
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// techniques. LTO also allows cross-compilation unit optimizations that should
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// result in faster and smaller code, at the expense of additional compilation
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// time.
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//
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// To properly build a module with LTO, the module and all recursive static
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// dependencies should be compiled with -flto which directs the compiler to emit
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// bitcode rather than native object files. These bitcode files are then passed
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// by the linker to the LLVM plugin for compilation at link time. Static
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// dependencies not built as bitcode will still function correctly but cannot be
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// optimized at link time and may not be compatible with features that require
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// LTO, such as CFI.
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//
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// This file adds support to soong to automatically propogate LTO options to a
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// new variant of all static dependencies for each module with LTO enabled.
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type LTOProperties struct {
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// Lto must violate capitialization style for acronyms so that it can be
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// referred to in blueprint files as "lto"
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Lto *bool `android:"arch_variant"`
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LTODep bool `blueprint:"mutated"`
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}
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type lto struct {
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Properties LTOProperties
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}
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func (lto *lto) props() []interface{} {
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return []interface{}{<o.Properties}
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}
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func (lto *lto) begin(ctx BaseModuleContext) {
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}
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func (lto *lto) deps(ctx BaseModuleContext, deps Deps) Deps {
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return deps
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}
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func (lto *lto) flags(ctx BaseModuleContext, flags Flags) Flags {
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if Bool(lto.Properties.Lto) {
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flags.CFlags = append(flags.CFlags, "-flto")
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flags.LdFlags = append(flags.LdFlags, "-flto")
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if ctx.Device() {
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// Work around bug in Clang that doesn't pass correct emulated
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// TLS option to target
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flags.LdFlags = append(flags.LdFlags, "-Wl,-plugin-opt,-emulated-tls")
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}
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flags.ArFlags = append(flags.ArFlags, " --plugin ${config.LLVMGoldPlugin}")
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}
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return flags
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}
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// Can be called with a null receiver
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func (lto *lto) LTO() bool {
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if lto == nil {
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return false
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}
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return Bool(lto.Properties.Lto)
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}
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// Propagate lto requirements down from binaries
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func ltoDepsMutator(mctx android.TopDownMutatorContext) {
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if c, ok := mctx.Module().(*Module); ok && c.lto.LTO() {
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mctx.VisitDepsDepthFirst(func(m blueprint.Module) {
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tag := mctx.OtherModuleDependencyTag(m)
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switch tag {
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case staticDepTag, staticExportDepTag, lateStaticDepTag, wholeStaticDepTag, objDepTag, reuseObjTag:
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if cc, ok := m.(*Module); ok && cc.lto != nil {
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cc.lto.Properties.LTODep = true
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}
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}
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})
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}
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}
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// Create lto variants for modules that need them
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func ltoMutator(mctx android.BottomUpMutatorContext) {
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if c, ok := mctx.Module().(*Module); ok && c.lto != nil {
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if c.lto.LTO() {
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mctx.SetDependencyVariation("lto")
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} else if c.lto.Properties.LTODep {
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modules := mctx.CreateVariations("", "lto")
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modules[0].(*Module).lto.Properties.Lto = boolPtr(false)
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modules[1].(*Module).lto.Properties.Lto = boolPtr(true)
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modules[0].(*Module).lto.Properties.LTODep = false
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modules[1].(*Module).lto.Properties.LTODep = false
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modules[1].(*Module).Properties.PreventInstall = true
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modules[1].(*Module).Properties.HideFromMake = true
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}
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c.lto.Properties.LTODep = false
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}
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}
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