cc/lto.go

// 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 (
	"fmt"

	"github.com/google/blueprint"
	"github.com/google/blueprint/proptools"

	"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 propagate LTO options to a
// new variant of all static dependencies for each module with LTO enabled.

type LTOProperties struct {
	// Lto must violate capitalization style for acronyms so that it can be
	// referred to in blueprint files as "lto"
	Lto struct {
		Never *bool `android:"arch_variant"`
		Thin  *bool `android:"arch_variant"`
	} `android:"arch_variant"`

	LtoEnabled bool `blueprint:"mutated"`
	LtoDefault bool `blueprint:"mutated"`

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

type lto struct {
	Properties LTOProperties
}

func (lto *lto) props() []interface{} {
	return []interface{}{&lto.Properties}
}

func (lto *lto) begin(ctx BaseModuleContext) {
	// First, determine the module independent default LTO mode.
	ltoDefault := true
	if ctx.Config().IsEnvTrue("DISABLE_LTO") {
		ltoDefault = false
	} else if lto.Never() {
		ltoDefault = false
	} else if ctx.Host() {
		// Performance and binary size are less important for host binaries.
		ltoDefault = false
	} else if ctx.Arch().ArchType.Multilib == "lib32" {
		// LP32 has many subtle issues and less test coverage.
		ltoDefault = false
	}

	// Then, determine the actual LTO mode to use. If different from `ltoDefault`, a variant needs
	// to be created.
	ltoEnabled := ltoDefault
	if lto.Never() {
		ltoEnabled = false
	} else if lto.ThinLTO() {
		// Module explicitly requests for LTO.
		ltoEnabled = true
	} else if ctx.testBinary() || ctx.testLibrary() {
		// Do not enable LTO for tests for better debugging.
		ltoEnabled = false
	}

	lto.Properties.LtoDefault = ltoDefault
	lto.Properties.LtoEnabled = ltoEnabled
}

func (lto *lto) flags(ctx ModuleContext, flags Flags) Flags {
	// TODO(b/131771163): CFI and Fuzzer controls LTO flags by themselves.
	// This has be checked late because these properties can be mutated.
	if ctx.isCfi() || ctx.isFuzzer() {
		return flags
	}
	if lto.Properties.LtoEnabled {
		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() {
			ltoLdFlags = append(ltoLdFlags, "-Wl,--lto-O0")
		}

		if Bool(lto.Properties.Whole_program_vtables) {
			ltoCFlags = append(ltoCFlags, "-fwhole-program-vtables")
		}

		if ctx.Config().IsEnvTrue("USE_THINLTO_CACHE") {
			// Set appropriate ThinLTO cache policy
			cacheDirFormat := "-Wl,--thinlto-cache-dir="
			cacheDir := android.PathForOutput(ctx, "thinlto-cache").String()
			ltoLdFlags = append(ltoLdFlags, cacheDirFormat+cacheDir)

			// Limit the size of the ThinLTO cache to the lesser of 10% of available
			// disk space and 10GB.
			cachePolicyFormat := "-Wl,--thinlto-cache-policy="
			policy := "cache_size=10%:cache_size_bytes=10g"
			ltoLdFlags = append(ltoLdFlags, cachePolicyFormat+policy)
		}

		// Reduce the inlining threshold for a better balance of binary size and
		// performance.
		if !ctx.Darwin() {
			if ctx.isAfdoCompile(ctx) {
				ltoLdFlags = append(ltoLdFlags, "-Wl,-plugin-opt,-import-instr-limit=40")
			} else {
				ltoLdFlags = append(ltoLdFlags, "-Wl,-plugin-opt,-import-instr-limit=5")
			}
		}

		if !ctx.Config().IsEnvFalse("THINLTO_USE_MLGO") {
			// Register allocation MLGO flags for ARM64.
			if ctx.Arch().ArchType == android.Arm64 && !ctx.optimizeForSize() {
				ltoLdFlags = append(ltoLdFlags, "-Wl,-mllvm,-regalloc-enable-advisor=release")
			}
			// Flags for training MLGO model.
			if ctx.Config().IsEnvTrue("THINLTO_EMIT_INDEXES_AND_IMPORTS") {
				ltoLdFlags = append(ltoLdFlags, "-Wl,--save-temps=import")
				ltoLdFlags = append(ltoLdFlags, "-Wl,--thinlto-emit-index-files")
			}
		}

		flags.Local.CFlags = append(flags.Local.CFlags, ltoCFlags...)
		flags.Local.AsFlags = append(flags.Local.AsFlags, ltoCFlags...)
		flags.Local.LdFlags = append(flags.Local.LdFlags, ltoCFlags...)
		flags.Local.LdFlags = append(flags.Local.LdFlags, ltoLdFlags...)
	}
	return flags
}

func (lto *lto) ThinLTO() bool {
	return lto != nil && proptools.Bool(lto.Properties.Lto.Thin)
}

func (lto *lto) Never() bool {
	return lto != nil && proptools.Bool(lto.Properties.Lto.Never)
}

func ltoPropagateViaDepTag(tag blueprint.DependencyTag) bool {
	libTag, isLibTag := tag.(libraryDependencyTag)
	// Do not recurse down non-static dependencies
	if isLibTag {
		return libTag.static()
	} else {
		return tag == objDepTag || tag == reuseObjTag || tag == staticVariantTag
	}
}

// ltoTransitionMutator creates LTO variants of cc modules.  Variant "" is the default variant, which may
// or may not have LTO enabled depending on the config and the module's type and properties.  "lto-thin" or
// "lto-none" variants are created when a module needs to compile in the non-default state for that module.
type ltoTransitionMutator struct{}

const LTO_NONE_VARIATION = "lto-none"
const LTO_THIN_VARIATION = "lto-thin"

func (l *ltoTransitionMutator) Split(ctx android.BaseModuleContext) []string {
	return []string{""}
}

func (l *ltoTransitionMutator) OutgoingTransition(ctx android.OutgoingTransitionContext, sourceVariation string) string {
	if m, ok := ctx.Module().(*Module); ok && m.lto != nil {
		if !ltoPropagateViaDepTag(ctx.DepTag()) {
			return ""
		}

		if sourceVariation != "" {
			return sourceVariation
		}

		// Always request an explicit variation, IncomingTransition will rewrite it back to the default variation
		// if necessary.
		if m.lto.Properties.LtoEnabled {
			return LTO_THIN_VARIATION
		} else {
			return LTO_NONE_VARIATION
		}
	}
	return ""
}

func (l *ltoTransitionMutator) IncomingTransition(ctx android.IncomingTransitionContext, incomingVariation string) string {
	if m, ok := ctx.Module().(*Module); ok && m.lto != nil {
		if m.lto.Never() {
			return ""
		}
		// Rewrite explicit variations back to the default variation if the default variation matches.
		if incomingVariation == LTO_THIN_VARIATION && m.lto.Properties.LtoDefault {
			return ""
		} else if incomingVariation == LTO_NONE_VARIATION && !m.lto.Properties.LtoDefault {
			return ""
		}
		return incomingVariation
	}
	return ""
}

func (l *ltoTransitionMutator) Mutate(ctx android.BottomUpMutatorContext, variation string) {
	// Default module which will be installed. Variation set above according to explicit LTO properties.
	if variation == "" {
		return
	}

	if m, ok := ctx.Module().(*Module); ok && m.lto != nil {
		// Non-default variation, set the LTO properties to match the variation.
		switch variation {
		case LTO_THIN_VARIATION:
			m.lto.Properties.LtoEnabled = true
		case LTO_NONE_VARIATION:
			m.lto.Properties.LtoEnabled = false
		default:
			panic(fmt.Errorf("unknown variation %s", variation))
		}
		// Non-default variations are never installed.
		m.Properties.PreventInstall = true
		m.Properties.HideFromMake = true
	}
}