Merge "Add comments to cc/cc.go" am: 47f0e63ba0

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/1515403

Change-Id: If3bf5a608443dd7335c601ce540a75d8cf86854b

cc/cc.go

@@ -91,6 +91,14 @@ func RegisterCCBuildComponents(ctx android.RegistrationContext) {
 	ctx.RegisterSingletonType("kythe_extract_all", kytheExtractAllFactory)
 }
 
+// Deps is a struct containing module names of dependencies, separated by the kind of dependency.
+// Mutators should use `AddVariationDependencies` or its sibling methods to add actual dependency
+// edges to these modules.
+// This object is constructed in DepsMutator, by calling to various module delegates to set
+// relevant fields. For example, `module.compiler.compilerDeps()` may append type-specific
+// dependencies.
+// This is then consumed by the same DepsMutator, which will call `ctx.AddVariationDependencies()`
+// (or its sibling methods) to set real dependencies on the given modules.
 type Deps struct {
 	SharedLibs, LateSharedLibs                  []string
 	StaticLibs, LateStaticLibs, WholeStaticLibs []string
@@ -103,6 +111,7 @@ type Deps struct {
 	// Used by DepsMutator to pass system_shared_libs information to check_elf_file.py.
 	SystemSharedLibs []string
 
+	// If true, statically link the unwinder into native libraries/binaries.
 	StaticUnwinderIfLegacy bool
 
 	ReexportSharedLibHeaders, ReexportStaticLibHeaders, ReexportHeaderLibHeaders []string
@@ -122,6 +131,11 @@ type Deps struct {
 	DynamicLinker   string
 }
 
+// PathDeps is a struct containing file paths to dependencies of a module.
+// It's constructed in depsToPath() by traversing the direct dependencies of the current module.
+// It's used to construct flags for various build statements (such as for compiling and linking).
+// It is then passed to module decorator functions responsible for registering build statements
+// (such as `module.compiler.compile()`).`
 type PathDeps struct {
 	// Paths to .so files
 	SharedLibs, EarlySharedLibs, LateSharedLibs android.Paths
@@ -182,8 +196,12 @@ type LocalOrGlobalFlags struct {
 	LdFlags         []string // Flags that apply to linker command lines
 }
 
+// Flags contains various types of command line flags (and settings) for use in building build
+// statements related to C++.
 type Flags struct {
-	Local  LocalOrGlobalFlags
+	// Local flags (which individual modules are responsible for). These may override global flags.
+	Local LocalOrGlobalFlags
+	// Global flags (which build system or toolchain is responsible for).
 	Global LocalOrGlobalFlags
 
 	aidlFlags     []string // Flags that apply to aidl source files
@@ -198,19 +216,23 @@ type Flags struct {
 	SystemIncludeFlags []string
 
 	Toolchain    config.Toolchain
-	Tidy         bool
-	GcovCoverage bool
-	SAbiDump     bool
+	Tidy         bool // True if clang-tidy is enabled.
+	GcovCoverage bool // True if coverage files should be generated.
+	SAbiDump     bool // True if header abi dumps should be generated.
 	EmitXrefs    bool // If true, generate Ninja rules to generate emitXrefs input files for Kythe
 
+	// The instruction set required for clang ("arm" or "thumb").
 	RequiredInstructionSet string
-	DynamicLinker          string
+	// The target-device system path to the dynamic linker.
+	DynamicLinker string
 
 	CFlagsDeps  android.Paths // Files depended on by compiler flags
 	LdFlagsDeps android.Paths // Files depended on by linker flags
 
+	// True if .s files should be processed with the c preprocessor.
 	AssemblerWithCpp bool
-	GroupStaticLibs  bool
+	// True if static libraries should be grouped (using `-Wl,--start-group` and `-Wl,--end-group`).
+	GroupStaticLibs bool
 
 	proto            android.ProtoFlags
 	protoC           bool // Whether to use C instead of C++
@@ -358,6 +380,10 @@ type VendorProperties struct {
 	Double_loadable *bool
 }
 
+// ModuleContextIntf is an interface (on a module context helper) consisting of functions related
+// to understanding  details about the type of the current module.
+// For example, one might call these functions to determine whether the current module is a static
+// library and/or is installed in vendor directories.
 type ModuleContextIntf interface {
 	static() bool
 	staticBinary() bool
@@ -412,6 +438,8 @@ type DepsContext interface {
 	ModuleContextIntf
 }
 
+// feature represents additional (optional) steps to building cc-related modules, such as invocation
+// of clang-tidy.
 type feature interface {
 	begin(ctx BaseModuleContext)
 	deps(ctx DepsContext, deps Deps) Deps
@@ -419,6 +447,9 @@ type feature interface {
 	props() []interface{}
 }
 
+// compiler is the interface for a compiler helper object. Different module decorators may implement
+// this helper differently. For example, compiling a `cc_library` may use a different build
+// statement than building a `toolchain_library`.
 type compiler interface {
 	compilerInit(ctx BaseModuleContext)
 	compilerDeps(ctx DepsContext, deps Deps) Deps
@@ -430,6 +461,9 @@ type compiler interface {
 	compile(ctx ModuleContext, flags Flags, deps PathDeps) Objects
 }
 
+// linker is the interface for a linker decorator object. Individual module types can provide
+// their own implementation for this decorator, and thus specify custom logic regarding build
+// statements pertaining to linking.
 type linker interface {
 	linkerInit(ctx BaseModuleContext)
 	linkerDeps(ctx DepsContext, deps Deps) Deps
@@ -445,15 +479,19 @@ type linker interface {
 	coverageOutputFilePath() android.OptionalPath
 
 	// Get the deps that have been explicitly specified in the properties.
-	// Only updates the
 	linkerSpecifiedDeps(specifiedDeps specifiedDeps) specifiedDeps
 }
 
+// specifiedDeps is a tuple struct representing dependencies of a linked binary owned by the linker.
 type specifiedDeps struct {
-	sharedLibs       []string
-	systemSharedLibs []string // Note nil and [] are semantically distinct.
+	sharedLibs []string
+	// Note nil and [] are semantically distinct. [] prevents linking against the defaults (usually
+	// libc, libm, etc.)
+	systemSharedLibs []string
 }
 
+// installer is the interface for an installer helper object. This helper is responsible for
+// copying build outputs to the appropriate locations so that they may be installed on device.
 type installer interface {
 	installerProps() []interface{}
 	install(ctx ModuleContext, path android.Path)
@@ -626,7 +664,18 @@ func IsTestPerSrcDepTag(depTag blueprint.DependencyTag) bool {
 
 // Module contains the properties and members used by all C/C++ module types, and implements
 // the blueprint.Module interface.  It delegates to compiler, linker, and installer interfaces
-// to construct the output file.  Behavior can be customized with a Customizer interface
+// to construct the output file.  Behavior can be customized with a Customizer, or "decorator",
+// interface.
+//
+// To define a C/C++ related module, construct a new Module object and point its delegates to
+// type-specific structs. These delegates will be invoked to register module-specific build
+// statements which may be unique to the module type. For example, module.compiler.compile() should
+// be defined so as to register build statements which are responsible for compiling the module.
+//
+// Another example: to construct a cc_binary module, one can create a `cc.binaryDecorator` struct
+// which implements the `linker` and `installer` interfaces, and points the `linker` and `installer`
+// members of the cc.Module to this decorator. Thus, a cc_binary module has custom linker and
+// installer logic.
 type Module struct {
 	android.ModuleBase
 	android.DefaultableModuleBase
@@ -643,18 +692,23 @@ type Module struct {
 	// Allowable SdkMemberTypes of this module type.
 	sdkMemberTypes []android.SdkMemberType
 
-	// delegates, initialize before calling Init
-	features  []feature
+	// decorator delegates, initialize before calling Init
+	// these may contain module-specific implementations, and effectively allow for custom
+	// type-specific logic. These members may reference different objects or the same object.
+	// Functions of these decorators will be invoked to initialize and register type-specific
+	// build statements.
 	compiler  compiler
 	linker    linker
 	installer installer
-	stl       *stl
-	sanitize  *sanitize
-	coverage  *coverage
-	sabi      *sabi
-	vndkdep   *vndkdep
-	lto       *lto
-	pgo       *pgo
+
+	features []feature
+	stl      *stl
+	sanitize *sanitize
+	coverage *coverage
+	sabi     *sabi
+	vndkdep  *vndkdep
+	lto      *lto
+	pgo      *pgo
 
 	library libraryInterface