[dev.unified] cmd/compile: implement simple inline body pruning heuristic

An important optimization in the existing export data format is the
pruning of unreachable inline bodies. That is, when re-exporting
transitively imported types, omitting the inline bodies for methods
that can't actually be needed due to importing that package.

The existing logic (implemented in typecheck/crawler.go) is fairly
sophisticated, but also relies on actually expanding inline bodies in
the process, which is undesirable. However, including all inline
bodies is also prohibitive for testing GOEXPERIMENT=unified against
very large Go code bases that impose size limits on build action
inputs.

As a short-term solution, this CL implements a simple heuristic for
GOEXPERIMENT=unified: include the inline bodies for all
locally-declared functions/methods, and for any imported
functions/methods that were inlined into this package.

Change-Id: I686964a0cd9262b77d3d5587f89cfbcfe8b2e521
Reviewed-on: https://go-review.googlesource.com/c/go/+/419675
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/noder/import.go

@@ -241,7 +241,7 @@ func readImportFile(path string, target *ir.Package, env *types2.Context, packag
 		pr := pkgbits.NewPkgDecoder(pkg1.Path, data)
 
 		// Read package descriptors for both types2 and compiler backend.
-		readPackage(newPkgReader(pr), pkg1)
+		readPackage(newPkgReader(pr), pkg1, false)
 		pkg2 = importer.ReadPackage(env, packages, pr)
 
 	case 'i':

src/cmd/compile/internal/noder/linker.go

@@ -38,8 +38,9 @@ import (
 type linker struct {
 	pw pkgbits.PkgEncoder
 
-	pkgs  map[string]pkgbits.Index
-	decls map[*types.Sym]pkgbits.Index
+	pkgs   map[string]pkgbits.Index
+	decls  map[*types.Sym]pkgbits.Index
+	bodies map[*types.Sym]pkgbits.Index
 }
 
 // relocAll ensures that all elements specified by pr and relocs are
@@ -170,21 +171,12 @@ func (l *linker) relocObj(pr *pkgReader, idx pkgbits.Index) pkgbits.Index {
 	l.relocCommon(pr, &wname, pkgbits.RelocName, idx)
 	l.relocCommon(pr, &wdict, pkgbits.RelocObjDict, idx)
 
-	var obj *ir.Name
-	if sym.Pkg == types.LocalPkg {
-		var ok bool
-		obj, ok = sym.Def.(*ir.Name)
+	// Generic types and functions won't have definitions, and imported
+	// objects may not either.
+	obj, _ := sym.Def.(*ir.Name)
+	local := sym.Pkg == types.LocalPkg
 
-		// Generic types and functions and declared constraint types won't
-		// have definitions.
-		// For now, just generically copy their extension data.
-		// TODO(mdempsky): Restore assertion.
-		if !ok && false {
-			base.Fatalf("missing definition for %v", sym)
-		}
-	}
-
-	if obj != nil {
+	if local && obj != nil {
 		wext.Sync(pkgbits.SyncObject1)
 		switch tag {
 		case pkgbits.ObjFunc:
@@ -199,9 +191,64 @@ func (l *linker) relocObj(pr *pkgReader, idx pkgbits.Index) pkgbits.Index {
 		l.relocCommon(pr, &wext, pkgbits.RelocObjExt, idx)
 	}
 
+	// Check if we need to export the inline bodies for functions and
+	// methods.
+	if obj != nil {
+		if obj.Op() == ir.ONAME && obj.Class == ir.PFUNC {
+			l.exportBody(obj, local)
+		}
+
+		if obj.Op() == ir.OTYPE {
+			if typ := obj.Type(); !typ.IsInterface() {
+				for _, method := range typ.Methods().Slice() {
+					l.exportBody(method.Nname.(*ir.Name), local)
+				}
+			}
+		}
+	}
+
 	return w.Idx
 }
 
+// exportBody exports the given function or method's body, if
+// appropriate. local indicates whether it's a local function or
+// method available on a locally declared type. (Due to cross-package
+// type aliases, a method may be imported, but still available on a
+// locally declared type.)
+func (l *linker) exportBody(obj *ir.Name, local bool) {
+	assert(obj.Op() == ir.ONAME && obj.Class == ir.PFUNC)
+
+	fn := obj.Func
+	if fn.Inl == nil {
+		return // not inlinable anyway
+	}
+
+	// As a simple heuristic, if the function was declared in this
+	// package or we inlined it somewhere in this package, then we'll
+	// (re)export the function body. This isn't perfect, but seems
+	// reasonable in practice. In particular, it has the nice property
+	// that in the worst case, adding a blank import ensures the
+	// function body is available for inlining.
+	//
+	// TODO(mdempsky): Reimplement the reachable method crawling logic
+	// from typecheck/crawler.go.
+	exportBody := local || fn.Inl.Body != nil
+	if !exportBody {
+		return
+	}
+
+	sym := obj.Sym()
+	if _, ok := l.bodies[sym]; ok {
+		// Due to type aliases, we might visit methods multiple times.
+		base.AssertfAt(obj.Type().Recv() != nil, obj.Pos(), "expected method: %v", obj)
+		return
+	}
+
+	pri, ok := bodyReaderFor(fn)
+	assert(ok)
+	l.bodies[sym] = l.relocIdx(pri.pr, pkgbits.RelocBody, pri.idx)
+}
+
 // relocCommon copies the specified element from pr into w,
 // recursively relocating any referenced elements as well.
 func (l *linker) relocCommon(pr *pkgReader, w *pkgbits.Encoder, k pkgbits.RelocKind, idx pkgbits.Index) {
@@ -240,10 +287,6 @@ func (l *linker) relocFuncExt(w *pkgbits.Encoder, name *ir.Name) {
 	if inl := name.Func.Inl; w.Bool(inl != nil) {
 		w.Len(int(inl.Cost))
 		w.Bool(inl.CanDelayResults)
-
-		pri, ok := bodyReader[name.Func]
-		assert(ok)
-		w.Reloc(pkgbits.RelocBody, l.relocIdx(pri.pr, pkgbits.RelocBody, pri.idx))
 	}
 
 	w.Sync(pkgbits.SyncEOF)

src/cmd/compile/internal/noder/reader.go

@@ -897,6 +897,8 @@ func (r *reader) funcExt(name *ir.Name) {
 	typecheck.Func(fn)
 
 	if r.Bool() {
+		assert(name.Defn == nil)
+
 		fn.ABI = obj.ABI(r.Uint64())
 
 		// Escape analysis.
@@ -911,7 +913,6 @@ func (r *reader) funcExt(name *ir.Name) {
 				Cost:            int32(r.Len()),
 				CanDelayResults: r.Bool(),
 			}
-			r.addBody(name.Func)
 		}
 	} else {
 		r.addBody(name.Func)
@@ -967,10 +968,26 @@ func (r *reader) pragmaFlag() ir.PragmaFlag {
 
 // @@@ Function bodies
 
-// bodyReader tracks where the serialized IR for a function's body can
-// be found.
+// bodyReader tracks where the serialized IR for a local or imported,
+// generic function's body can be found.
 var bodyReader = map[*ir.Func]pkgReaderIndex{}
 
+// importBodyReader tracks where the serialized IR for an imported,
+// static (i.e., non-generic) function body can be read.
+var importBodyReader = map[*types.Sym]pkgReaderIndex{}
+
+// bodyReaderFor returns the pkgReaderIndex for reading fn's
+// serialized IR, and whether one was found.
+func bodyReaderFor(fn *ir.Func) (pri pkgReaderIndex, ok bool) {
+	if fn.Nname.Defn != nil {
+		pri, ok = bodyReader[fn]
+		assert(ok) // must always be available
+	} else {
+		pri, ok = importBodyReader[fn.Sym()]
+	}
+	return
+}
+
 // todoBodies holds the list of function bodies that still need to be
 // constructed.
 var todoBodies []*ir.Func
@@ -978,15 +995,13 @@ var todoBodies []*ir.Func
 // addBody reads a function body reference from the element bitstream,
 // and associates it with fn.
 func (r *reader) addBody(fn *ir.Func) {
+	// addBody should only be called for local functions or imported
+	// generic functions; see comment in funcExt.
+	assert(fn.Nname.Defn != nil)
+
 	pri := pkgReaderIndex{r.p, r.Reloc(pkgbits.RelocBody), r.dict}
 	bodyReader[fn] = pri
 
-	if fn.Nname.Defn == nil {
-		// Don't read in function body for imported functions.
-		// See comment in funcExt.
-		return
-	}
-
 	if r.curfn == nil {
 		todoBodies = append(todoBodies, fn)
 		return
@@ -2225,7 +2240,7 @@ func InlineCall(call *ir.CallExpr, fn *ir.Func, inlIndex int) *ir.InlinedCallExp
 	// TODO(mdempsky): Turn callerfn into an explicit parameter.
 	callerfn := ir.CurFunc
 
-	pri, ok := bodyReader[fn]
+	pri, ok := bodyReaderFor(fn)
 	if !ok {
 		// TODO(mdempsky): Reconsider this diagnostic's wording, if it's
 		// to be included in Go 1.20.

src/cmd/compile/internal/noder/unified.go

@@ -85,7 +85,7 @@ func unified(noders []*noder) {
 	typecheck.TypecheckAllowed = true
 
 	localPkgReader = newPkgReader(pkgbits.NewPkgDecoder(types.LocalPkg.Path, data))
-	readPackage(localPkgReader, types.LocalPkg)
+	readPackage(localPkgReader, types.LocalPkg, true)
 
 	r := localPkgReader.newReader(pkgbits.RelocMeta, pkgbits.PrivateRootIdx, pkgbits.SyncPrivate)
 	r.pkgInit(types.LocalPkg, target)
@@ -226,29 +226,54 @@ func freePackage(pkg *types2.Package) {
 
 // readPackage reads package export data from pr to populate
 // importpkg.
-func readPackage(pr *pkgReader, importpkg *types.Pkg) {
-	r := pr.newReader(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
+//
+// localStub indicates whether pr is reading the stub export data for
+// the local package, as opposed to relocated export data for an
+// import.
+func readPackage(pr *pkgReader, importpkg *types.Pkg, localStub bool) {
+	{
+		r := pr.newReader(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
 
-	pkg := r.pkg()
-	base.Assertf(pkg == importpkg, "have package %q (%p), want package %q (%p)", pkg.Path, pkg, importpkg.Path, importpkg)
+		pkg := r.pkg()
+		base.Assertf(pkg == importpkg, "have package %q (%p), want package %q (%p)", pkg.Path, pkg, importpkg.Path, importpkg)
 
-	if r.Bool() {
-		sym := pkg.Lookup(".inittask")
-		task := ir.NewNameAt(src.NoXPos, sym)
-		task.Class = ir.PEXTERN
-		sym.Def = task
+		if r.Bool() {
+			sym := pkg.Lookup(".inittask")
+			task := ir.NewNameAt(src.NoXPos, sym)
+			task.Class = ir.PEXTERN
+			sym.Def = task
+		}
+
+		for i, n := 0, r.Len(); i < n; i++ {
+			r.Sync(pkgbits.SyncObject)
+			assert(!r.Bool())
+			idx := r.Reloc(pkgbits.RelocObj)
+			assert(r.Len() == 0)
+
+			path, name, code := r.p.PeekObj(idx)
+			if code != pkgbits.ObjStub {
+				objReader[types.NewPkg(path, "").Lookup(name)] = pkgReaderIndex{pr, idx, nil}
+			}
+		}
+
+		r.Sync(pkgbits.SyncEOF)
 	}
 
-	for i, n := 0, r.Len(); i < n; i++ {
-		r.Sync(pkgbits.SyncObject)
-		assert(!r.Bool())
-		idx := r.Reloc(pkgbits.RelocObj)
-		assert(r.Len() == 0)
+	if !localStub {
+		r := pr.newReader(pkgbits.RelocMeta, pkgbits.PrivateRootIdx, pkgbits.SyncPrivate)
 
-		path, name, code := r.p.PeekObj(idx)
-		if code != pkgbits.ObjStub {
-			objReader[types.NewPkg(path, "").Lookup(name)] = pkgReaderIndex{pr, idx, nil}
+		for i, n := 0, r.Len(); i < n; i++ {
+			path := r.String()
+			name := r.String()
+			idx := r.Reloc(pkgbits.RelocBody)
+
+			sym := types.NewPkg(path, "").Lookup(name)
+			if _, ok := importBodyReader[sym]; !ok {
+				importBodyReader[sym] = pkgReaderIndex{pr, idx, nil}
+			}
 		}
+
+		r.Sync(pkgbits.SyncEOF)
 	}
 }
 
@@ -258,12 +283,15 @@ func writeUnifiedExport(out io.Writer) {
 	l := linker{
 		pw: pkgbits.NewPkgEncoder(base.Debug.SyncFrames),
 
-		pkgs:  make(map[string]pkgbits.Index),
-		decls: make(map[*types.Sym]pkgbits.Index),
+		pkgs:   make(map[string]pkgbits.Index),
+		decls:  make(map[*types.Sym]pkgbits.Index),
+		bodies: make(map[*types.Sym]pkgbits.Index),
 	}
 
 	publicRootWriter := l.pw.NewEncoder(pkgbits.RelocMeta, pkgbits.SyncPublic)
+	privateRootWriter := l.pw.NewEncoder(pkgbits.RelocMeta, pkgbits.SyncPrivate)
 	assert(publicRootWriter.Idx == pkgbits.PublicRootIdx)
+	assert(privateRootWriter.Idx == pkgbits.PrivateRootIdx)
 
 	var selfPkgIdx pkgbits.Index
 
@@ -320,5 +348,29 @@ func writeUnifiedExport(out io.Writer) {
 		w.Flush()
 	}
 
+	{
+		type symIdx struct {
+			sym *types.Sym
+			idx pkgbits.Index
+		}
+		var bodies []symIdx
+		for sym, idx := range l.bodies {
+			bodies = append(bodies, symIdx{sym, idx})
+		}
+		sort.Slice(bodies, func(i, j int) bool { return bodies[i].idx < bodies[j].idx })
+
+		w := privateRootWriter
+
+		w.Len(len(bodies))
+		for _, body := range bodies {
+			w.String(body.sym.Pkg.Path)
+			w.String(body.sym.Name)
+			w.Reloc(pkgbits.RelocBody, body.idx)
+		}
+
+		w.Sync(pkgbits.SyncEOF)
+		w.Flush()
+	}
+
 	base.Ctxt.Fingerprint = l.pw.DumpTo(out)
 }