cmd/cgo,cmd/go: preallocate slices if they have known fixed capacities

This allows for more efficient use of memory.

src/cmd/cgo/ast.go

@@ -181,7 +181,7 @@ func (f *File) ParseGo(abspath string, src []byte) {
 // Like ast.CommentGroup's Text method but preserves
 // leading blank lines, so that line numbers line up.
 func commentText(g *ast.CommentGroup) string {
-	var pieces []string
+	pieces := make([]string, 0, len(g.List))
 	for _, com := range g.List {
 		c := com.Text
 		// Remove comment markers.

src/cmd/cgo/main.go

@@ -83,7 +83,7 @@ func (f *File) offset(p token.Pos) int {
 }
 
 func nameKeys(m map[string]*Name) []string {
-	var ks []string
+	ks := make([]string, 0, len(m))
 	for k := range m {
 		ks = append(ks, k)
 	}

src/cmd/go/internal/base/path.go

@@ -78,7 +78,7 @@ func relConservative(basepath, targpath string) (string, error) {
 // RelPaths returns a copy of paths with absolute paths
 // made relative to the current directory if they would be shorter.
 func RelPaths(paths []string) []string {
-	var out []string
+	out := make([]string, 0, len(paths))
 	for _, p := range paths {
 		rel, err := relConservative(Cwd(), p)
 		if err == nil && len(rel) < len(p) {

src/cmd/go/internal/envcmd/env.go

@@ -340,7 +340,7 @@ func runEnv(ctx context.Context, cmd *base.Command, args []string) {
 		// Show only the named vars.
 		if !*envChanged {
 			if *envJson {
-				var es []cfg.EnvVar
+				es := make([]cfg.EnvVar, 0, len(args))
 				for _, name := range args {
 					e := cfg.EnvVar{Name: name, Value: findEnv(env, name)}
 					es = append(es, e)

src/cmd/go/internal/fsys/fsys.go

@@ -761,7 +761,7 @@ func glob(dir, pattern string, matches []string) (m []string, e error) {
 		return // ignore I/O error
 	}
 
-	var names []string
+	names := make([]string, 0, len(list))
 	for _, info := range list {
 		names = append(names, info.Name())
 	}

src/cmd/go/internal/imports/scan.go

@@ -98,7 +98,7 @@ Files:
 var ErrNoGo = fmt.Errorf("no Go source files")
 
 func keys(m map[string]bool) []string {
-	var list []string
+	list := make([]string, 0, len(m))
 	for k := range m {
 		list = append(list, k)
 	}

src/cmd/go/internal/list/list.go

@@ -389,7 +389,7 @@ func (v *jsonFlag) Set(s string) error {
 }
 
 func (v *jsonFlag) String() string {
-	var fields []string
+	fields := make([]string, 0, len(*v))
 	for f := range *v {
 		fields = append(fields, f)
 	}

src/cmd/go/internal/load/godebug.go

@@ -94,7 +94,7 @@ func defaultGODEBUG(p *Package, directives, testDirectives, xtestDirectives []bu
 		m = defaults
 	}
 
-	var keys []string
+	keys := make([]string, 0, len(m))
 	for k := range m {
 		keys = append(keys, k)
 	}

src/cmd/go/internal/modfetch/codehost/git.go

@@ -319,7 +319,7 @@ func (r *gitRepo) Tags(ctx context.Context, prefix string) (*Tags, error) {
 // the absence of a specific module version.
 // The caller must supply refs, the result of a successful r.loadRefs.
 func (r *gitRepo) repoSum(refs map[string]string) string {
-	var list []string
+	list := make([]string, 0, len(refs))
 	for ref := range refs {
 		list = append(list, ref)
 	}

src/cmd/go/internal/modfetch/fetch.go

@@ -915,7 +915,7 @@ func tidyGoSum(data []byte, keep map[module.Version]bool) []byte {
 		}
 	}
 
-	var mods []module.Version
+	mods := make([]module.Version, 0, len(goSum.m))
 	for m := range goSum.m {
 		mods = append(mods, m)
 	}

src/cmd/go/internal/modget/get.go

@@ -763,8 +763,9 @@ func (r *resolver) performLocalQueries(ctx context.Context) {
 			pkgPattern, mainModule := modload.MainModules.DirImportPath(ctx, q.pattern)
 			if pkgPattern == "." {
 				modload.MustHaveModRoot()
-				var modRoots []string
-				for _, m := range modload.MainModules.Versions() {
+				versions := modload.MainModules.Versions()
+				modRoots := make([]string, 0, len(versions))
+				for _, m := range versions {
 					modRoots = append(modRoots, modload.MainModules.ModRoot(m))
 				}
 				var plural string

src/cmd/go/internal/modload/init.go

@@ -993,10 +993,11 @@ func loadModFile(ctx context.Context, opts *PackageOpts) (*Requirements, error)
 
 	if cfg.BuildMod == "vendor" {
 		readVendorList(VendorDir())
-		var indexes []*modFileIndex
-		var modFiles []*modfile.File
-		var modRoots []string
-		for _, m := range MainModules.Versions() {
+		versions := MainModules.Versions()
+		indexes := make([]*modFileIndex, 0, len(versions))
+		modFiles := make([]*modfile.File, 0, len(versions))
+		modRoots := make([]string, 0, len(versions))
+		for _, m := range versions {
 			indexes = append(indexes, MainModules.Index(m))
 			modFiles = append(modFiles, MainModules.ModFile(m))
 			modRoots = append(modRoots, MainModules.ModRoot(m))

src/cmd/go/internal/run/run.go

@@ -128,7 +128,7 @@ func runRun(ctx context.Context, cmd *base.Command, args []string) {
 			base.Fatalf("go: no packages loaded from %s", arg)
 		}
 		if len(pkgs) > 1 {
-			var names []string
+			names := make([]string, 0, len(pkgs))
 			for _, p := range pkgs {
 				names = append(names, p.ImportPath)
 			}

src/cmd/go/internal/search/search.go

@@ -361,8 +361,9 @@ func ImportPaths(patterns, modRoots []string) []*Match {
 
 // ImportPathsQuiet is like ImportPaths but does not warn about patterns with no matches.
 func ImportPathsQuiet(patterns, modRoots []string) []*Match {
-	var out []*Match
-	for _, a := range CleanPatterns(patterns) {
+	patterns = CleanPatterns(patterns)
+	out := make([]*Match, 0, len(patterns))
+	for _, a := range patterns {
 		m := NewMatch(a)
 		if m.IsLocal() {
 			m.MatchDirs(modRoots)
@@ -399,7 +400,7 @@ func CleanPatterns(patterns []string) []string {
 	if len(patterns) == 0 {
 		return []string{"."}
 	}
-	var out []string
+	out := make([]string, 0, len(patterns))
 	for _, a := range patterns {
 		var p, v string
 		if build.IsLocalImport(a) || filepath.IsAbs(a) {

src/cmd/go/internal/vcweb/vcstest/vcstest.go

@@ -101,7 +101,7 @@ func NewServer() (srv *Server, err error) {
 	vcs.VCSTestRepoURL = srv.HTTP.URL
 	vcs.VCSTestHosts = Hosts
 
-	var interceptors []web.Interceptor
+	interceptors := make([]web.Interceptor, 0, 2*len(Hosts))
 	for _, host := range Hosts {
 		interceptors = append(interceptors,
 			web.Interceptor{Scheme: "http", FromHost: host, ToHost: httpURL.Host, Client: srv.HTTP.Client()},

src/cmd/go/internal/work/action.go

@@ -208,7 +208,7 @@ func actionGraphJSON(a *Action) string {
 		}
 	}
 
-	var list []*actionJSON
+	list := make([]*actionJSON, 0, len(workq))
 	for id, a := range workq {
 		if a.json == nil {
 			a.json = &actionJSON{

src/cmd/go/internal/work/gc.go

@@ -469,7 +469,7 @@ func toolVerify(a *Action, b *Builder, p *load.Package, newTool string, ofile st
 }
 
 func (gcToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
-	var absOfiles []string
+	absOfiles := make([]string, 0, len(ofiles))
 	for _, f := range ofiles {
 		absOfiles = append(absOfiles, mkAbs(a.Objdir, f))
 	}

src/cmd/go/internal/work/gccgo.go

@@ -230,7 +230,7 @@ func (tools gccgoToolchain) pack(b *Builder, a *Action, afile string, ofiles []s
 	p := a.Package
 	sh := b.Shell(a)
 	objdir := a.Objdir
-	var absOfiles []string
+	absOfiles := make([]string, 0, len(ofiles))
 	for _, f := range ofiles {
 		absOfiles = append(absOfiles, mkAbs(objdir, f))
 	}