cmd/compile: remove most of the Lookupf users and garbage

Introduce garbage-free LookupN to replace most users of Lookupf.

Also, remove the string interning from LookupBytes which was hurting
more than helping.

name       old alloc/op    new alloc/op    delta
Template      63.0MB ± 0%     62.7MB ± 0%  -0.48%         (p=0.000 n=10+9)
Unicode       43.0MB ± 0%     43.0MB ± 0%  -0.17%         (p=0.000 n=10+7)
GoTypes        219MB ± 0%      218MB ± 0%  -0.14%        (p=0.000 n=10+10)
Compiler       992MB ± 0%      991MB ± 0%  -0.12%        (p=0.000 n=10+10)

name       old allocs/op   new allocs/op   delta
Template        683k ± 0%       681k ± 0%  -0.38%         (p=0.000 n=10+8)
Unicode         541k ± 0%       541k ± 0%  -0.11%        (p=0.000 n=10+10)
GoTypes        2.09M ± 0%      2.08M ± 0%  -0.40%        (p=0.000 n=10+10)
Compiler       9.28M ± 0%      9.24M ± 0%  -0.36%        (p=0.000 n=10+10)

Size of $GOROOT/pkg/darwin_amd64 drops from 40124 KB to 40100 KB too,
removing the zero padding as suggested by josharian.

Updates #6853

Change-Id: I3c557266e9325fe29c459cef8e5b8954913e7abb
Reviewed-on: https://go-review.googlesource.com/20931Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/closure.go

@@ -529,7 +529,7 @@ func makepartialcall(fn *Node, t0 *Type, meth *Sym) *Node {
 	var fld *Node
 	var n *Node
 	for _, t := range t0.Params().Fields().Slice() {
-		n = newname(Lookupf("a%d", i))
+		n = newname(LookupN("a%d", i))
 		i++
 		n.Class = PPARAM
 		xfunc.Func.Dcl = append(xfunc.Func.Dcl, n)
@@ -548,7 +548,7 @@ func makepartialcall(fn *Node, t0 *Type, meth *Sym) *Node {
 	l = nil
 	var retargs []*Node
 	for _, t := range t0.Results().Fields().Slice() {
-		n = newname(Lookupf("r%d", i))
+		n = newname(LookupN("r", i))
 		i++
 		n.Class = PPARAMOUT
 		xfunc.Func.Dcl = append(xfunc.Func.Dcl, n)

src/cmd/compile/internal/gc/dcl.go

@@ -613,7 +613,7 @@ func funcargs(nt *Node) {
 
 		if n.Left == nil {
 			// Name so that escape analysis can track it. ~r stands for 'result'.
-			n.Left = newname(Lookupf("~r%d", gen))
+			n.Left = newname(LookupN("~r", gen))
 			gen++
 		}
 
@@ -633,7 +633,7 @@ func funcargs(nt *Node) {
 
 			*nn = *n.Left
 			nn.Orig = nn
-			nn.Sym = Lookupf("~b%d", gen)
+			nn.Sym = LookupN("~b", gen)
 			gen++
 			n.Left = nn
 		}

src/cmd/compile/internal/gc/gen.go

@@ -594,7 +594,7 @@ func Tempname(nn *Node, t *Type) {
 
 	// give each tmp a different name so that there
 	// a chance to registerizer them
-	s := Lookupf("autotmp_%.4d", statuniqgen)
+	s := LookupN("autotmp_", statuniqgen)
 	statuniqgen++
 	n := Nod(ONAME, nil, nil)
 	n.Sym = s

src/cmd/compile/internal/gc/init.go

@@ -27,7 +27,7 @@ var renameinit_initgen int
 
 func renameinit() *Sym {
 	renameinit_initgen++
-	return Lookupf("init.%d", renameinit_initgen)
+	return LookupN("init.", renameinit_initgen)
 }
 
 // hand-craft the following initialization code
@@ -154,7 +154,7 @@ func fninit(n []*Node) {
 	// (9)
 	// could check that it is fn of no args/returns
 	for i := 1; ; i++ {
-		s := Lookupf("init.%d", i)
+		s := LookupN("init.", i)
 		if s.Def == nil {
 			break
 		}

src/cmd/compile/internal/gc/inl.go

@@ -851,7 +851,7 @@ func inlvar(var_ *Node) *Node {
 
 // Synthesize a variable to store the inlined function's results in.
 func retvar(t *Field, i int) *Node {
-	n := newname(Lookupf("~r%d", i))
+	n := newname(LookupN("~r", i))
 	n.Type = t.Type
 	n.Class = PAUTO
 	n.Used = true
@@ -863,7 +863,7 @@ func retvar(t *Field, i int) *Node {
 // Synthesize a variable to store the inlined function's arguments
 // when they come from a multiple return call.
 func argvar(t *Type, i int) *Node {
-	n := newname(Lookupf("~arg%d", i))
+	n := newname(LookupN("~arg", i))
 	n.Type = t.Type
 	n.Class = PAUTO
 	n.Used = true
@@ -876,7 +876,7 @@ var newlabel_inl_label int
 
 func newlabel_inl() *Node {
 	newlabel_inl_label++
-	n := newname(Lookupf(".inlret%.6d", newlabel_inl_label))
+	n := newname(LookupN(".inlret", newlabel_inl_label))
 	n.Etype = 1 // flag 'safe' for escape analysis (no backjumps)
 	return n
 }

src/cmd/compile/internal/gc/pgen.go

@@ -15,12 +15,12 @@ import (
 
 // "Portable" code generation.
 
-var makefuncdatasym_nsym int32
+var makefuncdatasym_nsym int
 
-func makefuncdatasym(namefmt string, funcdatakind int64) *Sym {
+func makefuncdatasym(nameprefix string, funcdatakind int64) *Sym {
 	var nod Node
 
-	sym := Lookupf(namefmt, makefuncdatasym_nsym)
+	sym := LookupN(nameprefix, makefuncdatasym_nsym)
 	makefuncdatasym_nsym++
 	pnod := newname(sym)
 	pnod.Class = PEXTERN
@@ -456,8 +456,8 @@ func compile(fn *Node) {
 
 	ginit()
 
-	gcargs := makefuncdatasym("gcargs·%d", obj.FUNCDATA_ArgsPointerMaps)
-	gclocals := makefuncdatasym("gclocals·%d", obj.FUNCDATA_LocalsPointerMaps)
+	gcargs := makefuncdatasym("gcargs·", obj.FUNCDATA_ArgsPointerMaps)
+	gclocals := makefuncdatasym("gclocals·", obj.FUNCDATA_LocalsPointerMaps)
 
 	if obj.Fieldtrack_enabled != 0 && len(Curfn.Func.FieldTrack) > 0 {
 		trackSyms := make([]*Sym, 0, len(Curfn.Func.FieldTrack))

src/cmd/compile/internal/gc/sinit.go

@@ -500,7 +500,7 @@ func staticassign(l *Node, r *Node, out *[]*Node) bool {
 // data statements for the constant
 // part of the composite literal.
 func staticname(t *Type, ctxt int) *Node {
-	n := newname(Lookupf("statictmp_%.4d", statuniqgen))
+	n := newname(LookupN("statictmp_", statuniqgen))
 	statuniqgen++
 	if ctxt == 0 {
 		n.Name.Readonly = true

src/cmd/compile/internal/gc/subr.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"os"
 	"sort"
+	"strconv"
 	"strings"
 	"unicode"
 	"unicode/utf8"
@@ -223,6 +224,15 @@ func LookupBytes(name []byte) *Sym {
 	return localpkg.LookupBytes(name)
 }
 
+// LookupN looks up the symbol starting with prefix and ending with
+// the decimal n. If prefix is too long, LookupN panics.
+func LookupN(prefix string, n int) *Sym {
+	var buf [20]byte // plenty long enough for all current users
+	copy(buf[:], prefix)
+	b := strconv.AppendInt(buf[:len(prefix)], int64(n), 10)
+	return LookupBytes(b)
+}
+
 var initSyms []*Sym
 
 var nopkg = &Pkg{