cmd/internal/obj: add LookupInit

There are some LSyms that are lazily initialized,
and which cannot be made eagerly initialized,
such as elements of a constant pool.

To avoid needing a mutex to protect the internals of
those LSyms, this CL introduces LookupInit,
which allows an LSym to be initialized only once.

By itself this is not fully concurrency-safe,
but Ctxt.Hash will need mutex protection anyway,
and that will be enough to support one-time LSym initialization.

Passes toolstash-check -all.

Updates #15756

Change-Id: Id7248dfdc4dfbdfe425fa31d0c0045018eeea1fa
Reviewed-on: https://go-review.googlesource.com/39990
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/sym.go

@@ -68,46 +68,52 @@ func Linknew(arch *LinkArch) *Link {
 	return ctxt
 }
 
+// Lookup looks up the symbol with name name and version v.
+// If it does not exist, it creates it.
 func (ctxt *Link) Lookup(name string, v int) *LSym {
+	return ctxt.LookupInit(name, v, nil)
+}
+
+// LookupInit looks up the symbol with name name and version v.
+// If it does not exist, it creates it and passes it to initfn for one-time initialization.
+func (ctxt *Link) LookupInit(name string, v int, init func(s *LSym)) *LSym {
 	s := ctxt.Hash[SymVer{name, v}]
 	if s != nil {
 		return s
 	}
 
-	s = &LSym{
-		Name:    name,
-		Type:    0,
-		Version: int16(v),
-		Size:    0,
-	}
+	s = &LSym{Name: name, Version: int16(v)}
 	ctxt.Hash[SymVer{name, v}] = s
+	if init != nil {
+		init(s)
+	}
 	return s
 }
 
 func (ctxt *Link) Float32Sym(f float32) *LSym {
 	i := math.Float32bits(f)
 	name := fmt.Sprintf("$f32.%08x", i)
-	s := ctxt.Lookup(name, 0)
+	return ctxt.LookupInit(name, 0, func(s *LSym) {
 		s.Size = 4
 		s.Set(AttrLocal, true)
-	return s
+	})
 }
 
 func (ctxt *Link) Float64Sym(f float64) *LSym {
 	i := math.Float64bits(f)
 	name := fmt.Sprintf("$f64.%016x", i)
-	s := ctxt.Lookup(name, 0)
+	return ctxt.LookupInit(name, 0, func(s *LSym) {
 		s.Size = 8
 		s.Set(AttrLocal, true)
-	return s
+	})
 }
 
 func (ctxt *Link) Int64Sym(i int64) *LSym {
 	name := fmt.Sprintf("$i64.%016x", uint64(i))
-	s := ctxt.Lookup(name, 0)
+	return ctxt.LookupInit(name, 0, func(s *LSym) {
 		s.Size = 8
 		s.Set(AttrLocal, true)
-	return s
+	})
 }
 
 func Linksymfmt(s *LSym) string {

src/cmd/internal/obj/x86/obj6.go

@@ -534,10 +534,10 @@ func rewriteToPcrel(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 	r := obj.Appendp(q, newprog)
 	r.RegTo2 = 1
 	q.As = obj.ACALL
-	q.To.Sym = ctxt.Lookup("__x86.get_pc_thunk."+strings.ToLower(rconv(int(dst))), 0)
+	thunkname := "__x86.get_pc_thunk." + strings.ToLower(rconv(int(dst)))
+	q.To.Sym = ctxt.LookupInit(thunkname, 0, func(s *obj.LSym) { s.Set(obj.AttrLocal, true) })
 	q.To.Type = obj.TYPE_MEM
 	q.To.Name = obj.NAME_EXTERN
-	q.To.Sym.Set(obj.AttrLocal, true)
 	r.As = p.As
 	r.Scond = p.Scond
 	r.From = p.From