cmd/internal/obj: remove Link.Plan9privates

Move it to the x86 package, matching our handling
of deferreturn in x86 and arm.
While we're here, improve the concurrency safety
of both Plan9privates and deferreturn
by eagerly initializing them in instinit.

Updates #15756

Change-Id: If3b1995c1e4ec816a5443a18f8d715631967a8b1
Reviewed-on: https://go-review.googlesource.com/40408
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/link.go

@@ -725,7 +725,6 @@ type Link struct {
 	PosTable      src.PosTable
 	InlTree       InlTree // global inlining tree used by gc/inl.go
 	Imports       []string
-	Plan9privates *LSym
 	DiagFunc      func(string, ...interface{})
 	DebugInfo     func(fn *LSym, curfn interface{}) []*dwarf.Var // if non-nil, curfn is a *gc.Node
 	Errors        int

src/cmd/internal/obj/x86/asm6.go

@@ -39,6 +39,11 @@ import (
 	"strings"
 )
 
+var (
+	plan9privates *obj.LSym
+	deferreturn   *obj.LSym
+)
+
 // Instruction layout.
 
 const (
@@ -1830,10 +1835,6 @@ func span6(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 	n := 0
 	var c int32
 	errors := ctxt.Errors
-	var deferreturn *obj.LSym
-	if ctxt.Headtype == obj.Hnacl {
-		deferreturn = ctxt.Lookup("runtime.deferreturn", 0)
-	}
 	for {
 		loop := int32(0)
 		for i := range s.R {
@@ -1973,6 +1974,13 @@ func instinit(ctxt *obj.Link) {
 		return
 	}
 
+	switch ctxt.Headtype {
+	case obj.Hplan9:
+		plan9privates = ctxt.Lookup("_privates", 0)
+	case obj.Hnacl:
+		deferreturn = ctxt.Lookup("runtime.deferreturn", 0)
+	}
+
 	for i := 1; optab[i].as != 0; i++ {
 		c := optab[i].as
 		if opindex[c&obj.AMask] != nil {
@@ -4108,13 +4116,10 @@ func (asmbuf *AsmBuf) doasm(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog) {
 								asmbuf.asmand(ctxt, cursym, p, &pp.From, &p.To)
 							}
 						case obj.Hplan9:
-							if ctxt.Plan9privates == nil {
-								ctxt.Plan9privates = ctxt.Lookup("_privates", 0)
-							}
 							pp.From = obj.Addr{}
 							pp.From.Type = obj.TYPE_MEM
 							pp.From.Name = obj.NAME_EXTERN
-							pp.From.Sym = ctxt.Plan9privates
+							pp.From.Sym = plan9privates
 							pp.From.Offset = 0
 							pp.From.Index = REG_NONE
 							asmbuf.Put1(0x8B)
@@ -4164,13 +4169,10 @@ func (asmbuf *AsmBuf) doasm(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog) {
 						asmbuf.PutInt32(0)
 
 					case obj.Hplan9:
-						if ctxt.Plan9privates == nil {
-							ctxt.Plan9privates = ctxt.Lookup("_privates", 0)
-						}
 						pp.From = obj.Addr{}
 						pp.From.Type = obj.TYPE_MEM
 						pp.From.Name = obj.NAME_EXTERN
-						pp.From.Sym = ctxt.Plan9privates
+						pp.From.Sym = plan9privates
 						pp.From.Offset = 0
 						pp.From.Index = REG_NONE
 						asmbuf.rexflag |= Pw

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

@@ -589,10 +589,6 @@ func nacladdr(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) {
 }
 
 func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
-	if ctxt.Headtype == obj.Hplan9 && ctxt.Plan9privates == nil {
-		ctxt.Plan9privates = ctxt.Lookup("_privates", 0)
-	}
-
 	if cursym.Text == nil || cursym.Text.Link == nil {
 		return
 	}