cmd/compile: de-dup the gclocals symbols in compiler too

These symbols are de-duplicated in the linker but the compiler generates quite
many duplicates too: 2425 of 13769 total symbols for runtime.a for example.
De-duplicating them in the compiler saves the linker a bit of work.

Fixes #14983

Change-Id: I5f18e5f9743563c795aad8f0a22d17a7ed147711
Reviewed-on: https://go-review.googlesource.com/22293Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -8,7 +8,6 @@ import (
 	"cmd/compile/internal/ssa"
 	"cmd/internal/obj"
 	"cmd/internal/sys"
-	"crypto/md5"
 	"fmt"
 	"sort"
 	"strings"
@@ -130,15 +129,6 @@ func removevardef(firstp *obj.Prog) {
 	}
 }
 
-func gcsymdup(s *Sym) {
-	ls := Linksym(s)
-	if len(ls.R) > 0 {
-		Fatalf("cannot rosymdup %s with relocations", ls.Name)
-	}
-	ls.Name = fmt.Sprintf("gclocals·%x", md5.Sum(ls.P))
-	ls.Dupok = true
-}
-
 func emitptrargsmap() {
 	if Curfn.Func.Nname.Sym.Name == "_" {
 		return
@@ -559,9 +549,6 @@ func genlegacy(ptxt *obj.Prog, gcargs, gclocals *Sym) {
 	// Emit garbage collection symbols.
 	liveness(Curfn, ptxt, gcargs, gclocals)
 
-	gcsymdup(gcargs)
-	gcsymdup(gclocals)
-
 	Thearch.Defframe(ptxt)
 
 	if Debug['f'] != 0 {

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

@@ -18,6 +18,7 @@ package gc
 import (
 	"cmd/internal/obj"
 	"cmd/internal/sys"
+	"crypto/md5"
 	"fmt"
 	"sort"
 	"strings"
@@ -1689,7 +1690,17 @@ func onebitwritesymbol(arr []Bvec, sym *Sym) {
 	}
 
 	duint32(sym, 0, uint32(i)) // number of bitmaps
-	ggloblsym(sym, int32(off), obj.RODATA)
+	ls := Linksym(sym)
+	ls.Name = fmt.Sprintf("gclocals·%x", md5.Sum(ls.P))
+	ls.Dupok = true
+	sv := obj.SymVer{ls.Name, 0}
+	ls2, ok := Ctxt.Hash[sv]
+	if ok {
+		sym.Lsym = ls2
+	} else {
+		Ctxt.Hash[sv] = ls
+		ggloblsym(sym, int32(off), obj.RODATA)
+	}
 }
 
 func printprog(p *obj.Prog) {

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

@@ -4005,8 +4005,6 @@ func genssa(f *ssa.Func, ptxt *obj.Prog, gcargs, gclocals *Sym) {
 
 	// Generate gc bitmaps.
 	liveness(Curfn, ptxt, gcargs, gclocals)
-	gcsymdup(gcargs)
-	gcsymdup(gclocals)
 
 	// Add frame prologue. Zero ambiguously live variables.
 	Thearch.Defframe(ptxt)