cmd/link: fix accidentally-quadratic library loading

Programs built from N libraries required O(N²) time to do the
deduplication checks, even if there were never any duplicates.
In most programs N is small enough not to worry, but this may
affect large programs.

Noticed by inspection, not any specific bug report.

Fixes #20578.

Change-Id: Ic4108f1058be39da990a79b1e0b8ce95fde44cef
Reviewed-on: https://go-review.googlesource.com/44852
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/ld.go

@@ -50,10 +50,8 @@ func addlib(ctxt *Link, src string, obj string, pathname string) *Library {
 	}
 
 	// already loaded?
-	for i := 0; i < len(ctxt.Library); i++ {
-		if ctxt.Library[i].Pkg == pkg {
-			return ctxt.Library[i]
-		}
+	if l := ctxt.LibraryByPkg[pkg]; l != nil {
+		return l
 	}
 
 	var pname string
@@ -97,18 +95,17 @@ func addlib(ctxt *Link, src string, obj string, pathname string) *Library {
  *	pkg: package import path, e.g. container/vector
  */
 func addlibpath(ctxt *Link, srcref string, objref string, file string, pkg string, shlibnamefile string) *Library {
-	for i := 0; i < len(ctxt.Library); i++ {
-		if pkg == ctxt.Library[i].Pkg {
-			return ctxt.Library[i]
-		}
+	if l := ctxt.LibraryByPkg[pkg]; l != nil {
+		return l
 	}
 
 	if ctxt.Debugvlog > 1 {
 		ctxt.Logf("%5.2f addlibpath: srcref: %s objref: %s file: %s pkg: %s shlibnamefile: %s\n", Cputime(), srcref, objref, file, pkg, shlibnamefile)
 	}
 
-	ctxt.Library = append(ctxt.Library, &Library{})
-	l := ctxt.Library[len(ctxt.Library)-1]
+	l := &Library{}
+	ctxt.LibraryByPkg[pkg] = l
+	ctxt.Library = append(ctxt.Library, l)
 	l.Objref = objref
 	l.Srcref = srcref
 	l.File = file

src/cmd/link/internal/ld/link.go

@@ -223,6 +223,7 @@ type Link struct {
 	Tlsg         *Symbol
 	Libdir       []string
 	Library      []*Library
+	LibraryByPkg map[string]*Library
 	Shlibs       []Shlib
 	Tlsoffset    int
 	Textp        []*Symbol

src/cmd/link/internal/ld/sym.go

@@ -48,6 +48,7 @@ func linknew(arch *sys.Arch) *Link {
 			Allsym: make([]*Symbol, 0, 100000),
 		},
 		Arch:         arch,
+		LibraryByPkg: make(map[string]*Library),
 	}
 
 	if objabi.GOARCH != arch.Name {