[dev.link] cmd/link/internal/loader: add 1-element object cache for use in toLocal

To speed up the loader.Loader.toLocal() method, cache the index of the
most recently accessed object file and check that object's sym range
in toLocal() before doing a full binary search over all object symbol
ranges. This speeds up relink of kubernetes/hyperkube by about 2%, and
improves compilebench (relative to the dev.link branch) by about 5%:

name                      old time/op       new time/op       delta
LinkCompiler                    1.62s ± 8%        1.50s ± 9%  -7.21%  (p=0.000 n=20+19)
LinkWithoutDebugCompiler        1.13s ± 8%        1.09s ±12%    ~     (p=0.052 n=20+20)

name                      old user-time/op  new user-time/op  delta
LinkCompiler                    1.94s ±18%        1.97s ±16%    ~     (p=0.813 n=19+20)
LinkWithoutDebugCompiler        1.15s ±16%        1.13s ±12%    ~     (p=0.547 n=20+20)

Change-Id: Id5a8a847b533858373c0462f03972d436eda6748
Reviewed-on: https://go-review.googlesource.com/c/go/+/204337Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/loader/loader.go

@@ -65,6 +65,7 @@ type oReader struct {
 type objIdx struct {
 	r *oReader
 	i Sym // start index
+	e Sym // end index
 }
 
 type nameVer struct {
@@ -98,6 +99,7 @@ type Loader struct {
 	extStart    Sym              // from this index on, the symbols are externally defined
 	extSyms     []nameVer        // externally defined symbols
 	builtinSyms []Sym            // global index of builtin symbols
+	ocache      int              // index (into 'objs') of most recent lookup
 
 	symsByName    [2]map[string]Sym // map symbol name to index, two maps are for ABI0 and ABIInternal
 	extStaticSyms map[nameVer]Sym   // externally defined static symbols, keyed by name
@@ -116,7 +118,7 @@ func NewLoader() *Loader {
 	nbuiltin := goobj2.NBuiltin()
 	return &Loader{
 		start:         make(map[*oReader]Sym),
-		objs:          []objIdx{{nil, 0}},
+		objs:          []objIdx{{nil, 0, 0}},
 		symsByName:    [2]map[string]Sym{make(map[string]Sym), make(map[string]Sym)},
 		objByPkg:      make(map[string]*oReader),
 		overwrite:     make(map[Sym]Sym),
@@ -143,7 +145,7 @@ func (l *Loader) addObj(pkg string, r *oReader) Sym {
 	n := r.NSym() + r.NNonpkgdef()
 	i := l.max + 1
 	l.start[r] = i
-	l.objs = append(l.objs, objIdx{r, i})
+	l.objs = append(l.objs, objIdx{r, i, i + Sym(n) - 1})
 	l.max += Sym(n)
 	return i
 }
@@ -249,12 +251,17 @@ func (l *Loader) toLocal(i Sym) (*oReader, int) {
 	if l.isExternal(i) {
 		return nil, int(i - l.extStart)
 	}
+	oc := l.ocache
+	if oc != 0 && i >= l.objs[oc].i && i <= l.objs[oc].e {
+		return l.objs[oc].r, int(i - l.objs[oc].i)
+	}
 	// Search for the local object holding index i.
 	// Below k is the first one that has its start index > i,
 	// so k-1 is the one we want.
 	k := sort.Search(len(l.objs), func(k int) bool {
 		return l.objs[k].i > i
 	})
+	l.ocache = k - 1
 	return l.objs[k-1].r, int(i - l.objs[k-1].i)
 }