[dev.link] cmd/link/internal/objfile: new loader method for reloc queries

Second change of several to update the loader API to reflect the final
consensus version of the loader API as described in Cherry's doc.
This piece:

 - define new loader.Relocs() method that returns a struct
   encapsulating a set of relocations on a global symbol

Old way of examining relocations:

  nreloc := loader.NReloc(someGlobalSymbolIndex)
  for i := 0; i < nreloc; i++ {
    tgtIdx := loader.RelocSym(someGlobalSymbolIndex, i))
    ... <do something with tgtIdx>
  }

New way of examining relocations:

  relocs := d.loader.Relocs(someGlobalSymbolIndex)
  for i := 0; i < relocs.Count; i++ {
    r := relocs.At(i).Sym
    ... <do something with r.Sym>
  }

Change-Id: I5bead1d729655ea13b3396647e53aafcd3e60f97
Reviewed-on: https://go-review.googlesource.com/c/go/+/200717
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/objfile/objfile2.go

@@ -27,6 +27,27 @@ var _ = fmt.Print
 // Go symbol. The 0-valued Sym is corresponds to an invalid symbol.
 type Sym int
 
+// Relocs encapsulates the set of relocations on a given symbol; an
+// instance of this type is returned by the Loader Relocs() method.
+type Relocs struct {
+	Count int // number of relocs
+
+	li int      // local index of symbol whose relocs we're examining
+	r  *oReader // object reader for containing package
+	l  *Loader  // loader
+}
+
+// Reloc contains the payload for a specific relocation.
+// TODO: replace this with sym.Reloc, once we change the
+// relocation target from "*sym.Symbol" to "loader.Sym" in sym.Reloc.
+type Reloc struct {
+	Off  int32            // offset to rewrite
+	Size uint8            // number of bytes to rewrite: 0, 1, 2, or 4
+	Type objabi.RelocType // the relocation type
+	Add  int64            // addend
+	Sym  Sym              // global index of symbol the reloc addresses
+}
+
 // oReader is a wrapper type of obj.Reader, along with some
 // extra information.
 // TODO: rename to objReader once the old one is gone?
@@ -295,6 +316,39 @@ func (l *Loader) InitReachable() {
 	l.Reachable = makeBitmap(l.NSym())
 }
 
+// At method returns the j-th reloc for a global symbol.
+func (relocs *Relocs) At(j int) Reloc {
+	rel := goobj2.Reloc{}
+	rel.Read(relocs.r.Reader, relocs.r.RelocOff(relocs.li, j))
+	target := relocs.l.Resolve(relocs.r, rel.Sym)
+	return Reloc{
+		Off:  rel.Off,
+		Size: rel.Siz,
+		Type: objabi.RelocType(rel.Type),
+		Add:  rel.Add,
+		Sym:  target,
+	}
+}
+
+// Relocs returns a Relocs object for the given global sym.
+func (l *Loader) Relocs(i Sym) Relocs {
+	r, li := l.ToLocal(i)
+	if r == nil {
+		return Relocs{}
+	}
+	return l.relocs(r, li)
+}
+
+// Relocs returns a Relocs object given a local sym index and reader.
+func (l *Loader) relocs(r *oReader, li int) Relocs {
+	return Relocs{
+		Count: r.NReloc(li),
+		li:    li,
+		r:     r,
+		l:     l,
+	}
+}
+
 // Preload a package: add autolibs, add symbols to the symbol table.
 // Does not read symbol data yet.
 func LoadNew(l *Loader, arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, lib *sym.Library, unit *sym.CompilationUnit, length int64, pn string, flags int) {
@@ -473,15 +527,14 @@ func loadObjReloc(l *Loader, r *oReader) {
 		s.Type = t
 		s.Unit = r.unit
 
-		// Reloc
-		nreloc := r.NReloc(i)
-		s.R = make([]sym.Reloc, nreloc)
+		// Relocs
+		relocs := l.relocs(r, i)
+		s.R = make([]sym.Reloc, relocs.Count)
 		for j := range s.R {
-			rel := goobj2.Reloc{}
-			rel.Read(r.Reader, r.RelocOff(i, j))
-			rs := l.Resolve(r, rel.Sym)
-			rt := objabi.RelocType(rel.Type)
-			sz := rel.Siz
+			r := relocs.At(j)
+			rs := r.Sym
+			sz := r.Size
+			rt := r.Type
 			if rt == objabi.R_METHODOFF {
 				if l.Reachable.Has(rs) {
 					rt = objabi.R_ADDROFF
@@ -495,13 +548,14 @@ func loadObjReloc(l *Loader, r *oReader) {
 				sz = 0
 			}
 			if rs != 0 && l.SymType(rs) == sym.SABIALIAS {
-				rs = l.RelocSym(rs, 0)
+				rsrelocs := l.Relocs(rs)
+				rs = rsrelocs.At(0).Sym
 			}
 			s.R[j] = sym.Reloc{
-				Off:  rel.Off,
+				Off:  r.Off,
 				Siz:  sz,
 				Type: rt,
-				Add:  rel.Add,
+				Add:  r.Add,
 				Sym:  l.Syms[rs],
 			}
 		}