cmd/link: address comments from CL 28540

Change-Id: I11899096c71ee0e24e902c87914601fcd7ffd7a9
Reviewed-on: https://go-review.googlesource.com/28967Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/amd64/asm.go

@@ -798,6 +798,9 @@ func tlsIEtoLE(s *ld.Symbol, off, size int) {
 	//
 	// To determine the instruction and register, we study the op codes.
 	// Consult an AMD64 instruction encoding guide to decipher this.
+	if off < 3 {
+		log.Fatal("R_X86_64_GOTTPOFF reloc not preceded by MOVQ or ADDQ instruction")
+	}
 	op := s.P[off-3 : off]
 	reg := op[2] >> 3
 

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

@@ -456,6 +456,9 @@ func relocsym(ctxt *Link, s *Symbol) {
 				}
 				Thearch.TLSIEtoLE(s, int(off), int(r.Siz))
 				o = int64(ctxt.Tlsoffset)
+				// TODO: o += r.Add when SysArch.Family != sys.AMD64?
+				// Why do we treat r.Add differently on AMD64?
+				// Is the external linker using Xadd at all?
 			} else {
 				log.Fatalf("cannot handle R_TLS_IE (sym %s) when linking internally", s.Name)
 			}