diff --git a/src/cmd/link/internal/amd64/asm.go b/src/cmd/link/internal/amd64/asm.go
index 9dd55727a0..fe145d0b67 100644
--- a/src/cmd/link/internal/amd64/asm.go
+++ b/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
 
diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index fd536181d7..04e6d3bdb7 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/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)
 			}