cmd/internal/obj/x86: implement -shared computation of TLS base

Somehow, terribly embarrassingly, I lost part of the "re-enable
-shared on amd64" patch when rebasing before it got submitted.
This restores it and also fixes the addend to be the necessary -4.

Now updated so that Git will not put the new case into the wrong
switch.

Change-Id: I1d628232771a6d6ce6d085adf379f94a377822c5
Reviewed-on: https://go-review.googlesource.com/7126
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/x86/asm6.go

@@ -3801,6 +3801,32 @@ func doasm(ctxt *obj.Link, p *obj.Prog) {
 					default:
 						log.Fatalf("unknown TLS base location for %s", obj.Headstr(ctxt.Headtype))
 
+					case obj.Hlinux:
+						if ctxt.Flag_shared == 0 {
+							log.Fatalf("unknown TLS base location for linux without -shared")
+						}
+						// Note that this is not generating the same insn as the other cases.
+						//     MOV TLS, R_to
+						// becomes
+						//     movq g@gottpoff(%rip), R_to
+						// which is encoded as
+						//     movq 0(%rip), R_to
+						// and a R_TLS_IE reloc. This all assumes the only tls variable we access
+						// is g, which we can't check here, but will when we assemble the second
+						// instruction.
+						ctxt.Rexflag = Pw | (regrex[p.To.Reg] & Rxr)
+
+						ctxt.Andptr[0] = 0x8B
+						ctxt.Andptr = ctxt.Andptr[1:]
+						ctxt.Andptr[0] = byte(0x05 | (reg[p.To.Reg] << 3))
+						ctxt.Andptr = ctxt.Andptr[1:]
+						r = obj.Addrel(ctxt.Cursym)
+						r.Off = int32(p.Pc + int64(-cap(ctxt.Andptr)+cap(ctxt.And[:])))
+						r.Type = obj.R_TLS_IE
+						r.Siz = 4
+						r.Add = -4
+						put4(ctxt, 0)
+
 					case obj.Hplan9:
 						if ctxt.Plan9privates == nil {
 							ctxt.Plan9privates = obj.Linklookup(ctxt, "_privates", 0)