cmd/internal/obj/x86: use LEAx rather than ADDx when calling DUFFxxxx via GOT

DUFFZERO on 386 is not marked as clobbering flags, but rewriteToUseGot rewrote
"ADUFFZERO $offset" to "MOVL runtime.duffxxx@GOT, CX; ADDL $offset, CX; CALL CX"
which does. Luckily the fix is easier than figuring out what the problem was:
replace the ADDL $offset, CX with LEAL $offset(CX), CX.

On amd64 DUFFZERO clobbers flags, on arm, arm64 and ppc64 ADD does not clobber
flags and s390x does not use the duff functions, so I'm fairly confident this
is the only fix required.

I don't know how to write a test though.

Change-Id: I69b0958f5f45771d61db5f5ecb4ded94e8960d4d
Reviewed-on: https://go-review.googlesource.com/41821
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/internal/obj/x86/obj6.go

@@ -296,15 +296,13 @@ func progedit(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 
 // Rewrite p, if necessary, to access global data via the global offset table.
 func rewriteToUseGot(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
-	var add, lea, mov obj.As
+	var lea, mov obj.As
 	var reg int16
 	if ctxt.Arch.Family == sys.AMD64 {
-		add = AADDQ
 		lea = ALEAQ
 		mov = AMOVQ
 		reg = REG_R15
 	} else {
-		add = AADDL
 		lea = ALEAL
 		mov = AMOVL
 		reg = REG_CX
@@ -321,8 +319,10 @@ func rewriteToUseGot(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 		//     ADUFFxxx $offset
 		// becomes
 		//     $MOV runtime.duffxxx@GOT, $reg
-		//     $ADD $offset, $reg
+		//     $LEA $offset($reg), $reg
 		//     CALL $reg
+		// (we use LEAx rather than ADDx because ADDx clobbers
+		// flags and duffzero on 386 does not otherwise do so)
 		var sym *obj.LSym
 		if p.As == obj.ADUFFZERO {
 			sym = ctxt.Lookup("runtime.duffzero")
@@ -339,9 +339,10 @@ func rewriteToUseGot(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 		p.To.Offset = 0
 		p.To.Sym = nil
 		p1 := obj.Appendp(p, newprog)
-		p1.As = add
-		p1.From.Type = obj.TYPE_CONST
+		p1.As = lea
+		p1.From.Type = obj.TYPE_MEM
 		p1.From.Offset = offset
+		p1.From.Reg = reg
 		p1.To.Type = obj.TYPE_REG
 		p1.To.Reg = reg
 		p2 := obj.Appendp(p1, newprog)