cmd/asm: for arm, rewrite argument shifted right by 0 to left by 0.

Right shift by 0 has bad semantics. Make sure if we try to right shift by 0,
do a left shift by 0 instead.

CL 549955 handled full instructions with this strange no-op encoding.
This CL handles the shift done to instruction register inputs.
(The former is implemented using the latter, but not until deep
inside the assembler.)

Update golang#64715

Change-Id: Ibfabb4b13e2595551e58b977162fe005aaaa0ad1
Reviewed-on: https://go-review.googlesource.com/c/go/+/550335
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/asm/internal/asm/testdata/arm.s

@@ -943,6 +943,20 @@ jmp_label_3:
 	SLL	R5, R7               // 1775a0e1
 	SLL.S	R5, R7               // 1775b0e1
 
+// Ops with zero shifts should encode as left shifts
+	ADD	R0<<0, R1, R2	     // 002081e0
+	ADD	R0>>0, R1, R2	     // 002081e0
+	ADD	R0->0, R1, R2	     // 002081e0
+	ADD	R0@>0, R1, R2	     // 002081e0
+	MOVW	R0<<0(R1), R2        // 002091e7
+	MOVW	R0>>0(R1), R2        // 002091e7
+	MOVW	R0->0(R1), R2        // 002091e7
+	MOVW	R0@>0(R1), R2        // 002091e7
+	MOVW	R0, R1<<0(R2)        // 010082e7
+	MOVW	R0, R1>>0(R2)        // 010082e7
+	MOVW	R0, R1->0(R2)        // 010082e7
+	MOVW	R0, R1@>0(R2)        // 010082e7
+
 // MULA / MULS
 	MULAWT		R1, R2, R3, R4       // c23124e1
 	MULAWB		R1, R2, R3, R4       // 823124e1

src/cmd/internal/obj/arm/asm5.go

@@ -1106,6 +1106,24 @@ func (c *ctxt5) oplook(p *obj.Prog) *Optab {
 		// TODO: rotate by 0? Not currently supported, but if we ever do then include it here.
 		p.As = ASLL
 	}
+	if p.As != AMOVB && p.As != AMOVBS && p.As != AMOVBU && p.As != AMOVH && p.As != AMOVHS && p.As != AMOVHU && p.As != AXTAB && p.As != AXTABU && p.As != AXTAH && p.As != AXTAHU {
+		// Same here, but for shifts encoded in Addrs.
+		// Don't do it for the extension ops, which
+		// need to keep their RR shifts.
+		fixShift := func(a *obj.Addr) {
+			if a.Type == obj.TYPE_SHIFT {
+				typ := a.Offset & SHIFT_RR
+				isConst := a.Offset&(1<<4) == 0
+				amount := a.Offset >> 7 & 0x1f
+				if isConst && amount == 0 && (typ == SHIFT_LR || typ == SHIFT_AR || typ == SHIFT_RR) {
+					a.Offset -= typ
+					a.Offset += SHIFT_LL
+				}
+			}
+		}
+		fixShift(&p.From)
+		fixShift(&p.To)
+	}
 
 	ops := oprange[p.As&obj.AMask]
 	c1 := &xcmp[a1]