cmd/asm: reject BSWAPW on amd64

Since BSWAP operation on 16-bit registers is undefined,
forbid the usage of BSWAPW. Users should rely on XCHGB instead.

This behavior is consistent with what GAS does.

Fixes #29167

Change-Id: I3b31e3dd2acfd039f7564a1c17e6068617bcde8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/174312
Run-TryBot: Iskander Sharipov <quasilyte@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/asm/internal/asm/line_test.go

@@ -28,6 +28,10 @@ func TestAMD64BadInstParser(t *testing.T) {
 		{"VADDPD.BCST.Z.SAE X0, X1, X2", `Z suffix should be the last; can't combine rounding/SAE and broadcast`},
 		{"VADDPD.SAE.SAE X0, X1, X2", `duplicate suffix "SAE"`},
 		{"VADDPD.RZ_SAE.SAE X0, X1, X2", `bad suffix combination`},
+
+		// BSWAP on 16-bit registers is undefined. See #29167,
+		{"BSWAPW DX", `unrecognized instruction`},
+		{"BSWAPW R11", `unrecognized instruction`},
 	})
 }
 

src/cmd/asm/internal/asm/testdata/amd64enc.s

@@ -588,8 +588,6 @@ TEXT asmtest(SB),DUPOK|NOSPLIT,$0
 	BSRQ (R11), R11                         // 4d0fbd1b
 	BSRQ DX, R11                            // 4c0fbdda
 	BSRQ R11, R11                           // 4d0fbddb
-	BSWAPW DX                               // 660fca
-	BSWAPW R11                              // 66410fcb
 	BSWAPL DX                               // 0fca
 	BSWAPL R11                              // 410fcb
 	BSWAPQ DX                               // 480fca

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

@@ -69,7 +69,6 @@ const (
 	ABSRW
 	ABSWAPL
 	ABSWAPQ
-	ABSWAPW
 	ABTCL
 	ABTCQ
 	ABTCW

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

@@ -67,7 +67,6 @@ var Anames = []string{
 	"BSRW",
 	"BSWAPL",
 	"BSWAPQ",
-	"BSWAPW",
 	"BTCL",
 	"BTCQ",
 	"BTCW",

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

@@ -961,7 +961,6 @@ var optab =
 	{ABSRL, yml_rl, Pm, opBytes{0xbd}},
 	{ABSRQ, yml_rl, Pw, opBytes{0x0f, 0xbd}},
 	{ABSRW, yml_rl, Pq, opBytes{0xbd}},
-	{ABSWAPW, ybswap, Pe, opBytes{0x0f, 0xc8}},
 	{ABSWAPL, ybswap, Px, opBytes{0x0f, 0xc8}},
 	{ABSWAPQ, ybswap, Pw, opBytes{0x0f, 0xc8}},
 	{ABTCL, ybtl, Pm, opBytes{0xba, 07, 0xbb}},

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

@@ -1206,7 +1206,6 @@ func stacksplit(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, newprog obj.ProgA
 var unaryDst = map[obj.As]bool{
 	ABSWAPL:     true,
 	ABSWAPQ:     true,
-	ABSWAPW:     true,
 	ACLFLUSH:    true,
 	ACLFLUSHOPT: true,
 	ACMPXCHG16B: true,