cmd/compile: optimize 386's math.bits.TrailingZeros16

This CL reverts CL 192097 and fixes the issue in CL 189277.

Change-Id: Icd271262e1f5019a8e01c91f91c12c1261eeb02b
Reviewed-on: https://go-review.googlesource.com/c/go/+/192519
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -3370,7 +3370,7 @@ func init() {
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			return s.newValue1(ssa.OpCtz16, types.Types[TINT], args[0])
 		},
-		sys.AMD64, sys.ARM, sys.ARM64, sys.Wasm)
+		sys.AMD64, sys.I386, sys.ARM, sys.ARM64, sys.Wasm)
 	addF("math/bits", "TrailingZeros16",
 		func(s *state, n *Node, args []*ssa.Value) *ssa.Value {
 			x := s.newValue1(ssa.OpZeroExt16to64, types.Types[TUINT64], args[0])

src/cmd/compile/internal/ssa/gen/386.rules

@@ -66,6 +66,9 @@
 
 (Sqrt x) -> (SQRTSD x)
 
+(Ctz16 x) -> (BSFL (ORLconst <typ.UInt32> [0x10000] x))
+(Ctz16NonZero x) -> (BSFL x)
+
 // Lowering extension
 (SignExt8to16  x) -> (MOVBLSX x)
 (SignExt8to32  x) -> (MOVBLSX x)

src/cmd/compile/internal/ssa/rewrite386.go

@@ -360,6 +360,10 @@ func rewriteValue386(v *Value) bool {
 		return rewriteValue386_OpConstBool_0(v)
 	case OpConstNil:
 		return rewriteValue386_OpConstNil_0(v)
+	case OpCtz16:
+		return rewriteValue386_OpCtz16_0(v)
+	case OpCtz16NonZero:
+		return rewriteValue386_OpCtz16NonZero_0(v)
 	case OpCvt32Fto32:
 		return rewriteValue386_OpCvt32Fto32_0(v)
 	case OpCvt32Fto64F:
@@ -20783,6 +20787,33 @@ func rewriteValue386_OpConstNil_0(v *Value) bool {
 		return true
 	}
 }
+func rewriteValue386_OpCtz16_0(v *Value) bool {
+	b := v.Block
+	typ := &b.Func.Config.Types
+	// match: (Ctz16 x)
+	// cond:
+	// result: (BSFL (ORLconst <typ.UInt32> [0x10000] x))
+	for {
+		x := v.Args[0]
+		v.reset(Op386BSFL)
+		v0 := b.NewValue0(v.Pos, Op386ORLconst, typ.UInt32)
+		v0.AuxInt = 0x10000
+		v0.AddArg(x)
+		v.AddArg(v0)
+		return true
+	}
+}
+func rewriteValue386_OpCtz16NonZero_0(v *Value) bool {
+	// match: (Ctz16NonZero x)
+	// cond:
+	// result: (BSFL x)
+	for {
+		x := v.Args[0]
+		v.reset(Op386BSFL)
+		v.AddArg(x)
+		return true
+	}
+}
 func rewriteValue386_OpCvt32Fto32_0(v *Value) bool {
 	// match: (Cvt32Fto32 x)
 	// cond:

test/codegen/mathbits.go

@@ -298,6 +298,7 @@ func TrailingZeros32(n uint32) int {
 
 func TrailingZeros16(n uint16) int {
 	// amd64:"BSFL","BTSL\\t\\$16"
+	// 386:"BSFL\t"
 	// arm:"ORR\t\\$65536","CLZ",-"MOVHU\tR"
 	// arm64:"ORR\t\\$65536","RBITW","CLZW",-"MOVHU\tR",-"RBIT\t",-"CLZ\t"
 	// s390x:"FLOGR","OR\t\\$65536"