e = stack[last]
	// Buggy compiler prints "Disproved Leq64" for the next line.
	stack = stack[:last]
	return e, nil
}

func sm1(b []int, x int) {
	// Test constant argument to slicemask.
	useSlice(b[2:8]) // ERROR "Proved slicemask not needed$"
	// Test non-constant argument with known limits.
	if cap(b) > 10 {
		useSlice(b[2:])
	}
}

func lim1(x, y, z int) {

		case OpSlicemask:
			// Replace OpSlicemask operations in b with constants where possible.
			x, delta := isConstDelta(v.Args[0])
			if x == nil {
				break
			}
			// slicemask(x + y)
			// if x is larger than -y (y is negative), then slicemask is -1.
			lim, ok := ft.limits[x.ID]
			if !ok {
				break
			}
			if lim.umin > uint64(-delta) {
				if v.Args[0].Op == OpAdd64 {
					v.reset(OpConst64)

(ZeroExt32to64        x) => (I64And x (I64Const [0xffffffff]))
(ZeroExt16to(64|32)   x) => (I64And x (I64Const [0xffff]))
(ZeroExt8to(64|32|16) x) => (I64And x (I64Const [0xff]))

(Slicemask x) => (I64ShrS (I64Sub (I64Const [0]) x) (I64Const [63]))

// Lowering truncation
// Because we ignore the high parts, truncates are just copies.
(Trunc64to(32|16|8) ...) => (Copy ...)
(Trunc32to(16|8)    ...) => (Copy ...)

(SignExt16to32 (Trunc32to16 x:(Rsh32x64 _ (Const64 [s])))) && s >= 16 => x
(SignExt8to16  (Trunc16to8  x:(Rsh16x64 _ (Const64 [s])))) && s >= 8 => x

(Slicemask (Const32 [x])) && x > 0 => (Const32 [-1])
(Slicemask (Const32 [0]))          => (Const32 [0])
(Slicemask (Const64 [x])) && x > 0 => (Const64 [-1])
(Slicemask (Const64 [0]))          => (Const64 [0])

// simplifications often used for lengths.  e.g. len(s[i:i+5])==5

// constants
(Const(64|32|16|8) [val]) => (MOVVconst [int64(val)])
(Const(32|64)F [val]) => (MOV(F|D)const [float64(val)])
(ConstNil) => (MOVVconst [0])
(ConstBool [t]) => (MOVVconst [int64(b2i(t))])

(Slicemask <t> x) => (SRAVconst (NEGV <t> x) [63])

// truncations
// Because we ignore high parts of registers, truncates are just copies.
(Trunc16to8 ...) => (Copy ...)
(Trunc32to8 ...) => (Copy ...)

(Cvt64Fto64 ...) => (FCVTLD ...)

(Cvt32Fto64F ...) => (FCVTDS ...)
(Cvt64Fto32F ...) => (FCVTSD ...)

(CvtBoolToUint8 ...) => (Copy ...)

(Round(32|64)F ...) => (LoweredRound(32|64)F ...)

(Slicemask <t> x) => (SRAI [63] (NEG <t> x))

// Truncations
// We ignore the unused high parts of registers, so truncates are just copies.
(Trunc16to8  ...) => (Copy ...)
(Trunc32to8  ...) => (Copy ...)

(SignExt8to16 ...)  => (MOVBreg ...)
(SignExt8to32 ...)  => (MOVBreg ...)
(SignExt16to32 ...) => (MOVHreg ...)

(Signmask x) => (SRAconst x [31])
(Zeromask x) => (NEG (SGTU x (MOVWconst [0])))
(Slicemask <t> x) => (SRAconst (NEG <t> x) [31])

// float-int conversion
(Cvt32to(32|64)F ...) => (MOVW(F|D) ...)
(Cvt(32|64)Fto32 ...) => (TRUNC(F|D)W ...)
(Cvt32Fto64F ...) => (MOVFD ...)
(Cvt64Fto32F ...) => (MOVDF ...)

// constants
(Const(64|32|16|8) [val]) => (MOVVconst [int64(val)])
(Const(32|64)F [val]) => (MOV(F|D)const [float64(val)])
(ConstNil) => (MOVVconst [0])
(ConstBool [t]) => (MOVVconst [int64(b2i(t))])

(Slicemask <t> x) => (SRAVconst (NEGV <t> x) [63])

// truncations
// Because we ignore high parts of registers, truncates are just copies.
(Trunc16to8 ...) => (Copy ...)
(Trunc32to8 ...) => (Copy ...)

	{name: "Signmask", argLength: 1, typ: "Int32"},  // 0 if arg0 >= 0, -1 if arg0 < 0
	{name: "Zeromask", argLength: 1, typ: "UInt32"}, // 0 if arg0 == 0, 0xffffffff if arg0 != 0
	{name: "Slicemask", argLength: 1},               // 0 if arg0 == 0, -1 if arg0 > 0, undef if arg0<0. Type is native int size.

	{name: "SpectreIndex", argLength: 2},      // arg0 if 0 <= arg0 < arg1, 0 otherwise. Type is native int size.

(ZeroExt8to32  ...) => (MOVBQZX ...)
(ZeroExt8to64  ...) => (MOVBQZX ...)
(ZeroExt16to32 ...) => (MOVWQZX ...)
(ZeroExt16to64 ...) => (MOVWQZX ...)
(ZeroExt32to64 ...) => (MOVLQZX ...)

(Slicemask <t> x) => (SARQconst (NEGQ <t> x) [63])

(SpectreIndex <t> x y) => (CMOVQCC x (MOVQconst [0]) (CMPQ x y))
(SpectreSliceIndex <t> x y) => (CMOVQHI x (MOVQconst [0]) (CMPQ x y))

// Lowering truncation