go.tools/ssa: fix regression in x<<y logic from CL 11011043

Details:
- reintroduce interp.asUint64: it's not sound to use only the
  low 32 bits of y, which is what asInt gives us, when GOARCH=386.
- instead, emit a uint64 conversion when y is not unsigned
  (i.e. a signed var, or an untyped constant).

Tested on 386 & x86-64.

R=gri
CC=golang-dev
https://golang.org/cl/11023043

ssa/emit.go

@@ -38,6 +38,11 @@ func emitArith(f *Function, op token.Token, x, y Value, t types.Type, pos token.
 	switch op {
 	case token.SHL, token.SHR:
 		x = emitConv(f, x, t)
+		// y may be signed or an 'untyped' constant.
+		// TODO(adonovan): whence signed values?
+		if b, ok := y.Type().Underlying().(*types.Basic); ok && b.Info()&types.IsUnsigned == 0 {
+			y = emitConv(f, y, types.Typ[types.Uint64])
+		}
 
 	case token.ADD, token.SUB, token.MUL, token.QUO, token.REM, token.AND, token.OR, token.XOR, token.AND_NOT:
 		x = emitConv(f, x, t)

ssa/interp/ops.go

@@ -30,6 +30,7 @@ func literalValue(l *ssa.Literal) value {
 	// By destination type:
 	switch t := l.Type().Underlying().(type) {
 	case *types.Basic:
+		// TODO(adonovan): eliminate untyped literals from SSA form.
 		switch t.Kind() {
 		case types.Bool, types.UntypedBool:
 			return exact.BoolVal(l.Value)
@@ -130,6 +131,26 @@ func asInt(x value) int {
 	panic(fmt.Sprintf("cannot convert %T to int", x))
 }
 
+// asUint64 converts x, which must be an unsigned integer, to a uint64
+// suitable for use as a bitwise shift count.
+func asUint64(x value) uint64 {
+	switch x := x.(type) {
+	case uint:
+		return uint64(x)
+	case uint8:
+		return uint64(x)
+	case uint16:
+		return uint64(x)
+	case uint32:
+		return uint64(x)
+	case uint64:
+		return x
+	case uintptr:
+		return uint64(x)
+	}
+	panic(fmt.Sprintf("cannot convert %T to uint64", x))
+}
+
 // zero returns a new "zero" value of the specified type.
 func zero(t types.Type) value {
 	switch t := t.(type) {
@@ -543,7 +564,7 @@ func binop(op token.Token, x, y value) value {
 		}
 
 	case token.SHL:
-		y := uint64(asInt(y))
+		y := asUint64(y)
 		switch x.(type) {
 		case int:
 			return x.(int) << y
@@ -570,7 +591,7 @@ func binop(op token.Token, x, y value) value {
 		}
 
 	case token.SHR:
-		y := uint64(asInt(y))
+		y := asUint64(y)
 		switch x.(type) {
 		case int:
 			return x.(int) >> y