blob: 55f3ea369f740ae418bd2f901e2b6092eae212b2 [file] [log] [blame]
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
/*
This file contains the code to check for suspicious shifts.
*/
package main
import (
"go/ast"
"go/constant"
"go/token"
"go/types"
)
func init() {
register("shift",
"check for useless shifts",
checkShift,
binaryExpr, assignStmt)
}
func checkShift(f *File, node ast.Node) {
switch node := node.(type) {
case *ast.BinaryExpr:
if node.Op == token.SHL || node.Op == token.SHR {
checkLongShift(f, node, node.X, node.Y)
}
case *ast.AssignStmt:
if len(node.Lhs) != 1 || len(node.Rhs) != 1 {
return
}
if node.Tok == token.SHL_ASSIGN || node.Tok == token.SHR_ASSIGN {
checkLongShift(f, node, node.Lhs[0], node.Rhs[0])
}
}
}
// checkLongShift checks if shift or shift-assign operations shift by more than
// the length of the underlying variable.
func checkLongShift(f *File, node ast.Node, x, y ast.Expr) {
if f.pkg.types[x].Value != nil {
// Ignore shifts of constants.
// These are frequently used for bit-twiddling tricks
// like ^uint(0) >> 63 for 32/64 bit detection and compatibility.
return
}
v := f.pkg.types[y].Value
if v == nil {
return
}
amt, ok := constant.Int64Val(v)
if !ok {
return
}
t := f.pkg.types[x].Type
if t == nil {
return
}
b, ok := t.Underlying().(*types.Basic)
if !ok {
return
}
var size int64
var msg string
switch b.Kind() {
case types.Uint8, types.Int8:
size = 8
case types.Uint16, types.Int16:
size = 16
case types.Uint32, types.Int32:
size = 32
case types.Uint64, types.Int64:
size = 64
case types.Int, types.Uint, types.Uintptr:
// These types may be as small as 32 bits, but no smaller.
size = 32
msg = "might be "
default:
return
}
if amt >= size {
ident := f.gofmt(x)
f.Badf(node.Pos(), "%s %stoo small for shift of %d", ident, msg, amt)
}
}