go.tools/cmd/vet: detect suspicious shifts

LGTM=josharian, dsymonds, r
R=golang-codereviews, josharian, minux, dsymonds, dave, axwalk, adg, r
CC=golang-codereviews
https://golang.org/cl/134780043

cmd/vet/doc.go

@@ -151,6 +151,12 @@ there is a uintptr-typed word in memory that holds a pointer value,
 because that word will be invisible to stack copying and to the garbage
 collector.
 
+Shifts
+
+Flag: -shift
+
+Shifts equal to or longer than the variable's length.
+
 Other flags
 
 These flags configure the behavior of vet:

cmd/vet/shift.go

@@ -0,0 +1,83 @@
+// 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/token"
+
+	"code.google.com/p/go.tools/go/exact"
+	"code.google.com/p/go.tools/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) {
+	v := f.pkg.types[y].Value
+	if v == nil {
+		return
+	}
+	amt, ok := exact.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)
+	}
+}

cmd/vet/testdata/shift.go

@@ -0,0 +1,78 @@
+// 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 tests for the suspicious shift checker.
+
+package testdata
+
+func ShiftTest() {
+	var i8 int8
+	_ = i8 << 7
+	_ = (i8 + 1) << 8 // ERROR "\(i8 \+ 1\) too small for shift of 8"
+	_ = i8 << (7 + 1) // ERROR "i8 too small for shift of 8"
+	_ = i8 >> 8       // ERROR "i8 too small for shift of 8"
+	i8 <<= 8          // ERROR "i8 too small for shift of 8"
+	i8 >>= 8          // ERROR "i8 too small for shift of 8"
+	var i16 int16
+	_ = i16 << 15
+	_ = i16 << 16 // ERROR "i16 too small for shift of 16"
+	_ = i16 >> 16 // ERROR "i16 too small for shift of 16"
+	i16 <<= 16    // ERROR "i16 too small for shift of 16"
+	i16 >>= 16    // ERROR "i16 too small for shift of 16"
+	var i32 int32
+	_ = i32 << 31
+	_ = i32 << 32 // ERROR "i32 too small for shift of 32"
+	_ = i32 >> 32 // ERROR "i32 too small for shift of 32"
+	i32 <<= 32    // ERROR "i32 too small for shift of 32"
+	i32 >>= 32    // ERROR "i32 too small for shift of 32"
+	var i64 int64
+	_ = i64 << 63
+	_ = i64 << 64 // ERROR "i64 too small for shift of 64"
+	_ = i64 >> 64 // ERROR "i64 too small for shift of 64"
+	i64 <<= 64    // ERROR "i64 too small for shift of 64"
+	i64 >>= 64    // ERROR "i64 too small for shift of 64"
+	var u8 uint8
+	_ = u8 << 7
+	_ = u8 << 8 // ERROR "u8 too small for shift of 8"
+	_ = u8 >> 8 // ERROR "u8 too small for shift of 8"
+	u8 <<= 8    // ERROR "u8 too small for shift of 8"
+	u8 >>= 8    // ERROR "u8 too small for shift of 8"
+	var u16 uint16
+	_ = u16 << 15
+	_ = u16 << 16 // ERROR "u16 too small for shift of 16"
+	_ = u16 >> 16 // ERROR "u16 too small for shift of 16"
+	u16 <<= 16    // ERROR "u16 too small for shift of 16"
+	u16 >>= 16    // ERROR "u16 too small for shift of 16"
+	var u32 uint32
+	_ = u32 << 31
+	_ = u32 << 32 // ERROR "u32 too small for shift of 32"
+	_ = u32 >> 32 // ERROR "u32 too small for shift of 32"
+	u32 <<= 32    // ERROR "u32 too small for shift of 32"
+	u32 >>= 32    // ERROR "u32 too small for shift of 32"
+	var u64 uint64
+	_ = u64 << 63
+	_ = u64 << 64  // ERROR "u64 too small for shift of 64"
+	_ = u64 >> 64  // ERROR "u64 too small for shift of 64"
+	u64 <<= 64     // ERROR "u64 too small for shift of 64"
+	u64 >>= 64     // ERROR "u64 too small for shift of 64"
+	_ = u64 << u64 // Non-constant shifts should succeed.
+	var i int
+	_ = i << 31
+	_ = i << 32 // ERROR "i might be too small for shift of 32"
+	_ = i >> 32 // ERROR "i might be too small for shift of 32"
+	i <<= 32    // ERROR "i might be too small for shift of 32"
+	i >>= 32    // ERROR "i might be too small for shift of 32"
+	var u uint
+	_ = u << 31
+	_ = u << 32 // ERROR "u might be too small for shift of 32"
+	_ = u >> 32 // ERROR "u might be too small for shift of 32"
+	u <<= 32    // ERROR "u might be too small for shift of 32"
+	u >>= 32    // ERROR "u might be too small for shift of 32"
+	var p uintptr
+	_ = p << 31
+	_ = p << 32 // ERROR "p might be too small for shift of 32"
+	_ = p >> 32 // ERROR "p might be too small for shift of 32"
+	p <<= 32    // ERROR "p might be too small for shift of 32"
+	p >>= 32    // ERROR "p might be too small for shift of 32"
+}