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// 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.
// Package shift defines an Analyzer that checks for shifts that exceed
// the width of an integer.
package shift
// TODO(adonovan): integrate with ctrflow (CFG-based) dead code analysis. May
// have impedance mismatch due to its (non-)treatment of constant
// expressions (such as runtime.GOARCH=="386").
import (
"go/ast"
"go/constant"
"go/token"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
"golang.org/x/tools/go/ast/inspector"
)
var Analyzer = &analysis.Analyzer{
Name: "shift",
Doc: "check for shifts that equal or exceed the width of the integer",
Requires: []*analysis.Analyzer{inspect.Analyzer},
Run: run,
}
func run(pass *analysis.Pass) (interface{}, error) {
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
// Do a complete pass to compute dead nodes.
dead := make(map[ast.Node]bool)
nodeFilter := []ast.Node{
(*ast.IfStmt)(nil),
(*ast.SwitchStmt)(nil),
}
inspect.Preorder(nodeFilter, func(n ast.Node) {
// TODO(adonovan): move updateDead into this file.
updateDead(pass.TypesInfo, dead, n)
})
nodeFilter = []ast.Node{
(*ast.AssignStmt)(nil),
(*ast.BinaryExpr)(nil),
}
inspect.Preorder(nodeFilter, func(node ast.Node) {
if dead[node] {
// Skip shift checks on unreachable nodes.
return
}
switch node := node.(type) {
case *ast.BinaryExpr:
if node.Op == token.SHL || node.Op == token.SHR {
checkLongShift(pass, 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(pass, node, node.Lhs[0], node.Rhs[0])
}
}
})
return nil, nil
}
// checkLongShift checks if shift or shift-assign operations shift by more than
// the length of the underlying variable.
func checkLongShift(pass *analysis.Pass, node ast.Node, x, y ast.Expr) {
if pass.TypesInfo.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 := pass.TypesInfo.Types[y].Value
if v == nil {
return
}
amt, ok := constant.Int64Val(v)
if !ok {
return
}
t := pass.TypesInfo.Types[x].Type
if t == nil {
return
}
size := 8 * pass.TypesSizes.Sizeof(t)
if amt >= size {
ident := analysisutil.Format(pass.Fset, x)
pass.Reportf(node.Pos(), "%s (%d bits) too small for shift of %d", ident, size, amt)
}
}
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