| 1 | // Copyright 2014 The Go Authors. All rights reserved. |
|---|---|
| 2 | // Use of this source code is governed by a BSD-style |
| 3 | // license that can be found in the LICENSE file. |
| 4 | |
| 5 | // Package shift defines an Analyzer that checks for shifts that exceed |
| 6 | // the width of an integer. |
| 7 | package shift |
| 8 | |
| 9 | // TODO(adonovan): integrate with ctrflow (CFG-based) dead code analysis. May |
| 10 | // have impedance mismatch due to its (non-)treatment of constant |
| 11 | // expressions (such as runtime.GOARCH=="386"). |
| 12 | |
| 13 | import ( |
| 14 | "go/ast" |
| 15 | "go/constant" |
| 16 | "go/token" |
| 17 | "go/types" |
| 18 | "math" |
| 19 | |
| 20 | "golang.org/x/tools/go/analysis" |
| 21 | "golang.org/x/tools/go/analysis/passes/inspect" |
| 22 | "golang.org/x/tools/go/analysis/passes/internal/analysisutil" |
| 23 | "golang.org/x/tools/go/ast/inspector" |
| 24 | "golang.org/x/tools/internal/typeparams" |
| 25 | ) |
| 26 | |
| 27 | const Doc = "check for shifts that equal or exceed the width of the integer" |
| 28 | |
| 29 | var Analyzer = &analysis.Analyzer{ |
| 30 | Name: "shift", |
| 31 | Doc: Doc, |
| 32 | Requires: []*analysis.Analyzer{inspect.Analyzer}, |
| 33 | Run: run, |
| 34 | } |
| 35 | |
| 36 | func run(pass *analysis.Pass) (interface{}, error) { |
| 37 | inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector) |
| 38 | |
| 39 | // Do a complete pass to compute dead nodes. |
| 40 | dead := make(map[ast.Node]bool) |
| 41 | nodeFilter := []ast.Node{ |
| 42 | (*ast.IfStmt)(nil), |
| 43 | (*ast.SwitchStmt)(nil), |
| 44 | } |
| 45 | inspect.Preorder(nodeFilter, func(n ast.Node) { |
| 46 | // TODO(adonovan): move updateDead into this file. |
| 47 | updateDead(pass.TypesInfo, dead, n) |
| 48 | }) |
| 49 | |
| 50 | nodeFilter = []ast.Node{ |
| 51 | (*ast.AssignStmt)(nil), |
| 52 | (*ast.BinaryExpr)(nil), |
| 53 | } |
| 54 | inspect.Preorder(nodeFilter, func(node ast.Node) { |
| 55 | if dead[node] { |
| 56 | // Skip shift checks on unreachable nodes. |
| 57 | return |
| 58 | } |
| 59 | |
| 60 | switch node := node.(type) { |
| 61 | case *ast.BinaryExpr: |
| 62 | if node.Op == token.SHL || node.Op == token.SHR { |
| 63 | checkLongShift(pass, node, node.X, node.Y) |
| 64 | } |
| 65 | case *ast.AssignStmt: |
| 66 | if len(node.Lhs) != 1 || len(node.Rhs) != 1 { |
| 67 | return |
| 68 | } |
| 69 | if node.Tok == token.SHL_ASSIGN || node.Tok == token.SHR_ASSIGN { |
| 70 | checkLongShift(pass, node, node.Lhs[0], node.Rhs[0]) |
| 71 | } |
| 72 | } |
| 73 | }) |
| 74 | return nil, nil |
| 75 | } |
| 76 | |
| 77 | // checkLongShift checks if shift or shift-assign operations shift by more than |
| 78 | // the length of the underlying variable. |
| 79 | func checkLongShift(pass *analysis.Pass, node ast.Node, x, y ast.Expr) { |
| 80 | if pass.TypesInfo.Types[x].Value != nil { |
| 81 | // Ignore shifts of constants. |
| 82 | // These are frequently used for bit-twiddling tricks |
| 83 | // like ^uint(0) >> 63 for 32/64 bit detection and compatibility. |
| 84 | return |
| 85 | } |
| 86 | |
| 87 | v := pass.TypesInfo.Types[y].Value |
| 88 | if v == nil { |
| 89 | return |
| 90 | } |
| 91 | amt, ok := constant.Int64Val(v) |
| 92 | if !ok { |
| 93 | return |
| 94 | } |
| 95 | t := pass.TypesInfo.Types[x].Type |
| 96 | if t == nil { |
| 97 | return |
| 98 | } |
| 99 | var structuralTypes []types.Type |
| 100 | switch t := t.(type) { |
| 101 | case *typeparams.TypeParam: |
| 102 | terms, err := typeparams.StructuralTerms(t) |
| 103 | if err != nil { |
| 104 | return // invalid type |
| 105 | } |
| 106 | for _, term := range terms { |
| 107 | structuralTypes = append(structuralTypes, term.Type()) |
| 108 | } |
| 109 | default: |
| 110 | structuralTypes = append(structuralTypes, t) |
| 111 | } |
| 112 | sizes := make(map[int64]struct{}) |
| 113 | for _, t := range structuralTypes { |
| 114 | size := 8 * pass.TypesSizes.Sizeof(t) |
| 115 | sizes[size] = struct{}{} |
| 116 | } |
| 117 | minSize := int64(math.MaxInt64) |
| 118 | for size := range sizes { |
| 119 | if size < minSize { |
| 120 | minSize = size |
| 121 | } |
| 122 | } |
| 123 | if amt >= minSize { |
| 124 | ident := analysisutil.Format(pass.Fset, x) |
| 125 | qualifier := "" |
| 126 | if len(sizes) > 1 { |
| 127 | qualifier = "may be " |
| 128 | } |
| 129 | pass.ReportRangef(node, "%s (%s%d bits) too small for shift of %d", ident, qualifier, minSize, amt) |
| 130 | } |
| 131 | } |
| 132 |
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