| // Copyright 2020 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 fieldalignment defines an Analyzer that detects structs that would take less |
| // memory if their fields were sorted. |
| package fieldalignment |
| |
| import ( |
| "bytes" |
| "fmt" |
| "go/ast" |
| "go/format" |
| "go/token" |
| "go/types" |
| "sort" |
| |
| "golang.org/x/tools/go/analysis" |
| "golang.org/x/tools/go/analysis/passes/inspect" |
| "golang.org/x/tools/go/ast/inspector" |
| ) |
| |
| const Doc = `find structs that would take less memory if their fields were sorted |
| |
| This analyzer find structs that can be rearranged to take less memory, and provides |
| a suggested edit with the optimal order. |
| ` |
| |
| var Analyzer = &analysis.Analyzer{ |
| Name: "fieldalignment", |
| Doc: Doc, |
| Requires: []*analysis.Analyzer{inspect.Analyzer}, |
| Run: run, |
| } |
| |
| func run(pass *analysis.Pass) (interface{}, error) { |
| inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector) |
| nodeFilter := []ast.Node{ |
| (*ast.StructType)(nil), |
| } |
| inspect.Preorder(nodeFilter, func(node ast.Node) { |
| var s *ast.StructType |
| var ok bool |
| if s, ok = node.(*ast.StructType); !ok { |
| return |
| } |
| if tv, ok := pass.TypesInfo.Types[s]; ok { |
| fieldalignment(pass, s, tv.Type.(*types.Struct)) |
| } |
| }) |
| return nil, nil |
| } |
| |
| var unsafePointerTyp = types.Unsafe.Scope().Lookup("Pointer").(*types.TypeName).Type() |
| |
| func fieldalignment(pass *analysis.Pass, node *ast.StructType, typ *types.Struct) { |
| wordSize := pass.TypesSizes.Sizeof(unsafePointerTyp) |
| maxAlign := pass.TypesSizes.Alignof(unsafePointerTyp) |
| |
| s := gcSizes{wordSize, maxAlign} |
| optimal, indexes := optimalOrder(typ, &s) |
| optsz, optptrs := s.Sizeof(optimal), s.ptrdata(optimal) |
| |
| var message string |
| if sz := s.Sizeof(typ); sz != optsz { |
| message = fmt.Sprintf("struct of size %d could be %d", sz, optsz) |
| } else if ptrs := s.ptrdata(typ); ptrs != optptrs { |
| message = fmt.Sprintf("struct with %d pointer bytes could be %d", ptrs, optptrs) |
| } else { |
| // Already optimal order. |
| return |
| } |
| |
| // Flatten the ast node since it could have multiple field names per list item while |
| // *types.Struct only have one item per field. |
| // TODO: Preserve multi-named fields instead of flattening. |
| var flat []*ast.Field |
| for _, f := range node.Fields.List { |
| // TODO: Preserve comment, for now get rid of them. |
| // See https://github.com/golang/go/issues/20744 |
| f.Comment = nil |
| if len(f.Names) <= 1 { |
| flat = append(flat, f) |
| continue |
| } |
| for _, name := range f.Names { |
| flat = append(flat, &ast.Field{ |
| Names: []*ast.Ident{name}, |
| Type: f.Type, |
| }) |
| } |
| } |
| |
| // Sort fields according to the optimal order. |
| var reordered []*ast.Field |
| for _, index := range indexes { |
| reordered = append(reordered, flat[index]) |
| } |
| |
| newStr := &ast.StructType{ |
| Fields: &ast.FieldList{ |
| List: reordered, |
| }, |
| } |
| |
| // Write the newly aligned struct node to get the content for suggested fixes. |
| var buf bytes.Buffer |
| if err := format.Node(&buf, token.NewFileSet(), newStr); err != nil { |
| return |
| } |
| |
| pass.Report(analysis.Diagnostic{ |
| Pos: node.Pos(), |
| End: node.Pos() + token.Pos(len("struct")), |
| Message: message, |
| SuggestedFixes: []analysis.SuggestedFix{{ |
| Message: "Rearrange fields", |
| TextEdits: []analysis.TextEdit{{ |
| Pos: node.Pos(), |
| End: node.End(), |
| NewText: buf.Bytes(), |
| }}, |
| }}, |
| }) |
| } |
| |
| func optimalOrder(str *types.Struct, sizes *gcSizes) (*types.Struct, []int) { |
| nf := str.NumFields() |
| |
| type elem struct { |
| index int |
| alignof int64 |
| sizeof int64 |
| ptrdata int64 |
| } |
| |
| elems := make([]elem, nf) |
| for i := 0; i < nf; i++ { |
| field := str.Field(i) |
| ft := field.Type() |
| elems[i] = elem{ |
| i, |
| sizes.Alignof(ft), |
| sizes.Sizeof(ft), |
| sizes.ptrdata(ft), |
| } |
| } |
| |
| sort.Slice(elems, func(i, j int) bool { |
| ei := &elems[i] |
| ej := &elems[j] |
| |
| // Place zero sized objects before non-zero sized objects. |
| zeroi := ei.sizeof == 0 |
| zeroj := ej.sizeof == 0 |
| if zeroi != zeroj { |
| return zeroi |
| } |
| |
| // Next, place more tightly aligned objects before less tightly aligned objects. |
| if ei.alignof != ej.alignof { |
| return ei.alignof > ej.alignof |
| } |
| |
| // Place pointerful objects before pointer-free objects. |
| noptrsi := ei.ptrdata == 0 |
| noptrsj := ej.ptrdata == 0 |
| if noptrsi != noptrsj { |
| return noptrsj |
| } |
| |
| if !noptrsi { |
| // If both have pointers... |
| |
| // ... then place objects with less trailing |
| // non-pointer bytes earlier. That is, place |
| // the field with the most trailing |
| // non-pointer bytes at the end of the |
| // pointerful section. |
| traili := ei.sizeof - ei.ptrdata |
| trailj := ej.sizeof - ej.ptrdata |
| if traili != trailj { |
| return traili < trailj |
| } |
| } |
| |
| // Lastly, order by size. |
| if ei.sizeof != ej.sizeof { |
| return ei.sizeof > ej.sizeof |
| } |
| |
| return false |
| }) |
| |
| fields := make([]*types.Var, nf) |
| indexes := make([]int, nf) |
| for i, e := range elems { |
| fields[i] = str.Field(e.index) |
| indexes[i] = e.index |
| } |
| return types.NewStruct(fields, nil), indexes |
| } |
| |
| // Code below based on go/types.StdSizes. |
| |
| type gcSizes struct { |
| WordSize int64 |
| MaxAlign int64 |
| } |
| |
| func (s *gcSizes) Alignof(T types.Type) int64 { |
| // For arrays and structs, alignment is defined in terms |
| // of alignment of the elements and fields, respectively. |
| switch t := T.Underlying().(type) { |
| case *types.Array: |
| // spec: "For a variable x of array type: unsafe.Alignof(x) |
| // is the same as unsafe.Alignof(x[0]), but at least 1." |
| return s.Alignof(t.Elem()) |
| case *types.Struct: |
| // spec: "For a variable x of struct type: unsafe.Alignof(x) |
| // is the largest of the values unsafe.Alignof(x.f) for each |
| // field f of x, but at least 1." |
| max := int64(1) |
| for i, nf := 0, t.NumFields(); i < nf; i++ { |
| if a := s.Alignof(t.Field(i).Type()); a > max { |
| max = a |
| } |
| } |
| return max |
| } |
| a := s.Sizeof(T) // may be 0 |
| // spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1." |
| if a < 1 { |
| return 1 |
| } |
| if a > s.MaxAlign { |
| return s.MaxAlign |
| } |
| return a |
| } |
| |
| var basicSizes = [...]byte{ |
| types.Bool: 1, |
| types.Int8: 1, |
| types.Int16: 2, |
| types.Int32: 4, |
| types.Int64: 8, |
| types.Uint8: 1, |
| types.Uint16: 2, |
| types.Uint32: 4, |
| types.Uint64: 8, |
| types.Float32: 4, |
| types.Float64: 8, |
| types.Complex64: 8, |
| types.Complex128: 16, |
| } |
| |
| func (s *gcSizes) Sizeof(T types.Type) int64 { |
| switch t := T.Underlying().(type) { |
| case *types.Basic: |
| k := t.Kind() |
| if int(k) < len(basicSizes) { |
| if s := basicSizes[k]; s > 0 { |
| return int64(s) |
| } |
| } |
| if k == types.String { |
| return s.WordSize * 2 |
| } |
| case *types.Array: |
| return t.Len() * s.Sizeof(t.Elem()) |
| case *types.Slice: |
| return s.WordSize * 3 |
| case *types.Struct: |
| nf := t.NumFields() |
| if nf == 0 { |
| return 0 |
| } |
| |
| var o int64 |
| max := int64(1) |
| for i := 0; i < nf; i++ { |
| ft := t.Field(i).Type() |
| a, sz := s.Alignof(ft), s.Sizeof(ft) |
| if a > max { |
| max = a |
| } |
| if i == nf-1 && sz == 0 && o != 0 { |
| sz = 1 |
| } |
| o = align(o, a) + sz |
| } |
| return align(o, max) |
| case *types.Interface: |
| return s.WordSize * 2 |
| } |
| return s.WordSize // catch-all |
| } |
| |
| // align returns the smallest y >= x such that y % a == 0. |
| func align(x, a int64) int64 { |
| y := x + a - 1 |
| return y - y%a |
| } |
| |
| func (s *gcSizes) ptrdata(T types.Type) int64 { |
| switch t := T.Underlying().(type) { |
| case *types.Basic: |
| switch t.Kind() { |
| case types.String, types.UnsafePointer: |
| return s.WordSize |
| } |
| return 0 |
| case *types.Chan, *types.Map, *types.Pointer, *types.Signature, *types.Slice: |
| return s.WordSize |
| case *types.Interface: |
| return 2 * s.WordSize |
| case *types.Array: |
| n := t.Len() |
| if n == 0 { |
| return 0 |
| } |
| a := s.ptrdata(t.Elem()) |
| if a == 0 { |
| return 0 |
| } |
| z := s.Sizeof(t.Elem()) |
| return (n-1)*z + a |
| case *types.Struct: |
| nf := t.NumFields() |
| if nf == 0 { |
| return 0 |
| } |
| |
| var o, p int64 |
| for i := 0; i < nf; i++ { |
| ft := t.Field(i).Type() |
| a, sz := s.Alignof(ft), s.Sizeof(ft) |
| fp := s.ptrdata(ft) |
| o = align(o, a) |
| if fp != 0 { |
| p = o + fp |
| } |
| o += sz |
| } |
| return p |
| } |
| |
| panic("impossible") |
| } |