+++ /dev/null
-// Copyright 2013 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 irutil
-
-// This file implements discovery of switch and type-switch constructs
-// from low-level control flow.
-//
-// Many techniques exist for compiling a high-level switch with
-// constant cases to efficient machine code. The optimal choice will
-// depend on the data type, the specific case values, the code in the
-// body of each case, and the hardware.
-// Some examples:
-// - a lookup table (for a switch that maps constants to constants)
-// - a computed goto
-// - a binary tree
-// - a perfect hash
-// - a two-level switch (to partition constant strings by their first byte).
-
-import (
- "bytes"
- "fmt"
- "go/token"
- "go/types"
-
- "honnef.co/go/tools/ir"
-)
-
-// A ConstCase represents a single constant comparison.
-// It is part of a Switch.
-type ConstCase struct {
- Block *ir.BasicBlock // block performing the comparison
- Body *ir.BasicBlock // body of the case
- Value *ir.Const // case comparand
-}
-
-// A TypeCase represents a single type assertion.
-// It is part of a Switch.
-type TypeCase struct {
- Block *ir.BasicBlock // block performing the type assert
- Body *ir.BasicBlock // body of the case
- Type types.Type // case type
- Binding ir.Value // value bound by this case
-}
-
-// A Switch is a logical high-level control flow operation
-// (a multiway branch) discovered by analysis of a CFG containing
-// only if/else chains. It is not part of the ir.Instruction set.
-//
-// One of ConstCases and TypeCases has length >= 2;
-// the other is nil.
-//
-// In a value switch, the list of cases may contain duplicate constants.
-// A type switch may contain duplicate types, or types assignable
-// to an interface type also in the list.
-// TODO(adonovan): eliminate such duplicates.
-//
-type Switch struct {
- Start *ir.BasicBlock // block containing start of if/else chain
- X ir.Value // the switch operand
- ConstCases []ConstCase // ordered list of constant comparisons
- TypeCases []TypeCase // ordered list of type assertions
- Default *ir.BasicBlock // successor if all comparisons fail
-}
-
-func (sw *Switch) String() string {
- // We represent each block by the String() of its
- // first Instruction, e.g. "print(42:int)".
- var buf bytes.Buffer
- if sw.ConstCases != nil {
- fmt.Fprintf(&buf, "switch %s {\n", sw.X.Name())
- for _, c := range sw.ConstCases {
- fmt.Fprintf(&buf, "case %s: %s\n", c.Value.Name(), c.Body.Instrs[0])
- }
- } else {
- fmt.Fprintf(&buf, "switch %s.(type) {\n", sw.X.Name())
- for _, c := range sw.TypeCases {
- fmt.Fprintf(&buf, "case %s %s: %s\n",
- c.Binding.Name(), c.Type, c.Body.Instrs[0])
- }
- }
- if sw.Default != nil {
- fmt.Fprintf(&buf, "default: %s\n", sw.Default.Instrs[0])
- }
- fmt.Fprintf(&buf, "}")
- return buf.String()
-}
-
-// Switches examines the control-flow graph of fn and returns the
-// set of inferred value and type switches. A value switch tests an
-// ir.Value for equality against two or more compile-time constant
-// values. Switches involving link-time constants (addresses) are
-// ignored. A type switch type-asserts an ir.Value against two or
-// more types.
-//
-// The switches are returned in dominance order.
-//
-// The resulting switches do not necessarily correspond to uses of the
-// 'switch' keyword in the source: for example, a single source-level
-// switch statement with non-constant cases may result in zero, one or
-// many Switches, one per plural sequence of constant cases.
-// Switches may even be inferred from if/else- or goto-based control flow.
-// (In general, the control flow constructs of the source program
-// cannot be faithfully reproduced from the IR.)
-//
-func Switches(fn *ir.Function) []Switch {
- // Traverse the CFG in dominance order, so we don't
- // enter an if/else-chain in the middle.
- var switches []Switch
- seen := make(map[*ir.BasicBlock]bool) // TODO(adonovan): opt: use ir.blockSet
- for _, b := range fn.DomPreorder() {
- if x, k := isComparisonBlock(b); x != nil {
- // Block b starts a switch.
- sw := Switch{Start: b, X: x}
- valueSwitch(&sw, k, seen)
- if len(sw.ConstCases) > 1 {
- switches = append(switches, sw)
- }
- }
-
- if y, x, T := isTypeAssertBlock(b); y != nil {
- // Block b starts a type switch.
- sw := Switch{Start: b, X: x}
- typeSwitch(&sw, y, T, seen)
- if len(sw.TypeCases) > 1 {
- switches = append(switches, sw)
- }
- }
- }
- return switches
-}
-
-func isSameX(x1 ir.Value, x2 ir.Value) bool {
- if x1 == x2 {
- return true
- }
- if x2, ok := x2.(*ir.Sigma); ok {
- return isSameX(x1, x2.X)
- }
- return false
-}
-
-func valueSwitch(sw *Switch, k *ir.Const, seen map[*ir.BasicBlock]bool) {
- b := sw.Start
- x := sw.X
- for isSameX(sw.X, x) {
- if seen[b] {
- break
- }
- seen[b] = true
-
- sw.ConstCases = append(sw.ConstCases, ConstCase{
- Block: b,
- Body: b.Succs[0],
- Value: k,
- })
- b = b.Succs[1]
- n := 0
- for _, instr := range b.Instrs {
- switch instr.(type) {
- case *ir.If, *ir.BinOp:
- n++
- case *ir.Sigma, *ir.Phi, *ir.DebugRef:
- default:
- n += 1000
- }
- }
- if n != 2 {
- // Block b contains not just 'if x == k' and σ/ϕ nodes,
- // so it may have side effects that
- // make it unsafe to elide.
- break
- }
- if len(b.Preds) != 1 {
- // Block b has multiple predecessors,
- // so it cannot be treated as a case.
- break
- }
- x, k = isComparisonBlock(b)
- }
- sw.Default = b
-}
-
-func typeSwitch(sw *Switch, y ir.Value, T types.Type, seen map[*ir.BasicBlock]bool) {
- b := sw.Start
- x := sw.X
- for isSameX(sw.X, x) {
- if seen[b] {
- break
- }
- seen[b] = true
-
- sw.TypeCases = append(sw.TypeCases, TypeCase{
- Block: b,
- Body: b.Succs[0],
- Type: T,
- Binding: y,
- })
- b = b.Succs[1]
- n := 0
- for _, instr := range b.Instrs {
- switch instr.(type) {
- case *ir.TypeAssert, *ir.Extract, *ir.If:
- n++
- case *ir.Sigma, *ir.Phi:
- default:
- n += 1000
- }
- }
- if n != 4 {
- // Block b contains not just
- // {TypeAssert; Extract #0; Extract #1; If}
- // so it may have side effects that
- // make it unsafe to elide.
- break
- }
- if len(b.Preds) != 1 {
- // Block b has multiple predecessors,
- // so it cannot be treated as a case.
- break
- }
- y, x, T = isTypeAssertBlock(b)
- }
- sw.Default = b
-}
-
-// isComparisonBlock returns the operands (v, k) if a block ends with
-// a comparison v==k, where k is a compile-time constant.
-//
-func isComparisonBlock(b *ir.BasicBlock) (v ir.Value, k *ir.Const) {
- if n := len(b.Instrs); n >= 2 {
- if i, ok := b.Instrs[n-1].(*ir.If); ok {
- if binop, ok := i.Cond.(*ir.BinOp); ok && binop.Block() == b && binop.Op == token.EQL {
- if k, ok := binop.Y.(*ir.Const); ok {
- return binop.X, k
- }
- if k, ok := binop.X.(*ir.Const); ok {
- return binop.Y, k
- }
- }
- }
- }
- return
-}
-
-// isTypeAssertBlock returns the operands (y, x, T) if a block ends with
-// a type assertion "if y, ok := x.(T); ok {".
-//
-func isTypeAssertBlock(b *ir.BasicBlock) (y, x ir.Value, T types.Type) {
- if n := len(b.Instrs); n >= 4 {
- if i, ok := b.Instrs[n-1].(*ir.If); ok {
- if ext1, ok := i.Cond.(*ir.Extract); ok && ext1.Block() == b && ext1.Index == 1 {
- if ta, ok := ext1.Tuple.(*ir.TypeAssert); ok && ta.Block() == b {
- // hack: relies upon instruction ordering.
- if ext0, ok := b.Instrs[n-3].(*ir.Extract); ok {
- return ext0, ta.X, ta.AssertedType
- }
- }
- }
- }
- }
- return
-}
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