234 lines
6.8 KiB
Go
234 lines
6.8 KiB
Go
// 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 ssautil
|
|
|
|
// 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/ssa"
|
|
)
|
|
|
|
// A ConstCase represents a single constant comparison.
|
|
// It is part of a Switch.
|
|
type ConstCase struct {
|
|
Block *ssa.BasicBlock // block performing the comparison
|
|
Body *ssa.BasicBlock // body of the case
|
|
Value *ssa.Const // case comparand
|
|
}
|
|
|
|
// A TypeCase represents a single type assertion.
|
|
// It is part of a Switch.
|
|
type TypeCase struct {
|
|
Block *ssa.BasicBlock // block performing the type assert
|
|
Body *ssa.BasicBlock // body of the case
|
|
Type types.Type // case type
|
|
Binding ssa.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 ssa.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 *ssa.BasicBlock // block containing start of if/else chain
|
|
X ssa.Value // the switch operand
|
|
ConstCases []ConstCase // ordered list of constant comparisons
|
|
TypeCases []TypeCase // ordered list of type assertions
|
|
Default *ssa.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, 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
|
|
// ssa.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 ssa.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 SSA representation.)
|
|
//
|
|
func Switches(fn *ssa.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[*ssa.BasicBlock]bool) // TODO(adonovan): opt: use ssa.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 valueSwitch(sw *Switch, k *ssa.Const, seen map[*ssa.BasicBlock]bool) {
|
|
b := sw.Start
|
|
x := sw.X
|
|
for x == sw.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]
|
|
if len(b.Instrs) > 2 {
|
|
// Block b contains not just 'if x == k',
|
|
// 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 ssa.Value, T types.Type, seen map[*ssa.BasicBlock]bool) {
|
|
b := sw.Start
|
|
x := sw.X
|
|
for x == sw.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]
|
|
if len(b.Instrs) > 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 *ssa.BasicBlock) (v ssa.Value, k *ssa.Const) {
|
|
if n := len(b.Instrs); n >= 2 {
|
|
if i, ok := b.Instrs[n-1].(*ssa.If); ok {
|
|
if binop, ok := i.Cond.(*ssa.BinOp); ok && binop.Block() == b && binop.Op == token.EQL {
|
|
if k, ok := binop.Y.(*ssa.Const); ok {
|
|
return binop.X, k
|
|
}
|
|
if k, ok := binop.X.(*ssa.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 *ssa.BasicBlock) (y, x ssa.Value, T types.Type) {
|
|
if n := len(b.Instrs); n >= 4 {
|
|
if i, ok := b.Instrs[n-1].(*ssa.If); ok {
|
|
if ext1, ok := i.Cond.(*ssa.Extract); ok && ext1.Block() == b && ext1.Index == 1 {
|
|
if ta, ok := ext1.Tuple.(*ssa.TypeAssert); ok && ta.Block() == b {
|
|
// hack: relies upon instruction ordering.
|
|
if ext0, ok := b.Instrs[n-3].(*ssa.Extract); ok {
|
|
return ext0, ta.X, ta.AssertedType
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
return
|
|
}
|