src/cmd/compile/internal/ssa/block.go - go - Git at Google

 // Copyright 2015 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 ssa

 import (
 	"cmd/internal/src"
 	"fmt"
 )

 // Block represents a basic block in the control flow graph of a function.
 type Block struct {
 	// A unique identifier for the block. The system will attempt to allocate
 	// these IDs densely, but no guarantees.
 	ID ID

 	// Source position for block's control operation
 	Pos src.XPos

 	// The kind of block this is.
 	Kind BlockKind

 	// Likely direction for branches.
 	// If BranchLikely, Succs[0] is the most likely branch taken.
 	// If BranchUnlikely, Succs[1] is the most likely branch taken.
 	// Ignored if len(Succs) < 2.
 	// Fatal if not BranchUnknown and len(Succs) > 2.
 	Likely BranchPrediction

 	// After flagalloc, records whether flags are live at the end of the block.
 	FlagsLiveAtEnd bool

 	// Subsequent blocks, if any. The number and order depend on the block kind.
 	Succs []Edge

 	// Inverse of successors.
 	// The order is significant to Phi nodes in the block.
 	// TODO: predecessors is a pain to maintain. Can we somehow order phi
 	// arguments by block id and have this field computed explicitly when needed?
 	Preds []Edge

 	// A list of values that determine how the block is exited. The number
 	// and type of control values depends on the Kind of the block. For
 	// instance, a BlockIf has a single boolean control value and BlockExit
 	// has a single memory control value.
 	//
 	// The ControlValues() method may be used to get a slice with the non-nil
 	// control values that can be ranged over.
 	//
 	// Controls[1] must be nil if Controls[0] is nil.
 	Controls [2]*Value

 	// Auxiliary info for the block. Its value depends on the Kind.
 	Aux    interface{}
 	AuxInt int64

 	// The unordered set of Values that define the operation of this block.
 	// After the scheduling pass, this list is ordered.
 	Values []*Value

 	// The containing function
 	Func *Func

 	// Storage for Succs, Preds and Values.
 	succstorage [2]Edge
 	predstorage [4]Edge
 	valstorage  [9]*Value
 }

 // Edge represents a CFG edge.
 // Example edges for b branching to either c or d.
 // (c and d have other predecessors.)
 //   b.Succs = [{c,3}, {d,1}]
 //   c.Preds = [?, ?, ?, {b,0}]
 //   d.Preds = [?, {b,1}, ?]
 // These indexes allow us to edit the CFG in constant time.
 // In addition, it informs phi ops in degenerate cases like:
 // b:
 //    if k then c else c
 // c:
 //    v = Phi(x, y)
 // Then the indexes tell you whether x is chosen from
 // the if or else branch from b.
 //   b.Succs = [{c,0},{c,1}]
 //   c.Preds = [{b,0},{b,1}]
 // means x is chosen if k is true.
 type Edge struct {
 	// block edge goes to (in a Succs list) or from (in a Preds list)
 	b *Block
 	// index of reverse edge.  Invariant:
 	//   e := x.Succs[idx]
 	//   e.b.Preds[e.i] = Edge{x,idx}
 	// and similarly for predecessors.
 	i int
 }

 func (e Edge) Block() *Block {
 	return e.b
 }
 func (e Edge) Index() int {
 	return e.i
 }

 //     kind          controls        successors
 //   ------------------------------------------
 //     Exit      [return mem]                []
 //    Plain                []            [next]
 //       If   [boolean Value]      [then, else]
 //    Defer             [mem]  [nopanic, panic]  (control opcode should be OpStaticCall to runtime.deferproc)
 type BlockKind int8

 // short form print
 func (b *Block) String() string {
 	return fmt.Sprintf("b%d", b.ID)
 }

 // long form print
 func (b *Block) LongString() string {
 	s := b.Kind.String()
 	if b.Aux != nil {
 		s += fmt.Sprintf(" {%s}", b.Aux)
 	}
 	if t := b.Kind.AuxIntType(); t != "" {
 		switch t {
 		case "Int8":
 			s += fmt.Sprintf(" [%v]", int8(b.AuxInt))
 		case "UInt8":
 			s += fmt.Sprintf(" [%v]", uint8(b.AuxInt))
 		default:
 			s += fmt.Sprintf(" [%v]", b.AuxInt)
 		}
 	}
 	for _, c := range b.ControlValues() {
 		s += fmt.Sprintf(" %s", c)
 	}
 	if len(b.Succs) > 0 {
 		s += " ->"
 		for _, c := range b.Succs {
 			s += " " + c.b.String()
 		}
 	}
 	switch b.Likely {
 	case BranchUnlikely:
 		s += " (unlikely)"
 	case BranchLikely:
 		s += " (likely)"
 	}
 	return s
 }

 // NumControls returns the number of non-nil control values the
 // block has.
 func (b *Block) NumControls() int {
 	if b.Controls[0] == nil {
 		return 0
 	}
 	if b.Controls[1] == nil {
 		return 1
 	}
 	return 2
 }

 // ControlValues returns a slice containing the non-nil control
 // values of the block. The index of each control value will be
 // the same as it is in the Controls property and can be used
 // in ReplaceControl calls.
 func (b *Block) ControlValues() []*Value {
 	if b.Controls[0] == nil {
 		return b.Controls[:0]
 	}
 	if b.Controls[1] == nil {
 		return b.Controls[:1]
 	}
 	return b.Controls[:2]
 }

 // SetControl removes all existing control values and then adds
 // the control value provided. The number of control values after
 // a call to SetControl will always be 1.
 func (b *Block) SetControl(v *Value) {
 	b.ResetControls()
 	b.Controls[0] = v
 	v.Uses++
 }

 // ResetControls sets the number of controls for the block to 0.
 func (b *Block) ResetControls() {
 	if b.Controls[0] != nil {
 		b.Controls[0].Uses--
 	}
 	if b.Controls[1] != nil {
 		b.Controls[1].Uses--
 	}
 	b.Controls = [2]*Value{} // reset both controls to nil
 }

 // AddControl appends a control value to the existing list of control values.
 func (b *Block) AddControl(v *Value) {
 	i := b.NumControls()
 	b.Controls[i] = v // panics if array is full
 	v.Uses++
 }

 // ReplaceControl exchanges the existing control value at the index provided
 // for the new value. The index must refer to a valid control value.
 func (b *Block) ReplaceControl(i int, v *Value) {
 	b.Controls[i].Uses--
 	b.Controls[i] = v
 	v.Uses++
 }

 // CopyControls replaces the controls for this block with those from the
 // provided block. The provided block is not modified.
 func (b *Block) CopyControls(from *Block) {
 	if b == from {
 		return
 	}
 	b.ResetControls()
 	for _, c := range from.ControlValues() {
 		b.AddControl(c)
 	}
 }

 // Reset sets the block to the provided kind and clears all the blocks control
 // and auxilliary values. Other properties of the block, such as its successors,
 // predecessors and values are left unmodified.
 func (b *Block) Reset(kind BlockKind) {
 	b.Kind = kind
 	b.ResetControls()
 	b.Aux = nil
 	b.AuxInt = 0
 }

 // AddEdgeTo adds an edge from block b to block c. Used during building of the
 // SSA graph; do not use on an already-completed SSA graph.
 func (b *Block) AddEdgeTo(c *Block) {
 	i := len(b.Succs)
 	j := len(c.Preds)
 	b.Succs = append(b.Succs, Edge{c, j})
 	c.Preds = append(c.Preds, Edge{b, i})
 	b.Func.invalidateCFG()
 }

 // removePred removes the ith input edge from b.
 // It is the responsibility of the caller to remove
 // the corresponding successor edge.
 func (b *Block) removePred(i int) {
 	n := len(b.Preds) - 1
 	if i != n {
 		e := b.Preds[n]
 		b.Preds[i] = e
 		// Update the other end of the edge we moved.
 		e.b.Succs[e.i].i = i
 	}
 	b.Preds[n] = Edge{}
 	b.Preds = b.Preds[:n]
 	b.Func.invalidateCFG()
 }

 // removeSucc removes the ith output edge from b.
 // It is the responsibility of the caller to remove
 // the corresponding predecessor edge.
 func (b *Block) removeSucc(i int) {
 	n := len(b.Succs) - 1
 	if i != n {
 		e := b.Succs[n]
 		b.Succs[i] = e
 		// Update the other end of the edge we moved.
 		e.b.Preds[e.i].i = i
 	}
 	b.Succs[n] = Edge{}
 	b.Succs = b.Succs[:n]
 	b.Func.invalidateCFG()
 }

 func (b *Block) swapSuccessors() {
 	if len(b.Succs) != 2 {
 		b.Fatalf("swapSuccessors with len(Succs)=%d", len(b.Succs))
 	}
 	e0 := b.Succs[0]
 	e1 := b.Succs[1]
 	b.Succs[0] = e1
 	b.Succs[1] = e0
 	e0.b.Preds[e0.i].i = 1
 	e1.b.Preds[e1.i].i = 0
 	b.Likely *= -1
 }

 // LackingPos indicates whether b is a block whose position should be inherited
 // from its successors.  This is true if all the values within it have unreliable positions
 // and if it is "plain", meaning that there is no control flow that is also very likely
 // to correspond to a well-understood source position.
 func (b *Block) LackingPos() bool {
 	// Non-plain predecessors are If or Defer, which both (1) have two successors,
 	// which might have different line numbers and (2) correspond to statements
 	// in the source code that have positions, so this case ought not occur anyway.
 	if b.Kind != BlockPlain {
 		return false
 	}
 	if b.Pos != src.NoXPos {
 		return false
 	}
 	for _, v := range b.Values {
 		if v.LackingPos() {
 			continue
 		}
 		return false
 	}
 	return true
 }

 func (b *Block) Logf(msg string, args ...interface{})   { b.Func.Logf(msg, args...) }
 func (b *Block) Log() bool                              { return b.Func.Log() }
 func (b *Block) Fatalf(msg string, args ...interface{}) { b.Func.Fatalf(msg, args...) }

 type BranchPrediction int8

 const (
 	BranchUnlikely = BranchPrediction(-1)
 	BranchUnknown  = BranchPrediction(0)
 	BranchLikely   = BranchPrediction(+1)
 )
	// Copyright 2015 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 ssa

	import (
	"cmd/internal/src"
	"fmt"
	)

	// Block represents a basic block in the control flow graph of a function.
	type Block struct {
	// A unique identifier for the block. The system will attempt to allocate
	// these IDs densely, but no guarantees.
	ID ID

	// Source position for block's control operation
	Pos src.XPos

	// The kind of block this is.
	Kind BlockKind

	// Likely direction for branches.
	// If BranchLikely, Succs[0] is the most likely branch taken.
	// If BranchUnlikely, Succs[1] is the most likely branch taken.
	// Ignored if len(Succs) < 2.
	// Fatal if not BranchUnknown and len(Succs) > 2.
	Likely BranchPrediction

	// After flagalloc, records whether flags are live at the end of the block.
	FlagsLiveAtEnd bool

	// Subsequent blocks, if any. The number and order depend on the block kind.
	Succs []Edge

	// Inverse of successors.
	// The order is significant to Phi nodes in the block.
	// TODO: predecessors is a pain to maintain. Can we somehow order phi
	// arguments by block id and have this field computed explicitly when needed?
	Preds []Edge

	// A list of values that determine how the block is exited. The number
	// and type of control values depends on the Kind of the block. For
	// instance, a BlockIf has a single boolean control value and BlockExit
	// has a single memory control value.
	//
	// The ControlValues() method may be used to get a slice with the non-nil
	// control values that can be ranged over.
	//
	// Controls[1] must be nil if Controls[0] is nil.
	Controls [2]*Value

	// Auxiliary info for the block. Its value depends on the Kind.
	Aux interface{}
	AuxInt int64

	// The unordered set of Values that define the operation of this block.
	// After the scheduling pass, this list is ordered.
	Values []*Value

	// The containing function
	Func *Func

	// Storage for Succs, Preds and Values.
	succstorage [2]Edge
	predstorage [4]Edge
	valstorage [9]*Value
	}

	// Edge represents a CFG edge.
	// Example edges for b branching to either c or d.
	// (c and d have other predecessors.)
	// b.Succs = [{c,3}, {d,1}]
	// c.Preds = [?, ?, ?, {b,0}]
	// d.Preds = [?, {b,1}, ?]
	// These indexes allow us to edit the CFG in constant time.
	// In addition, it informs phi ops in degenerate cases like:
	// b:
	// if k then c else c
	// c:
	// v = Phi(x, y)
	// Then the indexes tell you whether x is chosen from
	// the if or else branch from b.
	// b.Succs = [{c,0},{c,1}]
	// c.Preds = [{b,0},{b,1}]
	// means x is chosen if k is true.
	type Edge struct {
	// block edge goes to (in a Succs list) or from (in a Preds list)
	b *Block
	// index of reverse edge. Invariant:
	// e := x.Succs[idx]
	// e.b.Preds[e.i] = Edge{x,idx}
	// and similarly for predecessors.
	i int
	}

	func (e Edge) Block() *Block {
	return e.b
	}
	func (e Edge) Index() int {
	return e.i
	}

	// kind controls successors
	// ------------------------------------------
	// Exit [return mem] []
	// Plain [] [next]
	// If [boolean Value] [then, else]
	// Defer [mem] [nopanic, panic] (control opcode should be OpStaticCall to runtime.deferproc)
	type BlockKind int8

	// short form print
	func (b *Block) String() string {
	return fmt.Sprintf("b%d", b.ID)
	}

	// long form print
	func (b *Block) LongString() string {
	s := b.Kind.String()
	if b.Aux != nil {
	s += fmt.Sprintf(" {%s}", b.Aux)
	}
	if t := b.Kind.AuxIntType(); t != "" {
	switch t {
	case "Int8":
	s += fmt.Sprintf(" [%v]", int8(b.AuxInt))
	case "UInt8":
	s += fmt.Sprintf(" [%v]", uint8(b.AuxInt))
	default:
	s += fmt.Sprintf(" [%v]", b.AuxInt)
	}
	}
	for _, c := range b.ControlValues() {
	s += fmt.Sprintf(" %s", c)
	}
	if len(b.Succs) > 0 {
	s += " ->"
	for _, c := range b.Succs {
	s += " " + c.b.String()
	}
	}
	switch b.Likely {
	case BranchUnlikely:
	s += " (unlikely)"
	case BranchLikely:
	s += " (likely)"
	}
	return s
	}

	// NumControls returns the number of non-nil control values the
	// block has.
	func (b *Block) NumControls() int {
	if b.Controls[0] == nil {
	return 0
	}
	if b.Controls[1] == nil {
	return 1
	}
	return 2
	}

	// ControlValues returns a slice containing the non-nil control
	// values of the block. The index of each control value will be
	// the same as it is in the Controls property and can be used
	// in ReplaceControl calls.
	func (b Block) ControlValues() []Value {
	if b.Controls[0] == nil {
	return b.Controls[:0]
	}
	if b.Controls[1] == nil {
	return b.Controls[:1]
	}
	return b.Controls[:2]
	}

	// SetControl removes all existing control values and then adds
	// the control value provided. The number of control values after
	// a call to SetControl will always be 1.
	func (b Block) SetControl(v Value) {
	b.ResetControls()
	b.Controls[0] = v
	v.Uses++
	}

	// ResetControls sets the number of controls for the block to 0.
	func (b *Block) ResetControls() {
	if b.Controls[0] != nil {
	b.Controls[0].Uses--
	}
	if b.Controls[1] != nil {
	b.Controls[1].Uses--
	}
	b.Controls = [2]*Value{} // reset both controls to nil
	}

	// AddControl appends a control value to the existing list of control values.
	func (b Block) AddControl(v Value) {
	i := b.NumControls()
	b.Controls[i] = v // panics if array is full
	v.Uses++
	}

	// ReplaceControl exchanges the existing control value at the index provided
	// for the new value. The index must refer to a valid control value.
	func (b Block) ReplaceControl(i int, v Value) {
	b.Controls[i].Uses--
	b.Controls[i] = v
	v.Uses++
	}

	// CopyControls replaces the controls for this block with those from the
	// provided block. The provided block is not modified.
	func (b Block) CopyControls(from Block) {
	if b == from {
	return
	}
	b.ResetControls()
	for _, c := range from.ControlValues() {
	b.AddControl(c)
	}
	}

	// Reset sets the block to the provided kind and clears all the blocks control
	// and auxilliary values. Other properties of the block, such as its successors,
	// predecessors and values are left unmodified.
	func (b *Block) Reset(kind BlockKind) {
	b.Kind = kind
	b.ResetControls()
	b.Aux = nil
	b.AuxInt = 0
	}

	// AddEdgeTo adds an edge from block b to block c. Used during building of the
	// SSA graph; do not use on an already-completed SSA graph.
	func (b Block) AddEdgeTo(c Block) {
	i := len(b.Succs)
	j := len(c.Preds)
	b.Succs = append(b.Succs, Edge{c, j})
	c.Preds = append(c.Preds, Edge{b, i})
	b.Func.invalidateCFG()
	}

	// removePred removes the ith input edge from b.
	// It is the responsibility of the caller to remove
	// the corresponding successor edge.
	func (b *Block) removePred(i int) {
	n := len(b.Preds) - 1
	if i != n {
	e := b.Preds[n]
	b.Preds[i] = e
	// Update the other end of the edge we moved.
	e.b.Succs[e.i].i = i
	}
	b.Preds[n] = Edge{}
	b.Preds = b.Preds[:n]
	b.Func.invalidateCFG()
	}

	// removeSucc removes the ith output edge from b.
	// It is the responsibility of the caller to remove
	// the corresponding predecessor edge.
	func (b *Block) removeSucc(i int) {
	n := len(b.Succs) - 1
	if i != n {
	e := b.Succs[n]
	b.Succs[i] = e
	// Update the other end of the edge we moved.
	e.b.Preds[e.i].i = i
	}
	b.Succs[n] = Edge{}
	b.Succs = b.Succs[:n]
	b.Func.invalidateCFG()
	}

	func (b *Block) swapSuccessors() {
	if len(b.Succs) != 2 {
	b.Fatalf("swapSuccessors with len(Succs)=%d", len(b.Succs))
	}
	e0 := b.Succs[0]
	e1 := b.Succs[1]
	b.Succs[0] = e1
	b.Succs[1] = e0
	e0.b.Preds[e0.i].i = 1
	e1.b.Preds[e1.i].i = 0
	b.Likely *= -1
	}

	// LackingPos indicates whether b is a block whose position should be inherited
	// from its successors. This is true if all the values within it have unreliable positions
	// and if it is "plain", meaning that there is no control flow that is also very likely
	// to correspond to a well-understood source position.
	func (b *Block) LackingPos() bool {
	// Non-plain predecessors are If or Defer, which both (1) have two successors,
	// which might have different line numbers and (2) correspond to statements
	// in the source code that have positions, so this case ought not occur anyway.
	if b.Kind != BlockPlain {
	return false
	}
	if b.Pos != src.NoXPos {
	return false
	}
	for _, v := range b.Values {
	if v.LackingPos() {
	continue
	}
	return false
	}
	return true
	}

	func (b *Block) Logf(msg string, args ...interface{}) { b.Func.Logf(msg, args...) }
	func (b *Block) Log() bool { return b.Func.Log() }
	func (b *Block) Fatalf(msg string, args ...interface{}) { b.Func.Fatalf(msg, args...) }

	type BranchPrediction int8

	const (
	BranchUnlikely = BranchPrediction(-1)
	BranchUnknown = BranchPrediction(0)
	BranchLikely = BranchPrediction(+1)
	)