go/src/cmd/compile/internal/ir/copy.go

// 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 ir

import (
	"cmd/compile/internal/base"
	"cmd/internal/src"
)

// A Node may implement the Orig and SetOrig method to
// maintain a pointer to the "unrewritten" form of a Node.
// If a Node does not implement OrigNode, it is its own Orig.
//
// Note that both SepCopy and Copy have definitions compatible
// with a Node that does not implement OrigNode: such a Node
// is its own Orig, and in that case, that's what both want to return
// anyway (SepCopy unconditionally, and Copy only when the input
// is its own Orig as well, but if the output does not implement
// OrigNode, then neither does the input, making the condition true).
type OrigNode interface {
	Node
	Orig() Node
	SetOrig(Node)
}

// Orig returns the “original” node for n.
// If n implements OrigNode, Orig returns n.Orig().
// Otherwise Orig returns n itself.
func Orig(n Node) Node {
	if n, ok := n.(OrigNode); ok {
		o := n.Orig()
		if o == nil {
			Dump("Orig nil", n)
			base.Fatalf("Orig returned nil")
		}
		return o
	}
	return n
}

// SepCopy returns a separate shallow copy of n,
// breaking any Orig link to any other nodes.
func SepCopy(n Node) Node {
	n = n.rawCopy()
	if n, ok := n.(OrigNode); ok {
		n.SetOrig(n)
	}
	return n
}

// Copy returns a shallow copy of n.
// If Orig(n) == n, then Orig(Copy(n)) == the copy.
// Otherwise the Orig link is preserved as well.
//
// The specific semantics surrounding Orig are subtle but right for most uses.
// See issues #26855 and #27765 for pitfalls.
func Copy(n Node) Node {
	copy := n.rawCopy()
	if n, ok := n.(OrigNode); ok && n.Orig() == n {
		copy.(OrigNode).SetOrig(copy)
	}
	return copy
}

// A Node can implement DeepCopyNode to provide a custom implementation
// of DeepCopy. If the compiler only needs access to a Node's structure during
// DeepCopy, then a Node can implement DeepCopyNode instead of providing
// fine-grained mutable access with Left, SetLeft, Right, SetRight, and so on.
type DeepCopyNode interface {
	Node
	DeepCopy(pos src.XPos) Node
}

// DeepCopy returns a “deep” copy of n, with its entire structure copied
// (except for shared nodes like ONAME, ONONAME, OLITERAL, and OTYPE).
// If pos.IsKnown(), it sets the source position of newly allocated Nodes to pos.
//
// The default implementation is to traverse the Node graph, making
// a shallow copy of each node and then updating each field to point
// at shallow copies of children, recursively, using Left, SetLeft, and so on.
//
// If a Node wishes to provide an alternate implementation, it can
// implement a DeepCopy method: see the DeepCopyNode interface.
func DeepCopy(pos src.XPos, n Node) Node {
	if n == nil {
		return nil
	}

	if n, ok := n.(DeepCopyNode); ok {
		return n.DeepCopy(pos)
	}

	switch n.Op() {
	default:
		m := SepCopy(n)
		m.SetLeft(DeepCopy(pos, n.Left()))
		m.SetRight(DeepCopy(pos, n.Right()))
		m.PtrList().Set(deepCopyList(pos, n.List().Slice()))
		if pos.IsKnown() {
			m.SetPos(pos)
		}
		if m.Name() != nil {
			Dump("DeepCopy", n)
			base.Fatalf("DeepCopy Name")
		}
		return m

	case OPACK:
		// OPACK nodes are never valid in const value declarations,
		// but allow them like any other declared symbol to avoid
		// crashing (golang.org/issue/11361).
		fallthrough

	case ONAME, ONONAME, OLITERAL, ONIL, OTYPE:
		return n
	}
}

func deepCopyList(pos src.XPos, list []Node) []Node {
	var out []Node
	for _, n := range list {
		out = append(out, DeepCopy(pos, n))
	}
	return out
}