cmd/compile: move Strongly Connected Components code into new file

This logic is used by the current escape analysis pass, but otherwise
logically independent. Move (unchanged) into a separate file to make
that clearer, and to make it easier to replace esc.go later.

Updates #23109.

Change-Id: Iec8c0c47ea04c0008165791731c11d9104d5a474
Reviewed-on: https://go-review.googlesource.com/c/go/+/167715Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/esc.go

@@ -11,144 +11,6 @@ import (
 	"strings"
 )
 
-// Run analysis on minimal sets of mutually recursive functions
-// or single non-recursive functions, bottom up.
-//
-// Finding these sets is finding strongly connected components
-// by reverse topological order in the static call graph.
-// The algorithm (known as Tarjan's algorithm) for doing that is taken from
-// Sedgewick, Algorithms, Second Edition, p. 482, with two adaptations.
-//
-// First, a hidden closure function (n.Func.IsHiddenClosure()) cannot be the
-// root of a connected component. Refusing to use it as a root
-// forces it into the component of the function in which it appears.
-// This is more convenient for escape analysis.
-//
-// Second, each function becomes two virtual nodes in the graph,
-// with numbers n and n+1. We record the function's node number as n
-// but search from node n+1. If the search tells us that the component
-// number (min) is n+1, we know that this is a trivial component: one function
-// plus its closures. If the search tells us that the component number is
-// n, then there was a path from node n+1 back to node n, meaning that
-// the function set is mutually recursive. The escape analysis can be
-// more precise when analyzing a single non-recursive function than
-// when analyzing a set of mutually recursive functions.
-
-type bottomUpVisitor struct {
-	analyze  func([]*Node, bool)
-	visitgen uint32
-	nodeID   map[*Node]uint32
-	stack    []*Node
-}
-
-// visitBottomUp invokes analyze on the ODCLFUNC nodes listed in list.
-// It calls analyze with successive groups of functions, working from
-// the bottom of the call graph upward. Each time analyze is called with
-// a list of functions, every function on that list only calls other functions
-// on the list or functions that have been passed in previous invocations of
-// analyze. Closures appear in the same list as their outer functions.
-// The lists are as short as possible while preserving those requirements.
-// (In a typical program, many invocations of analyze will be passed just
-// a single function.) The boolean argument 'recursive' passed to analyze
-// specifies whether the functions on the list are mutually recursive.
-// If recursive is false, the list consists of only a single function and its closures.
-// If recursive is true, the list may still contain only a single function,
-// if that function is itself recursive.
-func visitBottomUp(list []*Node, analyze func(list []*Node, recursive bool)) {
-	var v bottomUpVisitor
-	v.analyze = analyze
-	v.nodeID = make(map[*Node]uint32)
-	for _, n := range list {
-		if n.Op == ODCLFUNC && !n.Func.IsHiddenClosure() {
-			v.visit(n)
-		}
-	}
-}
-
-func (v *bottomUpVisitor) visit(n *Node) uint32 {
-	if id := v.nodeID[n]; id > 0 {
-		// already visited
-		return id
-	}
-
-	v.visitgen++
-	id := v.visitgen
-	v.nodeID[n] = id
-	v.visitgen++
-	min := v.visitgen
-
-	v.stack = append(v.stack, n)
-	min = v.visitcodelist(n.Nbody, min)
-	if (min == id || min == id+1) && !n.Func.IsHiddenClosure() {
-		// This node is the root of a strongly connected component.
-
-		// The original min passed to visitcodelist was v.nodeID[n]+1.
-		// If visitcodelist found its way back to v.nodeID[n], then this
-		// block is a set of mutually recursive functions.
-		// Otherwise it's just a lone function that does not recurse.
-		recursive := min == id
-
-		// Remove connected component from stack.
-		// Mark walkgen so that future visits return a large number
-		// so as not to affect the caller's min.
-
-		var i int
-		for i = len(v.stack) - 1; i >= 0; i-- {
-			x := v.stack[i]
-			if x == n {
-				break
-			}
-			v.nodeID[x] = ^uint32(0)
-		}
-		v.nodeID[n] = ^uint32(0)
-		block := v.stack[i:]
-		// Run escape analysis on this set of functions.
-		v.stack = v.stack[:i]
-		v.analyze(block, recursive)
-	}
-
-	return min
-}
-
-func (v *bottomUpVisitor) visitcodelist(l Nodes, min uint32) uint32 {
-	for _, n := range l.Slice() {
-		min = v.visitcode(n, min)
-	}
-	return min
-}
-
-func (v *bottomUpVisitor) visitcode(n *Node, min uint32) uint32 {
-	if n == nil {
-		return min
-	}
-
-	min = v.visitcodelist(n.Ninit, min)
-	min = v.visitcode(n.Left, min)
-	min = v.visitcode(n.Right, min)
-	min = v.visitcodelist(n.List, min)
-	min = v.visitcodelist(n.Nbody, min)
-	min = v.visitcodelist(n.Rlist, min)
-
-	switch n.Op {
-	case OCALLFUNC, OCALLMETH:
-		fn := asNode(n.Left.Type.Nname())
-		if fn != nil && fn.Op == ONAME && fn.Class() == PFUNC && fn.Name.Defn != nil {
-			m := v.visit(fn.Name.Defn)
-			if m < min {
-				min = m
-			}
-		}
-
-	case OCLOSURE:
-		m := v.visit(n.Func.Closure)
-		if m < min {
-			min = m
-		}
-	}
-
-	return min
-}
-
 // Escape analysis.
 
 // An escape analysis pass for a set of functions. The

src/cmd/compile/internal/gc/scc.go

+// Copyright 2011 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 gc
+
+// Strongly connected components.
+//
+// Run analysis on minimal sets of mutually recursive functions
+// or single non-recursive functions, bottom up.
+//
+// Finding these sets is finding strongly connected components
+// by reverse topological order in the static call graph.
+// The algorithm (known as Tarjan's algorithm) for doing that is taken from
+// Sedgewick, Algorithms, Second Edition, p. 482, with two adaptations.
+//
+// First, a hidden closure function (n.Func.IsHiddenClosure()) cannot be the
+// root of a connected component. Refusing to use it as a root
+// forces it into the component of the function in which it appears.
+// This is more convenient for escape analysis.
+//
+// Second, each function becomes two virtual nodes in the graph,
+// with numbers n and n+1. We record the function's node number as n
+// but search from node n+1. If the search tells us that the component
+// number (min) is n+1, we know that this is a trivial component: one function
+// plus its closures. If the search tells us that the component number is
+// n, then there was a path from node n+1 back to node n, meaning that
+// the function set is mutually recursive. The escape analysis can be
+// more precise when analyzing a single non-recursive function than
+// when analyzing a set of mutually recursive functions.
+
+type bottomUpVisitor struct {
+	analyze  func([]*Node, bool)
+	visitgen uint32
+	nodeID   map[*Node]uint32
+	stack    []*Node
+}
+
+// visitBottomUp invokes analyze on the ODCLFUNC nodes listed in list.
+// It calls analyze with successive groups of functions, working from
+// the bottom of the call graph upward. Each time analyze is called with
+// a list of functions, every function on that list only calls other functions
+// on the list or functions that have been passed in previous invocations of
+// analyze. Closures appear in the same list as their outer functions.
+// The lists are as short as possible while preserving those requirements.
+// (In a typical program, many invocations of analyze will be passed just
+// a single function.) The boolean argument 'recursive' passed to analyze
+// specifies whether the functions on the list are mutually recursive.
+// If recursive is false, the list consists of only a single function and its closures.
+// If recursive is true, the list may still contain only a single function,
+// if that function is itself recursive.
+func visitBottomUp(list []*Node, analyze func(list []*Node, recursive bool)) {
+	var v bottomUpVisitor
+	v.analyze = analyze
+	v.nodeID = make(map[*Node]uint32)
+	for _, n := range list {
+		if n.Op == ODCLFUNC && !n.Func.IsHiddenClosure() {
+			v.visit(n)
+		}
+	}
+}
+
+func (v *bottomUpVisitor) visit(n *Node) uint32 {
+	if id := v.nodeID[n]; id > 0 {
+		// already visited
+		return id
+	}
+
+	v.visitgen++
+	id := v.visitgen
+	v.nodeID[n] = id
+	v.visitgen++
+	min := v.visitgen
+
+	v.stack = append(v.stack, n)
+	min = v.visitcodelist(n.Nbody, min)
+	if (min == id || min == id+1) && !n.Func.IsHiddenClosure() {
+		// This node is the root of a strongly connected component.
+
+		// The original min passed to visitcodelist was v.nodeID[n]+1.
+		// If visitcodelist found its way back to v.nodeID[n], then this
+		// block is a set of mutually recursive functions.
+		// Otherwise it's just a lone function that does not recurse.
+		recursive := min == id
+
+		// Remove connected component from stack.
+		// Mark walkgen so that future visits return a large number
+		// so as not to affect the caller's min.
+
+		var i int
+		for i = len(v.stack) - 1; i >= 0; i-- {
+			x := v.stack[i]
+			if x == n {
+				break
+			}
+			v.nodeID[x] = ^uint32(0)
+		}
+		v.nodeID[n] = ^uint32(0)
+		block := v.stack[i:]
+		// Run escape analysis on this set of functions.
+		v.stack = v.stack[:i]
+		v.analyze(block, recursive)
+	}
+
+	return min
+}
+
+func (v *bottomUpVisitor) visitcodelist(l Nodes, min uint32) uint32 {
+	for _, n := range l.Slice() {
+		min = v.visitcode(n, min)
+	}
+	return min
+}
+
+func (v *bottomUpVisitor) visitcode(n *Node, min uint32) uint32 {
+	if n == nil {
+		return min
+	}
+
+	min = v.visitcodelist(n.Ninit, min)
+	min = v.visitcode(n.Left, min)
+	min = v.visitcode(n.Right, min)
+	min = v.visitcodelist(n.List, min)
+	min = v.visitcodelist(n.Nbody, min)
+	min = v.visitcodelist(n.Rlist, min)
+
+	switch n.Op {
+	case OCALLFUNC, OCALLMETH:
+		fn := asNode(n.Left.Type.Nname())
+		if fn != nil && fn.Op == ONAME && fn.Class() == PFUNC && fn.Name.Defn != nil {
+			m := v.visit(fn.Name.Defn)
+			if m < min {
+				min = m
+			}
+		}
+
+	case OCLOSURE:
+		m := v.visit(n.Func.Closure)
+		if m < min {
+			min = m
+		}
+	}
+
+	return min
+}