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// Copyright 2016 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 cfg constructs a simple control-flow graph (CFG) of the
// statements and expressions within a single function.
//
// Use cfg.New to construct the CFG for a function body.
//
// The blocks of the CFG contain all the function's non-control
// statements. The CFG does not contain control statements such as If,
// Switch, Select, and Branch, but does contain their subexpressions.
// For example, this source code:
//
// if x := f(); x != nil {
// T()
// } else {
// F()
// }
//
// produces this CFG:
//
// 1: x := f()
// x != nil
// succs: 2, 3
// 2: T()
// succs: 4
// 3: F()
// succs: 4
// 4:
//
// The CFG does contain Return statements; even implicit returns are
// materialized (at the position of the function's closing brace).
//
// The CFG does not record conditions associated with conditional branch
// edges, nor the short-circuit semantics of the && and || operators,
// nor abnormal control flow caused by panic. If you need this
// information, use golang.org/x/tools/go/ssa instead.
//
package cfg
import (
"bytes"
"fmt"
"go/ast"
"go/format"
"go/token"
)
// A CFG represents the control-flow graph of a single function.
//
// The entry point is Blocks[0]; there may be multiple return blocks.
type CFG struct {
Blocks []*Block // block[0] is entry; order otherwise undefined
}
// A Block represents a basic block: a list of statements and
// expressions that are always evaluated sequentially.
//
// A block may have 0-2 successors: zero for a return block or a block
// that calls a function such as panic that never returns; one for a
// normal (jump) block; and two for a conditional (if) block.
type Block struct {
Nodes []ast.Node // statements, expressions, and ValueSpecs
Succs []*Block // successor nodes in the graph
Index int32 // index within CFG.Blocks
Live bool // block is reachable from entry
comment string // for debugging
succs2 [2]*Block // underlying array for Succs
}
// New returns a new control-flow graph for the specified function body,
// which must be non-nil.
//
// The CFG builder calls mayReturn to determine whether a given function
// call may return. For example, calls to panic, os.Exit, and log.Fatal
// do not return, so the builder can remove infeasible graph edges
// following such calls. The builder calls mayReturn only for a
// CallExpr beneath an ExprStmt.
func New(body *ast.BlockStmt, mayReturn func(*ast.CallExpr) bool) *CFG {
b := builder{
mayReturn: mayReturn,
cfg: new(CFG),
}
b.current = b.newBlock("entry")
b.stmt(body)
// Compute liveness (reachability from entry point), breadth-first.
q := make([]*Block, 0, len(b.cfg.Blocks))
q = append(q, b.cfg.Blocks[0]) // entry point
for len(q) > 0 {
b := q[len(q)-1]
q = q[:len(q)-1]
if !b.Live {
b.Live = true
q = append(q, b.Succs...)
}
}
// Does control fall off the end of the function's body?
// Make implicit return explicit.
if b.current != nil && b.current.Live {
b.add(&ast.ReturnStmt{
Return: body.End() - 1,
})
}
return b.cfg
}
func (b *Block) String() string {
return fmt.Sprintf("block %d (%s)", b.Index, b.comment)
}
// Return returns the return statement at the end of this block if present, nil otherwise.
func (b *Block) Return() (ret *ast.ReturnStmt) {
if len(b.Nodes) > 0 {
ret, _ = b.Nodes[len(b.Nodes)-1].(*ast.ReturnStmt)
}
return
}
// Format formats the control-flow graph for ease of debugging.
func (g *CFG) Format(fset *token.FileSet) string {
var buf bytes.Buffer
for _, b := range g.Blocks {
fmt.Fprintf(&buf, ".%d: # %s\n", b.Index, b.comment)
for _, n := range b.Nodes {
fmt.Fprintf(&buf, "\t%s\n", formatNode(fset, n))
}
if len(b.Succs) > 0 {
fmt.Fprintf(&buf, "\tsuccs:")
for _, succ := range b.Succs {
fmt.Fprintf(&buf, " %d", succ.Index)
}
buf.WriteByte('\n')
}
buf.WriteByte('\n')
}
return buf.String()
}
func formatNode(fset *token.FileSet, n ast.Node) string {
var buf bytes.Buffer
format.Node(&buf, fset, n)
// Indent secondary lines by a tab.
return string(bytes.Replace(buf.Bytes(), []byte("\n"), []byte("\n\t"), -1))
}
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