Current File : //usr/local/go119/src/cmd/compile/internal/escape/graph.go
// Copyright 2018 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 escape
import (
"cmd/compile/internal/base"
"cmd/compile/internal/ir"
"cmd/compile/internal/logopt"
"cmd/compile/internal/types"
"fmt"
)
// Below we implement the methods for walking the AST and recording
// data flow edges. Note that because a sub-expression might have
// side-effects, it's important to always visit the entire AST.
//
// For example, write either:
//
// if x {
// e.discard(n.Left)
// } else {
// e.value(k, n.Left)
// }
//
// or
//
// if x {
// k = e.discardHole()
// }
// e.value(k, n.Left)
//
// Do NOT write:
//
// // BAD: possibly loses side-effects within n.Left
// if !x {
// e.value(k, n.Left)
// }
// An location represents an abstract location that stores a Go
// variable.
type location struct {
n ir.Node // represented variable or expression, if any
curfn *ir.Func // enclosing function
edges []edge // incoming edges
loopDepth int // loopDepth at declaration
// resultIndex records the tuple index (starting at 1) for
// PPARAMOUT variables within their function's result type.
// For non-PPARAMOUT variables it's 0.
resultIndex int
// derefs and walkgen are used during walkOne to track the
// minimal dereferences from the walk root.
derefs int // >= -1
walkgen uint32
// dst and dstEdgeindex track the next immediate assignment
// destination location during walkone, along with the index
// of the edge pointing back to this location.
dst *location
dstEdgeIdx int
// queued is used by walkAll to track whether this location is
// in the walk queue.
queued bool
// escapes reports whether the represented variable's address
// escapes; that is, whether the variable must be heap
// allocated.
escapes bool
// transient reports whether the represented expression's
// address does not outlive the statement; that is, whether
// its storage can be immediately reused.
transient bool
// paramEsc records the represented parameter's leak set.
paramEsc leaks
captured bool // has a closure captured this variable?
reassigned bool // has this variable been reassigned?
addrtaken bool // has this variable's address been taken?
}
// An edge represents an assignment edge between two Go variables.
type edge struct {
src *location
derefs int // >= -1
notes *note
}
func (l *location) asHole() hole {
return hole{dst: l}
}
// leak records that parameter l leaks to sink.
func (l *location) leakTo(sink *location, derefs int) {
// If sink is a result parameter that doesn't escape (#44614)
// and we can fit return bits into the escape analysis tag,
// then record as a result leak.
if !sink.escapes && sink.isName(ir.PPARAMOUT) && sink.curfn == l.curfn {
ri := sink.resultIndex - 1
if ri < numEscResults {
// Leak to result parameter.
l.paramEsc.AddResult(ri, derefs)
return
}
}
// Otherwise, record as heap leak.
l.paramEsc.AddHeap(derefs)
}
func (l *location) isName(c ir.Class) bool {
return l.n != nil && l.n.Op() == ir.ONAME && l.n.(*ir.Name).Class == c
}
// A hole represents a context for evaluation of a Go
// expression. E.g., when evaluating p in "x = **p", we'd have a hole
// with dst==x and derefs==2.
type hole struct {
dst *location
derefs int // >= -1
notes *note
// addrtaken indicates whether this context is taking the address of
// the expression, independent of whether the address will actually
// be stored into a variable.
addrtaken bool
}
type note struct {
next *note
where ir.Node
why string
}
func (k hole) note(where ir.Node, why string) hole {
if where == nil || why == "" {
base.Fatalf("note: missing where/why")
}
if base.Flag.LowerM >= 2 || logopt.Enabled() {
k.notes = ¬e{
next: k.notes,
where: where,
why: why,
}
}
return k
}
func (k hole) shift(delta int) hole {
k.derefs += delta
if k.derefs < -1 {
base.Fatalf("derefs underflow: %v", k.derefs)
}
k.addrtaken = delta < 0
return k
}
func (k hole) deref(where ir.Node, why string) hole { return k.shift(1).note(where, why) }
func (k hole) addr(where ir.Node, why string) hole { return k.shift(-1).note(where, why) }
func (k hole) dotType(t *types.Type, where ir.Node, why string) hole {
if !t.IsInterface() && !types.IsDirectIface(t) {
k = k.shift(1)
}
return k.note(where, why)
}
func (b *batch) flow(k hole, src *location) {
if k.addrtaken {
src.addrtaken = true
}
dst := k.dst
if dst == &b.blankLoc {
return
}
if dst == src && k.derefs >= 0 { // dst = dst, dst = *dst, ...
return
}
if dst.escapes && k.derefs < 0 { // dst = &src
if base.Flag.LowerM >= 2 || logopt.Enabled() {
pos := base.FmtPos(src.n.Pos())
if base.Flag.LowerM >= 2 {
fmt.Printf("%s: %v escapes to heap:\n", pos, src.n)
}
explanation := b.explainFlow(pos, dst, src, k.derefs, k.notes, []*logopt.LoggedOpt{})
if logopt.Enabled() {
var e_curfn *ir.Func // TODO(mdempsky): Fix.
logopt.LogOpt(src.n.Pos(), "escapes", "escape", ir.FuncName(e_curfn), fmt.Sprintf("%v escapes to heap", src.n), explanation)
}
}
src.escapes = true
return
}
// TODO(mdempsky): Deduplicate edges?
dst.edges = append(dst.edges, edge{src: src, derefs: k.derefs, notes: k.notes})
}
func (b *batch) heapHole() hole { return b.heapLoc.asHole() }
func (b *batch) discardHole() hole { return b.blankLoc.asHole() }
func (b *batch) oldLoc(n *ir.Name) *location {
if n.Canonical().Opt == nil {
base.Fatalf("%v has no location", n)
}
return n.Canonical().Opt.(*location)
}
func (e *escape) newLoc(n ir.Node, transient bool) *location {
if e.curfn == nil {
base.Fatalf("e.curfn isn't set")
}
if n != nil && n.Type() != nil && n.Type().NotInHeap() {
base.ErrorfAt(n.Pos(), "%v is incomplete (or unallocatable); stack allocation disallowed", n.Type())
}
if n != nil && n.Op() == ir.ONAME {
if canon := n.(*ir.Name).Canonical(); n != canon {
base.Fatalf("newLoc on non-canonical %v (canonical is %v)", n, canon)
}
}
loc := &location{
n: n,
curfn: e.curfn,
loopDepth: e.loopDepth,
transient: transient,
}
e.allLocs = append(e.allLocs, loc)
if n != nil {
if n.Op() == ir.ONAME {
n := n.(*ir.Name)
if n.Class == ir.PPARAM && n.Curfn == nil {
// ok; hidden parameter
} else if n.Curfn != e.curfn {
base.Fatalf("curfn mismatch: %v != %v for %v", n.Curfn, e.curfn, n)
}
if n.Opt != nil {
base.Fatalf("%v already has a location", n)
}
n.Opt = loc
}
}
return loc
}
// teeHole returns a new hole that flows into each hole of ks,
// similar to the Unix tee(1) command.
func (e *escape) teeHole(ks ...hole) hole {
if len(ks) == 0 {
return e.discardHole()
}
if len(ks) == 1 {
return ks[0]
}
// TODO(mdempsky): Optimize if there's only one non-discard hole?
// Given holes "l1 = _", "l2 = **_", "l3 = *_", ..., create a
// new temporary location ltmp, wire it into place, and return
// a hole for "ltmp = _".
loc := e.newLoc(nil, true)
for _, k := range ks {
// N.B., "p = &q" and "p = &tmp; tmp = q" are not
// semantically equivalent. To combine holes like "l1
// = _" and "l2 = &_", we'd need to wire them as "l1 =
// *ltmp" and "l2 = ltmp" and return "ltmp = &_"
// instead.
if k.derefs < 0 {
base.Fatalf("teeHole: negative derefs")
}
e.flow(k, loc)
}
return loc.asHole()
}
// later returns a new hole that flows into k, but some time later.
// Its main effect is to prevent immediate reuse of temporary
// variables introduced during Order.
func (e *escape) later(k hole) hole {
loc := e.newLoc(nil, false)
e.flow(k, loc)
return loc.asHole()
}
// Fmt is called from node printing to print information about escape analysis results.
func Fmt(n ir.Node) string {
text := ""
switch n.Esc() {
case ir.EscUnknown:
break
case ir.EscHeap:
text = "esc(h)"
case ir.EscNone:
text = "esc(no)"
case ir.EscNever:
text = "esc(N)"
default:
text = fmt.Sprintf("esc(%d)", n.Esc())
}
if n.Op() == ir.ONAME {
n := n.(*ir.Name)
if loc, ok := n.Opt.(*location); ok && loc.loopDepth != 0 {
if text != "" {
text += " "
}
text += fmt.Sprintf("ld(%d)", loc.loopDepth)
}
}
return text
}
Mini Shell