Current File : //usr/local/go119/src/cmd/compile/internal/escape/solve.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/internal/src"
"fmt"
"strings"
)
// walkAll computes the minimal dereferences between all pairs of
// locations.
func (b *batch) walkAll() {
// We use a work queue to keep track of locations that we need
// to visit, and repeatedly walk until we reach a fixed point.
//
// We walk once from each location (including the heap), and
// then re-enqueue each location on its transition from
// transient->!transient and !escapes->escapes, which can each
// happen at most once. So we take Θ(len(e.allLocs)) walks.
// LIFO queue, has enough room for e.allLocs and e.heapLoc.
todo := make([]*location, 0, len(b.allLocs)+1)
enqueue := func(loc *location) {
if !loc.queued {
todo = append(todo, loc)
loc.queued = true
}
}
for _, loc := range b.allLocs {
enqueue(loc)
}
enqueue(&b.heapLoc)
var walkgen uint32
for len(todo) > 0 {
root := todo[len(todo)-1]
todo = todo[:len(todo)-1]
root.queued = false
walkgen++
b.walkOne(root, walkgen, enqueue)
}
}
// walkOne computes the minimal number of dereferences from root to
// all other locations.
func (b *batch) walkOne(root *location, walkgen uint32, enqueue func(*location)) {
// The data flow graph has negative edges (from addressing
// operations), so we use the Bellman-Ford algorithm. However,
// we don't have to worry about infinite negative cycles since
// we bound intermediate dereference counts to 0.
root.walkgen = walkgen
root.derefs = 0
root.dst = nil
todo := []*location{root} // LIFO queue
for len(todo) > 0 {
l := todo[len(todo)-1]
todo = todo[:len(todo)-1]
derefs := l.derefs
// If l.derefs < 0, then l's address flows to root.
addressOf := derefs < 0
if addressOf {
// For a flow path like "root = &l; l = x",
// l's address flows to root, but x's does
// not. We recognize this by lower bounding
// derefs at 0.
derefs = 0
// If l's address flows to a non-transient
// location, then l can't be transiently
// allocated.
if !root.transient && l.transient {
l.transient = false
enqueue(l)
}
}
if b.outlives(root, l) {
// l's value flows to root. If l is a function
// parameter and root is the heap or a
// corresponding result parameter, then record
// that value flow for tagging the function
// later.
if l.isName(ir.PPARAM) {
if (logopt.Enabled() || base.Flag.LowerM >= 2) && !l.escapes {
if base.Flag.LowerM >= 2 {
fmt.Printf("%s: parameter %v leaks to %s with derefs=%d:\n", base.FmtPos(l.n.Pos()), l.n, b.explainLoc(root), derefs)
}
explanation := b.explainPath(root, l)
if logopt.Enabled() {
var e_curfn *ir.Func // TODO(mdempsky): Fix.
logopt.LogOpt(l.n.Pos(), "leak", "escape", ir.FuncName(e_curfn),
fmt.Sprintf("parameter %v leaks to %s with derefs=%d", l.n, b.explainLoc(root), derefs), explanation)
}
}
l.leakTo(root, derefs)
}
// If l's address flows somewhere that
// outlives it, then l needs to be heap
// allocated.
if addressOf && !l.escapes {
if logopt.Enabled() || base.Flag.LowerM >= 2 {
if base.Flag.LowerM >= 2 {
fmt.Printf("%s: %v escapes to heap:\n", base.FmtPos(l.n.Pos()), l.n)
}
explanation := b.explainPath(root, l)
if logopt.Enabled() {
var e_curfn *ir.Func // TODO(mdempsky): Fix.
logopt.LogOpt(l.n.Pos(), "escape", "escape", ir.FuncName(e_curfn), fmt.Sprintf("%v escapes to heap", l.n), explanation)
}
}
l.escapes = true
enqueue(l)
continue
}
}
for i, edge := range l.edges {
if edge.src.escapes {
continue
}
d := derefs + edge.derefs
if edge.src.walkgen != walkgen || edge.src.derefs > d {
edge.src.walkgen = walkgen
edge.src.derefs = d
edge.src.dst = l
edge.src.dstEdgeIdx = i
todo = append(todo, edge.src)
}
}
}
}
// explainPath prints an explanation of how src flows to the walk root.
func (b *batch) explainPath(root, src *location) []*logopt.LoggedOpt {
visited := make(map[*location]bool)
pos := base.FmtPos(src.n.Pos())
var explanation []*logopt.LoggedOpt
for {
// Prevent infinite loop.
if visited[src] {
if base.Flag.LowerM >= 2 {
fmt.Printf("%s: warning: truncated explanation due to assignment cycle; see golang.org/issue/35518\n", pos)
}
break
}
visited[src] = true
dst := src.dst
edge := &dst.edges[src.dstEdgeIdx]
if edge.src != src {
base.Fatalf("path inconsistency: %v != %v", edge.src, src)
}
explanation = b.explainFlow(pos, dst, src, edge.derefs, edge.notes, explanation)
if dst == root {
break
}
src = dst
}
return explanation
}
func (b *batch) explainFlow(pos string, dst, srcloc *location, derefs int, notes *note, explanation []*logopt.LoggedOpt) []*logopt.LoggedOpt {
ops := "&"
if derefs >= 0 {
ops = strings.Repeat("*", derefs)
}
print := base.Flag.LowerM >= 2
flow := fmt.Sprintf(" flow: %s = %s%v:", b.explainLoc(dst), ops, b.explainLoc(srcloc))
if print {
fmt.Printf("%s:%s\n", pos, flow)
}
if logopt.Enabled() {
var epos src.XPos
if notes != nil {
epos = notes.where.Pos()
} else if srcloc != nil && srcloc.n != nil {
epos = srcloc.n.Pos()
}
var e_curfn *ir.Func // TODO(mdempsky): Fix.
explanation = append(explanation, logopt.NewLoggedOpt(epos, "escflow", "escape", ir.FuncName(e_curfn), flow))
}
for note := notes; note != nil; note = note.next {
if print {
fmt.Printf("%s: from %v (%v) at %s\n", pos, note.where, note.why, base.FmtPos(note.where.Pos()))
}
if logopt.Enabled() {
var e_curfn *ir.Func // TODO(mdempsky): Fix.
explanation = append(explanation, logopt.NewLoggedOpt(note.where.Pos(), "escflow", "escape", ir.FuncName(e_curfn),
fmt.Sprintf(" from %v (%v)", note.where, note.why)))
}
}
return explanation
}
func (b *batch) explainLoc(l *location) string {
if l == &b.heapLoc {
return "{heap}"
}
if l.n == nil {
// TODO(mdempsky): Omit entirely.
return "{temp}"
}
if l.n.Op() == ir.ONAME {
return fmt.Sprintf("%v", l.n)
}
return fmt.Sprintf("{storage for %v}", l.n)
}
// outlives reports whether values stored in l may survive beyond
// other's lifetime if stack allocated.
func (b *batch) outlives(l, other *location) bool {
// The heap outlives everything.
if l.escapes {
return true
}
// We don't know what callers do with returned values, so
// pessimistically we need to assume they flow to the heap and
// outlive everything too.
if l.isName(ir.PPARAMOUT) {
// Exception: Directly called closures can return
// locations allocated outside of them without forcing
// them to the heap. For example:
//
// var u int // okay to stack allocate
// *(func() *int { return &u }()) = 42
if containsClosure(other.curfn, l.curfn) && l.curfn.ClosureCalled() {
return false
}
return true
}
// If l and other are within the same function, then l
// outlives other if it was declared outside other's loop
// scope. For example:
//
// var l *int
// for {
// l = new(int)
// }
if l.curfn == other.curfn && l.loopDepth < other.loopDepth {
return true
}
// If other is declared within a child closure of where l is
// declared, then l outlives it. For example:
//
// var l *int
// func() {
// l = new(int)
// }
if containsClosure(l.curfn, other.curfn) {
return true
}
return false
}
// containsClosure reports whether c is a closure contained within f.
func containsClosure(f, c *ir.Func) bool {
// Common case.
if f == c {
return false
}
// Closures within function Foo are named like "Foo.funcN..."
// TODO(mdempsky): Better way to recognize this.
fn := f.Sym().Name
cn := c.Sym().Name
return len(cn) > len(fn) && cn[:len(fn)] == fn && cn[len(fn)] == '.'
}
Mini Shell