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// 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
import (
"cmd/compile/internal/types"
"fmt"
)
func escapes(all []*Node) {
visitBottomUp(all, escapeFuncs)
}
const (
EscFuncUnknown = 0 + iota
EscFuncPlanned
EscFuncStarted
EscFuncTagged
)
func min8(a, b int8) int8 {
if a < b {
return a
}
return b
}
func max8(a, b int8) int8 {
if a > b {
return a
}
return b
}
const (
EscUnknown = iota
EscNone // Does not escape to heap, result, or parameters.
EscHeap // Reachable from the heap
EscNever // By construction will not escape.
)
// funcSym returns fn.Func.Nname.Sym if no nils are encountered along the way.
func funcSym(fn *Node) *types.Sym {
if fn == nil || fn.Func.Nname == nil {
return nil
}
return fn.Func.Nname.Sym
}
// Mark labels that have no backjumps to them as not increasing e.loopdepth.
// Walk hasn't generated (goto|label).Left.Sym.Label yet, so we'll cheat
// and set it to one of the following two. Then in esc we'll clear it again.
var (
looping Node
nonlooping Node
)
func isSliceSelfAssign(dst, src *Node) bool {
// Detect the following special case.
//
// func (b *Buffer) Foo() {
// n, m := ...
// b.buf = b.buf[n:m]
// }
//
// This assignment is a no-op for escape analysis,
// it does not store any new pointers into b that were not already there.
// However, without this special case b will escape, because we assign to OIND/ODOTPTR.
// Here we assume that the statement will not contain calls,
// that is, that order will move any calls to init.
// Otherwise base ONAME value could change between the moments
// when we evaluate it for dst and for src.
// dst is ONAME dereference.
if dst.Op != ODEREF && dst.Op != ODOTPTR || dst.Left.Op != ONAME {
return false
}
// src is a slice operation.
switch src.Op {
case OSLICE, OSLICE3, OSLICESTR:
// OK.
case OSLICEARR, OSLICE3ARR:
// Since arrays are embedded into containing object,
// slice of non-pointer array will introduce a new pointer into b that was not already there
// (pointer to b itself). After such assignment, if b contents escape,
// b escapes as well. If we ignore such OSLICEARR, we will conclude
// that b does not escape when b contents do.
//
// Pointer to an array is OK since it's not stored inside b directly.
// For slicing an array (not pointer to array), there is an implicit OADDR.
// We check that to determine non-pointer array slicing.
if src.Left.Op == OADDR {
return false
}
default:
return false
}
// slice is applied to ONAME dereference.
if src.Left.Op != ODEREF && src.Left.Op != ODOTPTR || src.Left.Left.Op != ONAME {
return false
}
// dst and src reference the same base ONAME.
return dst.Left == src.Left.Left
}
// isSelfAssign reports whether assignment from src to dst can
// be ignored by the escape analysis as it's effectively a self-assignment.
func isSelfAssign(dst, src *Node) bool {
if isSliceSelfAssign(dst, src) {
return true
}
// Detect trivial assignments that assign back to the same object.
//
// It covers these cases:
// val.x = val.y
// val.x[i] = val.y[j]
// val.x1.x2 = val.x1.y2
// ... etc
//
// These assignments do not change assigned object lifetime.
if dst == nil || src == nil || dst.Op != src.Op {
return false
}
switch dst.Op {
case ODOT, ODOTPTR:
// Safe trailing accessors that are permitted to differ.
case OINDEX:
if mayAffectMemory(dst.Right) || mayAffectMemory(src.Right) {
return false
}
default:
return false
}
// The expression prefix must be both "safe" and identical.
return samesafeexpr(dst.Left, src.Left)
}
// mayAffectMemory reports whether evaluation of n may affect the program's
// memory state. If the expression can't affect memory state, then it can be
// safely ignored by the escape analysis.
func mayAffectMemory(n *Node) bool {
// We may want to use a list of "memory safe" ops instead of generally
// "side-effect free", which would include all calls and other ops that can
// allocate or change global state. For now, it's safer to start with the latter.
//
// We're ignoring things like division by zero, index out of range,
// and nil pointer dereference here.
switch n.Op {
case ONAME, OCLOSUREVAR, OLITERAL:
return false
// Left+Right group.
case OINDEX, OADD, OSUB, OOR, OXOR, OMUL, OLSH, ORSH, OAND, OANDNOT, ODIV, OMOD:
return mayAffectMemory(n.Left) || mayAffectMemory(n.Right)
// Left group.
case ODOT, ODOTPTR, ODEREF, OCONVNOP, OCONV, OLEN, OCAP,
ONOT, OBITNOT, OPLUS, ONEG, OALIGNOF, OOFFSETOF, OSIZEOF:
return mayAffectMemory(n.Left)
default:
return true
}
}
// heapAllocReason returns the reason the given Node must be heap
// allocated, or the empty string if it doesn't.
func heapAllocReason(n *Node) string {
if n.Type == nil {
return ""
}
// Parameters are always passed via the stack.
if n.Op == ONAME && (n.Class() == PPARAM || n.Class() == PPARAMOUT) {
return ""
}
if n.Type.Width > maxStackVarSize {
return "too large for stack"
}
if (n.Op == ONEW || n.Op == OPTRLIT) && n.Type.Elem().Width >= maxImplicitStackVarSize {
return "too large for stack"
}
if n.Op == OCLOSURE && closureType(n).Size() >= maxImplicitStackVarSize {
return "too large for stack"
}
if n.Op == OCALLPART && partialCallType(n).Size() >= maxImplicitStackVarSize {
return "too large for stack"
}
if n.Op == OMAKESLICE {
r := n.Right
if r == nil {
r = n.Left
}
if !smallintconst(r) {
return "non-constant size"
}
if t := n.Type; t.Elem().Width != 0 && r.Int64Val() >= maxImplicitStackVarSize/t.Elem().Width {
return "too large for stack"
}
}
return ""
}
// addrescapes tags node n as having had its address taken
// by "increasing" the "value" of n.Esc to EscHeap.
// Storage is allocated as necessary to allow the address
// to be taken.
func addrescapes(n *Node) {
switch n.Op {
default:
// Unexpected Op, probably due to a previous type error. Ignore.
case ODEREF, ODOTPTR:
// Nothing to do.
case ONAME:
if n == nodfp {
break
}
// if this is a tmpname (PAUTO), it was tagged by tmpname as not escaping.
// on PPARAM it means something different.
if n.Class() == PAUTO && n.Esc == EscNever {
break
}
// If a closure reference escapes, mark the outer variable as escaping.
if n.Name.IsClosureVar() {
addrescapes(n.Name.Defn)
break
}
if n.Class() != PPARAM && n.Class() != PPARAMOUT && n.Class() != PAUTO {
break
}
// This is a plain parameter or local variable that needs to move to the heap,
// but possibly for the function outside the one we're compiling.
// That is, if we have:
//
// func f(x int) {
// func() {
// global = &x
// }
// }
//
// then we're analyzing the inner closure but we need to move x to the
// heap in f, not in the inner closure. Flip over to f before calling moveToHeap.
oldfn := Curfn
Curfn = n.Name.Curfn
if Curfn.Func.Closure != nil && Curfn.Op == OCLOSURE {
Curfn = Curfn.Func.Closure
}
ln := lineno
lineno = Curfn.Pos
moveToHeap(n)
Curfn = oldfn
lineno = ln
// ODOTPTR has already been introduced,
// so these are the non-pointer ODOT and OINDEX.
// In &x[0], if x is a slice, then x does not
// escape--the pointer inside x does, but that
// is always a heap pointer anyway.
case ODOT, OINDEX, OPAREN, OCONVNOP:
if !n.Left.Type.IsSlice() {
addrescapes(n.Left)
}
}
}
// moveToHeap records the parameter or local variable n as moved to the heap.
func moveToHeap(n *Node) {
if Debug.r != 0 {
Dump("MOVE", n)
}
if compiling_runtime {
yyerror("%v escapes to heap, not allowed in runtime", n)
}
if n.Class() == PAUTOHEAP {
Dump("n", n)
Fatalf("double move to heap")
}
// Allocate a local stack variable to hold the pointer to the heap copy.
// temp will add it to the function declaration list automatically.
heapaddr := temp(types.NewPtr(n.Type))
heapaddr.Sym = lookup("&" + n.Sym.Name)
heapaddr.Orig.Sym = heapaddr.Sym
heapaddr.Pos = n.Pos
// Unset AutoTemp to persist the &foo variable name through SSA to
// liveness analysis.
// TODO(mdempsky/drchase): Cleaner solution?
heapaddr.Name.SetAutoTemp(false)
// Parameters have a local stack copy used at function start/end
// in addition to the copy in the heap that may live longer than
// the function.
if n.Class() == PPARAM || n.Class() == PPARAMOUT {
if n.Xoffset == BADWIDTH {
Fatalf("addrescapes before param assignment")
}
// We rewrite n below to be a heap variable (indirection of heapaddr).
// Preserve a copy so we can still write code referring to the original,
// and substitute that copy into the function declaration list
// so that analyses of the local (on-stack) variables use it.
stackcopy := newname(n.Sym)
stackcopy.Type = n.Type
stackcopy.Xoffset = n.Xoffset
stackcopy.SetClass(n.Class())
stackcopy.Name.Param.Heapaddr = heapaddr
if n.Class() == PPARAMOUT {
// Make sure the pointer to the heap copy is kept live throughout the function.
// The function could panic at any point, and then a defer could recover.
// Thus, we need the pointer to the heap copy always available so the
// post-deferreturn code can copy the return value back to the stack.
// See issue 16095.
heapaddr.Name.SetIsOutputParamHeapAddr(true)
}
n.Name.Param.Stackcopy = stackcopy
// Substitute the stackcopy into the function variable list so that
// liveness and other analyses use the underlying stack slot
// and not the now-pseudo-variable n.
found := false
for i, d := range Curfn.Func.Dcl {
if d == n {
Curfn.Func.Dcl[i] = stackcopy
found = true
break
}
// Parameters are before locals, so can stop early.
// This limits the search even in functions with many local variables.
if d.Class() == PAUTO {
break
}
}
if !found {
Fatalf("cannot find %v in local variable list", n)
}
Curfn.Func.Dcl = append(Curfn.Func.Dcl, n)
}
// Modify n in place so that uses of n now mean indirection of the heapaddr.
n.SetClass(PAUTOHEAP)
n.Xoffset = 0
n.Name.Param.Heapaddr = heapaddr
n.Esc = EscHeap
if Debug.m != 0 {
Warnl(n.Pos, "moved to heap: %v", n)
}
}
// This special tag is applied to uintptr variables
// that we believe may hold unsafe.Pointers for
// calls into assembly functions.
const unsafeUintptrTag = "unsafe-uintptr"
// This special tag is applied to uintptr parameters of functions
// marked go:uintptrescapes.
const uintptrEscapesTag = "uintptr-escapes"
func (e *Escape) paramTag(fn *Node, narg int, f *types.Field) string {
name := func() string {
if f.Sym != nil {
return f.Sym.Name
}
return fmt.Sprintf("arg#%d", narg)
}
if fn.Nbody.Len() == 0 {
// Assume that uintptr arguments must be held live across the call.
// This is most important for syscall.Syscall.
// See golang.org/issue/13372.
// This really doesn't have much to do with escape analysis per se,
// but we are reusing the ability to annotate an individual function
// argument and pass those annotations along to importing code.
if f.Type.IsUintptr() {
if Debug.m != 0 {
Warnl(f.Pos, "assuming %v is unsafe uintptr", name())
}
return unsafeUintptrTag
}
if !f.Type.HasPointers() { // don't bother tagging for scalars
return ""
}
var esc EscLeaks
// External functions are assumed unsafe, unless
// //go:noescape is given before the declaration.
if fn.Func.Pragma&Noescape != 0 {
if Debug.m != 0 && f.Sym != nil {
Warnl(f.Pos, "%v does not escape", name())
}
} else {
if Debug.m != 0 && f.Sym != nil {
Warnl(f.Pos, "leaking param: %v", name())
}
esc.AddHeap(0)
}
return esc.Encode()
}
if fn.Func.Pragma&UintptrEscapes != 0 {
if f.Type.IsUintptr() {
if Debug.m != 0 {
Warnl(f.Pos, "marking %v as escaping uintptr", name())
}
return uintptrEscapesTag
}
if f.IsDDD() && f.Type.Elem().IsUintptr() {
// final argument is ...uintptr.
if Debug.m != 0 {
Warnl(f.Pos, "marking %v as escaping ...uintptr", name())
}
return uintptrEscapesTag
}
}
if !f.Type.HasPointers() { // don't bother tagging for scalars
return ""
}
// Unnamed parameters are unused and therefore do not escape.
if f.Sym == nil || f.Sym.IsBlank() {
var esc EscLeaks
return esc.Encode()
}
n := asNode(f.Nname)
loc := e.oldLoc(n)
esc := loc.paramEsc
esc.Optimize()
if Debug.m != 0 && !loc.escapes {
if esc.Empty() {
Warnl(f.Pos, "%v does not escape", name())
}
if x := esc.Heap(); x >= 0 {
if x == 0 {
Warnl(f.Pos, "leaking param: %v", name())
} else {
// TODO(mdempsky): Mention level=x like below?
Warnl(f.Pos, "leaking param content: %v", name())
}
}
for i := 0; i < numEscResults; i++ {
if x := esc.Result(i); x >= 0 {
res := fn.Type.Results().Field(i).Sym
Warnl(f.Pos, "leaking param: %v to result %v level=%d", name(), res, x)
}
}
}
return esc.Encode()
}