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// 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/ir"
"cmd/compile/internal/typecheck"
)
func isSliceSelfAssign(dst, src ir.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.
var dstX ir.Node
switch dst.Op() {
default:
return false
case ir.ODEREF:
dst := dst.(*ir.StarExpr)
dstX = dst.X
case ir.ODOTPTR:
dst := dst.(*ir.SelectorExpr)
dstX = dst.X
}
if dstX.Op() != ir.ONAME {
return false
}
// src is a slice operation.
switch src.Op() {
case ir.OSLICE, ir.OSLICE3, ir.OSLICESTR:
// OK.
case ir.OSLICEARR, ir.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.
src := src.(*ir.SliceExpr)
if src.X.Op() == ir.OADDR {
return false
}
default:
return false
}
// slice is applied to ONAME dereference.
var baseX ir.Node
switch base := src.(*ir.SliceExpr).X; base.Op() {
default:
return false
case ir.ODEREF:
base := base.(*ir.StarExpr)
baseX = base.X
case ir.ODOTPTR:
base := base.(*ir.SelectorExpr)
baseX = base.X
}
if baseX.Op() != ir.ONAME {
return false
}
// dst and src reference the same base ONAME.
return dstX.(*ir.Name) == baseX.(*ir.Name)
}
// 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 ir.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
}
// The expression prefix must be both "safe" and identical.
switch dst.Op() {
case ir.ODOT, ir.ODOTPTR:
// Safe trailing accessors that are permitted to differ.
dst := dst.(*ir.SelectorExpr)
src := src.(*ir.SelectorExpr)
return ir.SameSafeExpr(dst.X, src.X)
case ir.OINDEX:
dst := dst.(*ir.IndexExpr)
src := src.(*ir.IndexExpr)
if mayAffectMemory(dst.Index) || mayAffectMemory(src.Index) {
return false
}
return ir.SameSafeExpr(dst.X, src.X)
default:
return false
}
}
// 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 ir.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.
// TODO(rsc): It seems like it should be possible to replace this with
// an ir.Any looking for any op that's not the ones in the case statement.
// But that produces changes in the compiled output detected by buildall.
switch n.Op() {
case ir.ONAME, ir.OLITERAL, ir.ONIL:
return false
case ir.OADD, ir.OSUB, ir.OOR, ir.OXOR, ir.OMUL, ir.OLSH, ir.ORSH, ir.OAND, ir.OANDNOT, ir.ODIV, ir.OMOD:
n := n.(*ir.BinaryExpr)
return mayAffectMemory(n.X) || mayAffectMemory(n.Y)
case ir.OINDEX:
n := n.(*ir.IndexExpr)
return mayAffectMemory(n.X) || mayAffectMemory(n.Index)
case ir.OCONVNOP, ir.OCONV:
n := n.(*ir.ConvExpr)
return mayAffectMemory(n.X)
case ir.OLEN, ir.OCAP, ir.ONOT, ir.OBITNOT, ir.OPLUS, ir.ONEG, ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
n := n.(*ir.UnaryExpr)
return mayAffectMemory(n.X)
case ir.ODOT, ir.ODOTPTR:
n := n.(*ir.SelectorExpr)
return mayAffectMemory(n.X)
case ir.ODEREF:
n := n.(*ir.StarExpr)
return mayAffectMemory(n.X)
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 ir.Node) string {
if n == nil || n.Type() == nil {
return ""
}
// Parameters are always passed via the stack.
if n.Op() == ir.ONAME {
n := n.(*ir.Name)
if n.Class == ir.PPARAM || n.Class == ir.PPARAMOUT {
return ""
}
}
if n.Type().Width > ir.MaxStackVarSize {
return "too large for stack"
}
if (n.Op() == ir.ONEW || n.Op() == ir.OPTRLIT) && n.Type().Elem().Width > ir.MaxImplicitStackVarSize {
return "too large for stack"
}
if n.Op() == ir.OCLOSURE && typecheck.ClosureType(n.(*ir.ClosureExpr)).Size() > ir.MaxImplicitStackVarSize {
return "too large for stack"
}
if n.Op() == ir.OMETHVALUE && typecheck.MethodValueType(n.(*ir.SelectorExpr)).Size() > ir.MaxImplicitStackVarSize {
return "too large for stack"
}
if n.Op() == ir.OMAKESLICE {
n := n.(*ir.MakeExpr)
r := n.Cap
if r == nil {
r = n.Len
}
if !ir.IsSmallIntConst(r) {
return "non-constant size"
}
if t := n.Type(); t.Elem().Width != 0 && ir.Int64Val(r) > ir.MaxImplicitStackVarSize/t.Elem().Width {
return "too large for stack"
}
}
return ""
}
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