--- /dev/null
- if (n.Op() == ir.ONEW || n.Op() == ir.OPTRLIT) && n.Type().Elem().Width >= ir.MaxImplicitStackVarSize {
+// 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.OCLOSURE && typecheck.ClosureType(n.(*ir.ClosureExpr)).Size() >= ir.MaxImplicitStackVarSize {
++ if (n.Op() == ir.ONEW || n.Op() == ir.OPTRLIT) && n.Type().Elem().Width > ir.MaxImplicitStackVarSize {
+ return "too large for stack"
+ }
+
- if n.Op() == ir.OMETHVALUE && typecheck.PartialCallType(n.(*ir.SelectorExpr)).Size() >= ir.MaxImplicitStackVarSize {
++ if n.Op() == ir.OCLOSURE && typecheck.ClosureType(n.(*ir.ClosureExpr)).Size() > ir.MaxImplicitStackVarSize {
+ return "too large for stack"
+ }
- if t := n.Type(); t.Elem().Width != 0 && ir.Int64Val(r) >= ir.MaxImplicitStackVarSize/t.Elem().Width {
++ if n.Op() == ir.OMETHVALUE && typecheck.PartialCallType(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 ""
+}