src/cmd/compile/internal/escape/utils.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/ir"
	"cmd/compile/internal/typecheck"
	"cmd/compile/internal/types"
)

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:
		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().Size() > ir.MaxStackVarSize {
		return "too large for stack"
	}
	if n.Type().Alignment() > int64(types.PtrSize) {
		return "too aligned for stack"
	}

	if (n.Op() == ir.ONEW || n.Op() == ir.OPTRLIT) && n.Type().Elem().Size() > ir.MaxImplicitStackVarSize {
		return "too large for stack"
	}
	if (n.Op() == ir.ONEW || n.Op() == ir.OPTRLIT) && n.Type().Elem().Alignment() > int64(types.PtrSize) {
		return "too aligned 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 n.Cap == nil {
			r = n.Len
		}

		elem := n.Type().Elem()
		if elem.Size() == 0 {
			// TODO: stack allocate these? See #65685.
			return "zero-sized element"
		}
		if !ir.IsSmallIntConst(r) {
			// For non-constant sizes, we do a hybrid approach:
			//
			// if cap <= K {
			//     var backing [K]E
			//     s = backing[:len:cap]
			// } else {
			//     s = makeslice(E, len, cap)
			// }
			//
			// It costs a constant amount of stack space, but may
			// avoid a heap allocation.
			// Note we have to be careful that assigning s[i] = v
			// still escapes v, because we forbid heap->stack pointers.
			// Implementation is in ../walk/builtin.go:walkMakeSlice.
			return ""
		}
		if ir.Int64Val(r) > ir.MaxImplicitStackVarSize/elem.Size() {
			return "too large for stack"
		}
	}

	return ""
}