before they become visible as GC roots. Otherwise, the GC may
   observe stale heap pointers. See "Zero-initialization versus
   zeroing".

Zero-initialization versus zeroing
==================================

There are two types of zeroing in the runtime, depending on whether
the memory is already initialized to a type-safe state.

If memory is not in a type-safe state, meaning it potentially contains

	REG_AR14
	REG_AR15

	REG_RESERVED // end of allocated registers

	REGARG  = -1      // -1 disables passing the first argument in register
	REGRT1  = REG_R3  // used during zeroing of the stack - not reserved
	REGRT2  = REG_R4  // used during zeroing of the stack - not reserved
	REGTMP  = REG_R10 // scratch register used in the assembler and linker
	REGTMP2 = REG_R11 // scratch register used in the assembler and linker

	var to unsafe.Pointer
	if !et.Pointers() {
		to = mallocgc(tomem, nil, false)
		if copymem < tomem {
			memclrNoHeapPointers(add(to, copymem), tomem-copymem)
		}
	} else {
		// Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory.
		to = mallocgc(tomem, et, true)
		if copymem > 0 && writeBarrier.enabled {
			// Only shade the pointers in old.array since we know the destination slice to

		(I64Store32 destptr (I64Const [0]) mem))
(Zero [7] destptr mem) =>
	(I64Store32 [3] destptr (I64Const [0])
		(I64Store32 destptr (I64Const [0]) mem))

// Strip off any fractional word zeroing.
(Zero [s] destptr mem) && s%8 != 0 && s > 8 && s < 32 =>
	(Zero [s-s%8] (OffPtr <destptr.Type> destptr [s%8])
		(I64Store destptr (I64Const [0]) mem))

// Zero small numbers of words directly.

	if goexperiment.CgoCheck2 {
		cgoCheckMemmove2(typ, dst, src, 0, typ.Size_)
	}
}

// wbZero performs the write barrier operations necessary before
// zeroing a region of memory at address dst of type typ.
// Does not actually do the zeroing.
//
//go:nowritebarrierrec
//go:nosplit
func wbZero(typ *_type, dst unsafe.Pointer) {
	// This always copies a full value of type typ so it's safe

	fn := typecheck.LookupRuntime("mapclear", t.Key(), t.Elem())
	n := mkcallstmt1(fn, rtyp, m)
	return walkStmt(typecheck.Stmt(n))
}

// Lower n into runtime·memclr if possible, for
// fast zeroing of slices and arrays (issue 5373).
// Look for instances of
//
//	for i := range a {
//		a[i] = zero
//	}
//
// in which the evaluation of a is side-effect-free.
//

			(MOVVstore [0] ptr (MOVVconst [0]) mem)))

// medium zeroing uses a duff device
// 8, and 128 are magic constants, see runtime/mkduff.go
(Zero [s] {t} ptr mem)
	&& s%8 == 0 && s > 24 && s <= 8*128
	&& t.Alignment()%8 == 0 && !config.noDuffDevice =>
	(DUFFZERO [8 * (128 - s/8)] ptr mem)

// large or unaligned zeroing uses a loop
(Zero [s] {t} ptr mem)

	} else {
		n.(*ir.AssignStmt).X = left
	}
	as := n.(*ir.AssignStmt)

	if oaslit(as, init) {
		return ir.NewBlockStmt(as.Pos(), nil)
	}

	if as.Y == nil {
		// TODO(austin): Check all "implicit zeroing"
		return as
	}

	if !base.Flag.Cfg.Instrumenting && ir.IsZero(as.Y) {
		return as
	}

	switch as.Y.Op() {
	default:
		as.Y = walkExpr(as.Y, init)

	case ir.ORECV:

			(MOVVstore [0] ptr (MOVVconst [0]) mem)))

// medium zeroing uses a duff device
// 8, and 128 are magic constants, see runtime/mkduff.go
(Zero [s] {t} ptr mem)
	&& s%8 == 0 && s > 24 && s <= 8*128
	&& t.Alignment()%8 == 0 && !config.noDuffDevice =>
	(DUFFZERO [8 * (128 - s/8)] ptr mem)

// large or unaligned zeroing uses a loop
(Zero [s] {t} ptr mem)

	// Sort pointer-typed before non-pointer types.
	// Keeps the stack's GC bitmap compact.
	ap := a.Type().HasPointers()
	bp := b.Type().HasPointers()
	if ap != bp {
		return ap
	}

	// Group variables that need zeroing, so we can efficiently zero
	// them altogether.
	ap = a.Needzero()
	bp = b.Needzero()
	if ap != bp {
		return ap
	}

	// Sort variables in descending alignment order, so we can optimally