// which prevents us from allocating more stack.
//
//go:nosplit
func sysAlloc(n uintptr, sysStat *sysMemStat) unsafe.Pointer {
	sysStat.add(int64(n))
	gcController.mappedReady.Add(int64(n))
	return sysAllocOS(n)
}

// sysUnused transitions a memory region from Ready to Prepared. It notifies the
// operating system that the physical pages backing this memory region are no

func (s *scavengeIndex) sysInit(test bool, sysStat *sysMemStat) (mappedReady uintptr) {
	if test {
		// Set up the scavenge index via sysAlloc so the test can free it later.
		scavIndexSize := uintptr(len(scavengeIndexArray)) * unsafe.Sizeof(atomicScavChunkData{})
		s.chunks = ((*[(1 << heapAddrBits) / pallocChunkBytes]atomicScavChunkData)(sysAlloc(scavIndexSize, sysStat)))[:]
		mappedReady = scavIndexSize
	} else {
		// Set up the scavenge index.

	var heapFree, heapAlloc, mappedReady uint64
	for {
		heapFree = c.heapFree.load()                         // Free and unscavenged memory.
		heapAlloc = c.totalAlloc.Load() - c.totalFree.Load() // Heap object bytes in use.
		mappedReady = c.mappedReady.Load()                   // Total unreleased mapped memory.
		if heapFree+heapAlloc <= mappedReady {
			break
		}

		}
		// Also check that mappedReady lines up with totalMapped - released.
		// This isn't really the same type of "make sure consistent stats line up" situation,
		// but this is an opportune time to check.
		if gcController.mappedReady.Load() != totalMapped-uint64(consStats.released) {
			print("runtime: mappedReady=", gcController.mappedReady.Load(), "\n")
			print("runtime: totalMapped=", totalMapped, "\n")

	memoryLimitGoal := uint64(float64(memoryLimit) * (1 - reduceExtraPercent/100.0))

	// mappedReady is comparable to memoryLimit, and represents how much total memory
	// the Go runtime has committed now (estimated).
	mappedReady := gcController.mappedReady.Load()

	// If we're below the goal already indicate that we don't need the background

	// Subtract back out whatever we mapped for the summaries.
	// sysUsed adds to p.sysStat and memstats.mappedReady no matter what
	// (and in anger should actually be accounted for), and there's no other
	// way to figure out how much we actually mapped.
	gcController.mappedReady.Add(-int64(p.summaryMappedReady))
	testSysStat.add(-int64(p.summaryMappedReady))

	// Free the mapped space for chunks.

	if limit := gcController.memoryLimit.Load(); !gcCPULimiter.limiting() {
		// Assist with scavenging to maintain the memory limit by the amount
		// that we expect to page in.
		inuse := gcController.mappedReady.Load()
		// Be careful about overflow, especially with uintptrs. Even on 32-bit platforms
		// someone can set a really big memory limit that isn't maxInt64.
		if uint64(scav)+inuse > uint64(limit) {