runtime: make sysMemStats' methods nosplit

sysMemStats are updated early on in runtime initialization, so
triggering a stack growth would be bad. Mark them nosplit.

Thank you so much to [email protected] for finding this fix!

Fixes #42218.

Change-Id: Ic62db76e6a4f829355d7eaabed1727c51adfbd0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/265157
Trust: Michael Knyszek <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: mknyszek

src/runtime/mstats.go

@@ -720,11 +720,17 @@ func flushallmcaches() {
 type sysMemStat uint64
 
 // load atomically reads the value of the stat.
+//
+// Must be nosplit as it is called in runtime initialization, e.g. newosproc0.
+//go:nosplit
 func (s *sysMemStat) load() uint64 {
 	return atomic.Load64((*uint64)(s))
 }
 
 // add atomically adds the sysMemStat by n.
+//
+// Must be nosplit as it is called in runtime initialization, e.g. newosproc0.
+//go:nosplit
 func (s *sysMemStat) add(n int64) {
 	if s == nil {
 		return