cmd/compile: allow all of the preamble to be preemptible

We currently make some parts of the preamble unpreemptible because
it confuses morestack. See comments in the code.

Instead, have morestack handle those weird cases so we can
remove unpreemptible marks from most places.

This CL makes user functions preemptible everywhere if they have no
write barriers (at least, on x86). In cmd/go the fraction of functions
that need preemptible markings drops from 82% to 36%. Makes the cmd/go
binary 0.3% smaller.

Update #35470

Change-Id: Ic83d5eabfd0f6d239a92e65684bcce7e67ff30bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/648518
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/stack.go

@@ -1075,6 +1075,29 @@ func newstack() {
 		gopreempt_m(gp) // never return
 	}
 
+	if stackguard0 == gp.stack.lo+stackGuard && gp.preemptRecent {
+		// The case happens because of an interaction between synchronous
+		// and asynchronous preemption. First, we set the cooperative
+		// preemption signal (g.stackguard0 = stackPreempt), and as a
+		// result the function fails the stack check and enters its
+		// morestack path. If it gets suspended at that point, we might
+		// give up waiting for it and send an async preempt. That async
+		// preempt gets processed and clears the cooperative preemption
+		// signal (g.stackguard0 = g.stack.lo+stackGuard) and resumes
+		// the function. But even though the cooperative preemption
+		// signal is cleared, we're already on the morestack path and
+		// can't avoid calling morestack. See issue 35470.
+		//
+		// To avoid this problem, if we've been preempted recently,
+		// clear the "preempted recently" flag and resume the G.
+		// If we really did need more stack, the morestack check will
+		// immediately fail and we'll get back here to try again (with
+		// preemptRecent==false, so we don't take this case the
+		// second time).
+		gp.preemptRecent = false
+		gogo(&gp.sched) // never return
+	}
+
 	// Allocate a bigger segment and move the stack.
 	oldsize := gp.stack.hi - gp.stack.lo
 	newsize := oldsize * 2