runtime: separate stack freeing from stack shrinking

Currently, shrinkstack will free the stack if the goroutine is dead.
There are only two places that call shrinkstack: scanstack, which will
never call it if the goroutine is dead; and markrootFreeGStacks, which
only calls it on dead goroutines.

Clean this up by separating stack freeing out of shrinkstack.

Change-Id: I7d7891e620550c32a2220833923a025704986681
Reviewed-on: https://go-review.googlesource.com/c/go/+/170890
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mgcmark.go

@@ -270,7 +270,9 @@ func markrootFreeGStacks() {
 	// Free stacks.
 	q := gQueue{list.head, list.head}
 	for gp := list.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
-		shrinkstack(gp)
+		stackfree(gp.stack)
+		gp.stack.lo = 0
+		gp.stack.hi = 0
 		// Manipulate the queue directly since the Gs are
 		// already all linked the right way.
 		q.tail.set(gp)

src/runtime/stack.go

@@ -1077,16 +1077,6 @@ func gostartcallfn(gobuf *gobuf, fv *funcval) {
 // gp must be stopped, but the world need not be.
 func shrinkstack(gp *g) {
 	gstatus := readgstatus(gp)
-	if gstatus&^_Gscan == _Gdead {
-		if gp.stack.lo != 0 {
-			// Free whole stack - it will get reallocated
-			// if G is used again.
-			stackfree(gp.stack)
-			gp.stack.lo = 0
-			gp.stack.hi = 0
-		}
-		return
-	}
 	if gp.stack.lo == 0 {
 		throw("missing stack in shrinkstack")
 	}