runtime: steal space for stack barrier tracking from stack

The stack barrier code will need a bookkeeping structure to keep track
of the overwritten return PCs. This commit introduces and allocates
this structure, but does not yet use the structure.

We don't want to allocate space for this structure during garbage
collection, so this commit allocates it along with the allocation of
the corresponding stack. However, we can't do a regular allocation in
newstack because mallocgc may itself grow the stack (which would lead
to a recursive allocation). Hence, this commit makes the bookkeeping
structure part of the stack allocation itself by stealing the
necessary space from the top of the stack allocation. Since the size
of this bookkeeping structure is logarithmic in the size of the stack,
this has minimal impact on stack behavior.

Change-Id: Ia14408be06aafa9ca4867f4e70bddb3fe0e96665
Reviewed-on: https://go-review.googlesource.com/10313
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/runtime2.go

@@ -202,6 +202,12 @@ type stack struct {
 	hi uintptr
 }
 
+// stkbar records the state of a G's stack barrier.
+type stkbar struct {
+	savedLRPtr uintptr // location overwritten by stack barrier PC
+	savedLRVal uintptr // value overwritten at savedLRPtr
+}
+
 type g struct {
 	// Stack parameters.
 	// stack describes the actual stack memory: [stack.lo, stack.hi).
@@ -220,6 +226,8 @@ type g struct {
 	sched        gobuf
 	syscallsp    uintptr        // if status==Gsyscall, syscallsp = sched.sp to use during gc
 	syscallpc    uintptr        // if status==Gsyscall, syscallpc = sched.pc to use during gc
+	stkbar       []stkbar       // stack barriers, from low to high
+	stkbarPos    uintptr        // index of lowest stack barrier not hit
 	param        unsafe.Pointer // passed parameter on wakeup
 	atomicstatus uint32
 	goid         int64