runtime: update debug call protocol for register ABI

The debug call tests currently assume that the target Go function is
ABI0; this is clearly no longer true when we switch to the new ABI, so
make the tests set up argument register state in the debug call handler
and copy back results returned in registers.

A small snag in calling a Go function that follows the new ABI is that
the debug call protocol depends on the AX register being set to a
specific value as it bounces in and out of the handler, but this
register is part of the new register ABI, so results end up being
clobbered. Use R12 instead.

Next, the new desugaring behavior for "go" statements means that
newosproc1 must always call a function with no frame; if it takes any
arguments, it closes over them and they're passed in the context
register. Currently when debugCallWrap creates a new goroutine, it uses
newosproc1 directly and passes a non-zero-sized frame, so that needs to
be updated. To fix this, briefly use the g's param field which is
otherwise only used for channels to pass an explicitly allocated object
containing the "closed over" variables. While we could manually do the
desugaring ourselves (we cannot do so automatically because the Go
compiler prevents heap-allocated closures in the runtime), that bakes in
more ABI details in a place that really doesn't need to care about them.

Finally, there's an old bug here where the context register was set up
in CX, so technically closure calls never worked. Oops. It was otherwise
harmless for other types of calls before, but now CX is an argument
register, so now that interferes with regular calls, too.

For #40724.

Change-Id: I652c25ed56a25741bb04c24cfb603063c099edde
Reviewed-on: https://go-review.googlesource.com/c/go/+/309169
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/runtime/runtime2.go

@@ -412,14 +412,25 @@ type g struct {
 	stackguard0 uintptr // offset known to liblink
 	stackguard1 uintptr // offset known to liblink
 
-	_panic       *_panic // innermost panic - offset known to liblink
-	_defer       *_defer // innermost defer
-	m            *m      // current m; offset known to arm liblink
-	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
-	stktopsp     uintptr        // expected sp at top of stack, to check in traceback
-	param        unsafe.Pointer // passed parameter on wakeup
+	_panic    *_panic // innermost panic - offset known to liblink
+	_defer    *_defer // innermost defer
+	m         *m      // current m; offset known to arm liblink
+	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
+	stktopsp  uintptr // expected sp at top of stack, to check in traceback
+	// param is a generic pointer parameter field used to pass
+	// values in particular contexts where other storage for the
+	// parameter would be difficult to find. It is currently used
+	// in three ways:
+	// 1. When a channel operation wakes up a blocked goroutine, it sets param to
+	//    point to the sudog of the completed blocking operation.
+	// 2. By gcAssistAlloc1 to signal back to its caller that the goroutine completed
+	//    the GC cycle. It is unsafe to do so in any other way, because the goroutine's
+	//    stack may have moved in the meantime.
+	// 3. By debugCallWrap to pass parameters to a new goroutine because allocating a
+	//    closure in the runtime is forbidden.
+	param        unsafe.Pointer
 	atomicstatus uint32
 	stackLock    uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
 	goid         int64