runtime: refactor obtaining the system stack

The system stack is only needed when we're not on it. So we can directly
call task.SystemStack() without problems.

This also saves a tiny bit of binary size.

builder/sizes_test.go

@@ -42,9 +42,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 4560, 280, 0, 2268},
+		{"hifive1b", "examples/echo", 4556, 280, 0, 2268},
-		{"microbit", "examples/serial", 2924, 388, 8, 2272},
+		{"microbit", "examples/serial", 2920, 388, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 7383, 1489, 116, 6912},
+		{"wioterminal", "examples/pininterrupt", 7379, 1489, 116, 6912},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the

src/internal/task/task_none.go

@@ -36,3 +36,9 @@ func OnSystemStack() bool {
 	// This scheduler does not do any stack switching.
 	return true
 }
+
+func SystemStack() uintptr {
+	// System stack is the current stack, so this shouldn't be called.
+	runtimePanic("scheduler is disabled")
+	return 0 // unreachable
+}

src/runtime/gc_stack_raw.go

@@ -20,7 +20,7 @@ func markStack() {
 
 	if !task.OnSystemStack() {
 		// Mark system stack.
-		markRoots(getSystemStackPointer(), stackTop)
+		markRoots(task.SystemStack(), stackTop)
 	}
 }
 

src/runtime/scheduler_none.go

@@ -73,9 +73,3 @@ func scheduler(returnAtDeadlock bool) {
 	// this code should be unreachable.
 	runtimePanic("unreachable: scheduler must not be called with the 'none' scheduler")
 }
-
-// getSystemStackPointer returns the current stack pointer of the system stack.
-// This is always the current stack pointer.
-func getSystemStackPointer() uintptr {
-	return getCurrentStackPointer()
-}

src/runtime/scheduler_tasks.go

@@ -1,17 +0,0 @@
-//go:build scheduler.tasks
-
-package runtime
-
-import "internal/task"
-
-// getSystemStackPointer returns the current stack pointer of the system stack.
-// This is not necessarily the same as the current stack pointer.
-func getSystemStackPointer() uintptr {
-	// TODO: this always returns the correct stack on Cortex-M, so don't bother
-	// comparing against 0.
-	sp := task.SystemStack()
-	if sp == 0 {
-		sp = getCurrentStackPointer()
-	}
-	return sp
-}