riscv: switch to tasks-based scheduler

This is only supported for RV32 at the moment. RV64 can be added at a
later time.

src/internal/task/task_stack_tinygoriscv.go

@@ -0,0 +1,66 @@
+// +build scheduler.tasks,tinygo.riscv
+
+package task
+
+import "unsafe"
+
+var systemStack uintptr
+
+// calleeSavedRegs is the list of registers that must be saved and restored when
+// switching between tasks. Also see scheduler_riscv.S that relies on the
+// exact layout of this struct.
+type calleeSavedRegs struct {
+	s0  uintptr // x8 (fp)
+	s1  uintptr // x9
+	s2  uintptr // x18
+	s3  uintptr // x19
+	s4  uintptr // x20
+	s5  uintptr // x21
+	s6  uintptr // x22
+	s7  uintptr // x23
+	s8  uintptr // x24
+	s9  uintptr // x25
+	s10 uintptr // x26
+	s11 uintptr // x27
+
+	pc uintptr
+}
+
+// archInit runs architecture-specific setup for the goroutine startup.
+func (s *state) archInit(r *calleeSavedRegs, fn uintptr, args unsafe.Pointer) {
+	// Store the initial sp for the startTask function (implemented in assembly).
+	s.sp = uintptr(unsafe.Pointer(r))
+
+	// Initialize the registers.
+	// These will be popped off of the stack on the first resume of the goroutine.
+
+	// Start the function at tinygo_startTask (defined in src/internal/task/task_stack_riscv.S).
+	// This assembly code calls a function (passed in s0) with a single argument
+	// (passed in s1). After the function returns, it calls Pause().
+	r.pc = uintptr(unsafe.Pointer(&startTask))
+
+	// Pass the function to call in s0.
+	// This function is a compiler-generated wrapper which loads arguments out
+	// of a struct pointer. See createGoroutineStartWrapper (defined in
+	// compiler/goroutine.go) for more information.
+	r.s0 = fn
+
+	// Pass the pointer to the arguments struct in s1.
+	r.s1 = uintptr(args)
+}
+
+func (s *state) resume() {
+	swapTask(s.sp, &systemStack)
+}
+
+func (s *state) pause() {
+	newStack := systemStack
+	systemStack = 0
+	swapTask(newStack, &s.sp)
+}
+
+// SystemStack returns the system stack pointer when called from a task stack.
+// When called from the system stack, it returns 0.
+func SystemStack() uintptr {
+	return systemStack
+}