internal/task: swap stack chain when switching goroutines

This change swaps the stack chain when switching goroutines, ensuring that the chain is maintained consistently.
This is only really currently necessary with asyncify on wasm.

compiler/testdata/channel.ll

@@ -5,7 +5,8 @@ target triple = "wasm32-unknown-wasi"
 
 %runtime.channel = type { i32, i32, i8, %runtime.channelBlockedList*, i32, i32, i32, i8* }
 %runtime.channelBlockedList = type { %runtime.channelBlockedList*, %"internal/task.Task"*, %runtime.chanSelectState*, { %runtime.channelBlockedList*, i32, i32 } }
-%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.state" }
+%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.gcData", %"internal/task.state" }
+%"internal/task.gcData" = type { i8* }
 %"internal/task.state" = type { i32, i8*, %"internal/task.stackState", i1 }
 %"internal/task.stackState" = type { i32, i32 }
 %runtime.chanSelectState = type { %runtime.channel*, i8* }

compiler/testdata/goroutine-cortex-m-qemu-tasks.ll

@@ -5,7 +5,8 @@ target triple = "thumbv7m-unknown-unknown-eabi"
 
 %runtime.channel = type { i32, i32, i8, %runtime.channelBlockedList*, i32, i32, i32, i8* }
 %runtime.channelBlockedList = type { %runtime.channelBlockedList*, %"internal/task.Task"*, %runtime.chanSelectState*, { %runtime.channelBlockedList*, i32, i32 } }
-%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.state" }
+%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.gcData", %"internal/task.state" }
+%"internal/task.gcData" = type {}
 %"internal/task.state" = type { i32, i32* }
 %runtime.chanSelectState = type { %runtime.channel*, i8* }
 

compiler/testdata/goroutine-wasm-asyncify.ll

@@ -5,7 +5,8 @@ target triple = "wasm32-unknown-wasi"
 
 %runtime.channel = type { i32, i32, i8, %runtime.channelBlockedList*, i32, i32, i32, i8* }
 %runtime.channelBlockedList = type { %runtime.channelBlockedList*, %"internal/task.Task"*, %runtime.chanSelectState*, { %runtime.channelBlockedList*, i32, i32 } }
-%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.state" }
+%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.gcData", %"internal/task.state" }
+%"internal/task.gcData" = type { i8* }
 %"internal/task.state" = type { i32, i8*, %"internal/task.stackState", i1 }
 %"internal/task.stackState" = type { i32, i32 }
 %runtime.chanSelectState = type { %runtime.channel*, i8* }

compiler/testdata/goroutine-wasm-coroutines.ll

@@ -6,7 +6,8 @@ target triple = "wasm32-unknown-wasi"
 %runtime.funcValueWithSignature = type { i32, i8* }
 %runtime.channel = type { i32, i32, i8, %runtime.channelBlockedList*, i32, i32, i32, i8* }
 %runtime.channelBlockedList = type { %runtime.channelBlockedList*, %"internal/task.Task"*, %runtime.chanSelectState*, { %runtime.channelBlockedList*, i32, i32 } }
-%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.state" }
+%"internal/task.Task" = type { %"internal/task.Task"*, i8*, i64, %"internal/task.gcData", %"internal/task.state" }
+%"internal/task.gcData" = type {}
 %"internal/task.state" = type { i8* }
 %runtime.chanSelectState = type { %runtime.channel*, i8* }
 

src/internal/task/gc_stack_chain.go

@@ -0,0 +1,19 @@
+//go:build (gc.conservative || gc.extalloc) && tinygo.wasm && !scheduler.coroutines
+// +build gc.conservative gc.extalloc
+// +build tinygo.wasm
+// +build !scheduler.coroutines
+
+package task
+
+import "unsafe"
+
+//go:linkname swapStackChain runtime.swapStackChain
+func swapStackChain(dst *unsafe.Pointer)
+
+type gcData struct {
+	stackChain unsafe.Pointer
+}
+
+func (gcd *gcData) swap() {
+	swapStackChain(&gcd.stackChain)
+}

src/internal/task/gc_stack_noop.go

@@ -0,0 +1,9 @@
+//go:build (!gc.conservative && !gc.extalloc) || !tinygo.wasm || scheduler.coroutines
+// +build !gc.conservative,!gc.extalloc !tinygo.wasm scheduler.coroutines
+
+package task
+
+type gcData struct{}
+
+func (gcd *gcData) swap() {
+}

src/internal/task/task.go

@@ -15,6 +15,9 @@ type Task struct {
 	// Data is a field which can be used for storing state information.
 	Data uint64
 
+	// gcData holds data for the GC.
+	gcData gcData
+
 	// state is the underlying running state of the task.
 	state state
 }

src/internal/task/task_asyncify.go

@@ -104,6 +104,7 @@ func (*stackState) unwind()
 func (t *Task) Resume() {
 	// The current task must be saved and restored because this can nest on WASM with JS.
 	prevTask := currentTask
+	t.gcData.swap()
 	currentTask = t
 	if !t.state.launched {
 		t.state.launch()
@@ -112,6 +113,7 @@ func (t *Task) Resume() {
 		t.state.rewind()
 	}
 	currentTask = prevTask
+	t.gcData.swap()
 	if t.state.asyncifysp > t.state.csp {
 		runtimePanic("stack overflow")
 	}

src/internal/task/task_stack.go

@@ -1,3 +1,4 @@
+//go:build scheduler.tasks
 // +build scheduler.tasks
 
 package task
@@ -54,7 +55,9 @@ func pause() {
 // This may only be called from the scheduler.
 func (t *Task) Resume() {
 	currentTask = t
+	t.gcData.swap()
 	t.state.resume()
+	t.gcData.swap()
 	currentTask = nil
 }
 

src/runtime/gc_stack_portable.go

@@ -1,3 +1,4 @@
+//go:build (gc.conservative || gc.extalloc) && tinygo.wasm
 // +build gc.conservative gc.extalloc
 // +build tinygo.wasm
 
@@ -37,3 +38,9 @@ func markStack() {
 // construction. Calls to it are later replaced with regular stack bookkeeping
 // code.
 func trackPointer(ptr unsafe.Pointer)
+
+// swapStackChain swaps the stack chain.
+// This is called from internal/task when switching goroutines.
+func swapStackChain(dst **stackChainObject) {
+	*dst, stackChainStart = stackChainStart, *dst
+}