Add -gc=custom option (#3302)

* Add -gc=custom option

src/internal/task/gc_stack_chain.go

@@ -1,4 +1,4 @@
-//go:build (gc.conservative || gc.precise) && tinygo.wasm
+//go:build (gc.conservative || gc.custom || gc.precise) && tinygo.wasm
 
 package task
 

src/internal/task/gc_stack_noop.go

@@ -1,4 +1,4 @@
-//go:build !(gc.conservative || gc.precise) || !tinygo.wasm
+//go:build !(gc.conservative || gc.custom || gc.precise) || !tinygo.wasm
 
 package task
 

src/runtime/gc_custom.go

@@ -0,0 +1,55 @@
+//go:build gc.custom
+// +build gc.custom
+
+package runtime
+
+// This GC strategy allows an external GC to be plugged in instead of the builtin
+// implementations.
+//
+// The interface defined in this file is not stable and can be broken at anytime, even
+// across minor versions.
+//
+// runtime.markStack() must be called at the beginning of any GC cycle. //go:linkname
+// on a function without a body can be used to access this internal function.
+//
+// The custom implementation must provide the following functions in the runtime package
+// using the go:linkname directive:
+//
+// - func initHeap()
+// - func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer
+// - func free(ptr unsafe.Pointer)
+// - func markRoots(start, end uintptr)
+// - func GC()
+//
+//
+// In addition, if targeting wasi, the following functions should be exported for interoperability
+// with wasi libraries that use them. Note, this requires the export directive, not go:linkname.
+//
+// - func malloc(size uintptr) unsafe.Pointer
+// - func free(ptr unsafe.Pointer)
+// - func calloc(nmemb, size uintptr) unsafe.Pointer
+// - func realloc(oldPtr unsafe.Pointer, size uintptr) unsafe.Pointer
+
+import (
+	"unsafe"
+)
+
+// initHeap is called when the heap is first initialized at program start.
+func initHeap()
+
+// alloc is called to allocate memory. layout is currently not used.
+func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer
+
+// free is called to explicitly free a previously allocated pointer.
+func free(ptr unsafe.Pointer)
+
+// markRoots is called with the start and end addresses to scan for references.
+// It is currently only called with the top and bottom of the stack.
+func markRoots(start, end uintptr)
+
+// GC is called to explicitly run garbage collection.
+func GC()
+
+func setHeapEnd(newHeapEnd uintptr) {
+	// Heap is in custom GC so ignore for when called from wasm initialization.
+}

src/runtime/gc_stack_portable.go

@@ -1,4 +1,4 @@
-//go:build (gc.conservative || gc.precise) && tinygo.wasm
+//go:build (gc.conservative || gc.custom || gc.precise) && tinygo.wasm
 
 package runtime