runtime: precisely scan Wasm globals

src/runtime/gc_custom.go

@@ -48,7 +48,6 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.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.

src/runtime/gc_globals_blocks.go

@@ -0,0 +1,22 @@
+//go:build (gc.conservative || gc.precise) && tinygo.wasm
+
+package runtime
+
+import "unsafe"
+
+func markGlobals() {
+	for i := uintptr(0); i < gcGlobalRootCount(); i++ {
+		addr := gcGlobalRoot(i)
+		size := gcGlobalRootSize(i)
+		for offset := uintptr(0); offset < size; offset += unsafe.Sizeof(uintptr(0)) {
+			slot := unsafe.Add(addr, offset)
+			markRoot(uintptr(slot), *(*uintptr)(slot))
+		}
+	}
+}
+
+// These functions are generated by the compiler from the pointer layouts of
+// mutable globals. Each range contains only pointer slots.
+func gcGlobalRootCount() uintptr
+func gcGlobalRoot(index uintptr) unsafe.Pointer
+func gcGlobalRootSize(index uintptr) uintptr

src/runtime/gc_globals_custom.go

@@ -0,0 +1,18 @@
+//go:build gc.custom && tinygo.wasm
+
+package runtime
+
+import "unsafe"
+
+func markGlobals() {
+	for i := uintptr(0); i < gcGlobalRootCount(); i++ {
+		start := uintptr(gcGlobalRoot(i))
+		markRoots(start, start+gcGlobalRootSize(i))
+	}
+}
+
+// These functions are generated by the compiler from the pointer layouts of
+// mutable globals. Each range contains only pointer slots.
+func gcGlobalRootCount() uintptr
+func gcGlobalRoot(index uintptr) unsafe.Pointer
+func gcGlobalRootSize(index uintptr) uintptr

src/runtime/gc_globals_range.go

@@ -0,0 +1,40 @@
+//go:build gc.boehm && tinygo.wasm
+
+package runtime
+
+import "unsafe"
+
+func markGlobals() {
+	rangeCount := gcGlobalRootCount()
+	if rangeCount == 0 {
+		return
+	}
+
+	var rootCount uintptr
+	for i := uintptr(0); i < rangeCount; i++ {
+		rootCount += gcGlobalRootSize(i) / unsafe.Sizeof(uintptr(0))
+	}
+
+	// markRoots only accepts a range, so copy all global pointers into
+	// contiguous scratch space for marking.
+	roots := unsafe.Slice((*uintptr)(gcGlobalRootValues()), rootCount)
+	var rootIndex uintptr
+	for i := uintptr(0); i < rangeCount; i++ {
+		addr := gcGlobalRoot(i)
+		size := gcGlobalRootSize(i)
+		for offset := uintptr(0); offset < size; offset += unsafe.Sizeof(uintptr(0)) {
+			roots[rootIndex] = *(*uintptr)(unsafe.Add(addr, offset))
+			rootIndex++
+		}
+	}
+
+	start := uintptr(unsafe.Pointer(&roots[0]))
+	markRoots(start, start+rootCount*unsafe.Sizeof(roots[0]))
+}
+
+// These functions are generated by the compiler from the pointer layouts of
+// mutable globals. Each range contains only pointer slots.
+func gcGlobalRootCount() uintptr
+func gcGlobalRoot(index uintptr) unsafe.Pointer
+func gcGlobalRootSize(index uintptr) uintptr
+func gcGlobalRootValues() unsafe.Pointer

src/runtime/gc_stack_portable.go

@@ -10,7 +10,7 @@ import (
 
 func gcMarkReachable() {
 	markStack()
-	findGlobals(markRoots)
+	markGlobals()
 }
 
 //go:extern runtime.stackChainStart