wasm: add Boehm GC support

This adds support for `-gc=boehm` on `-target=wasip1` and `-target=wasm`
(in a browser or NodeJS). Notably it does *not* add Boehm GC support for
`-target=wasip2`, since that target doesn't have a real libc.

src/runtime/arch_tinygowasm_malloc.go

@@ -1,4 +1,4 @@
-//go:build tinygo.wasm && !(custommalloc || wasm_unknown)
+//go:build tinygo.wasm && !(custommalloc || wasm_unknown || gc.boehm)
 
 package runtime
 

src/runtime/gc_boehm.c

@@ -0,0 +1,37 @@
+//go:build none
+
+// This file is included in the build on systems that support the Boehm GC,
+// despite the //go:build line above.
+
+#include <stdint.h>
+
+typedef void (* GC_push_other_roots_proc)(void);
+void GC_set_push_other_roots(GC_push_other_roots_proc);
+
+typedef void(* GC_warn_proc)(const char *msg, uintptr_t arg);
+void GC_set_warn_proc(GC_warn_proc p);
+
+void tinygo_runtime_bdwgc_callback(void);
+
+static void callback(void) {
+    tinygo_runtime_bdwgc_callback();
+}
+
+static void warn_proc(const char *msg, uintptr_t arg) {
+}
+
+void tinygo_runtime_bdwgc_init(void) {
+    GC_set_push_other_roots(callback);
+#if defined(__wasm__)
+    // There are a lot of warnings on WebAssembly in the form:
+    //
+    //     GC Warning: Repeated allocation of very large block (appr. size 68 KiB):
+    //         May lead to memory leak and poor performance
+    //
+    // The usual advice is to use something like GC_malloc_ignore_off_page but
+    // unfortunately for most allocations that's not allowed: Go allocations can
+    // legitimately hold pointers further than one page in the allocation. So
+    // instead we just disable the warning.
+    GC_set_warn_proc(warn_proc);
+#endif
+}

src/runtime/gc_boehm.go

@@ -20,7 +20,6 @@ package runtime
 
 import (
 	"internal/gclayout"
-	"internal/reflectlite"
 	"internal/task"
 	"unsafe"
 )
@@ -39,22 +38,30 @@ var needsResumeWorld bool
 func initHeap() {
 	libgc_init()
 
-	libgc_set_push_other_roots(gcCallbackPtr)
+	// Call GC_set_push_other_roots(gcCallback) in C because of function
+	// signature differences that do matter in WebAssembly.
+	gcInit()
 }
 
-var gcCallbackPtr = reflectlite.ValueOf(gcCallback).UnsafePointer()
+//export tinygo_runtime_bdwgc_init
+func gcInit()
 
+//export tinygo_runtime_bdwgc_callback
 func gcCallback() {
 	// Mark globals and all stacks, and stop the world if we're using threading.
 	gcMarkReachable()
 
-	if needsResumeWorld {
-		// Should never happen, check for it anyway.
-		runtimePanic("gc: world already stopped")
-	}
+	// If we use a scheduler with parallelism (the threads scheduler for
+	// example), we need to call gcResumeWorld() after scanning has finished.
+	if hasParallelism {
+		if needsResumeWorld {
+			// Should never happen, check for it anyway.
+			runtimePanic("gc: world already stopped")
+		}
 
-	// Note that we need to resume the world after finishing the GC call.
-	needsResumeWorld = true
+		// Note that we need to resume the world after finishing the GC call.
+		needsResumeWorld = true
+	}
 }
 
 func markRoots(start, end uintptr) {

src/runtime/gc_stack_portable.go

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