wasm: scan globals conservatively

Make the GC globals scan phase conservative instead of precise on
WebAssembly. This reduces code size at the risk of introducing some
false positives.

This is a stopgap measure to mitigate an issue with the precise scanning
of globals that doesn't track all pointers. It works for regular globals
but globals created in the interp package don't always have a type and
therefore may be missed by the AddGlobalsBitmap pass.
The same issue is present on Linux and macOS, but is not as noticeable
there.

src/runtime/arch_wasm.go

@@ -14,6 +14,9 @@ const TargetBits = 32
 //go:extern __heap_base
 var heapStartSymbol [0]byte
 
+//go:extern __global_base
+var globalsStartSymbol [0]byte
+
 //export llvm.wasm.memory.size.i32
 func wasm_memory_size(index int32) int32
 
@@ -21,8 +24,10 @@ func wasm_memory_size(index int32) int32
 func wasm_memory_grow(index int32, delta int32) int32
 
 var (
-	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
-	heapEnd   = uintptr(wasm_memory_size(0) * wasmPageSize)
+	heapStart    = uintptr(unsafe.Pointer(&heapStartSymbol))
+	heapEnd      = uintptr(wasm_memory_size(0) * wasmPageSize)
+	globalsStart = uintptr(unsafe.Pointer(&globalsStartSymbol))
+	globalsEnd   = uintptr(unsafe.Pointer(&heapStartSymbol))
 )
 
 const wasmPageSize = 64 * 1024

src/runtime/gc_conservative.go

@@ -401,7 +401,7 @@ func markRoots(start, end uintptr) {
 		}
 	}
 
-	for addr := start; addr != end; addr += unsafe.Alignof(addr) {
+	for addr := start; addr < end; addr += unsafe.Alignof(addr) {
 		root := *(*uintptr)(unsafe.Pointer(addr))
 		markRoot(addr, root)
 	}

src/runtime/gc_globals_conservative.go

@@ -1,5 +1,5 @@
 // +build gc.conservative gc.extalloc
-// +build baremetal
+// +build baremetal wasm
 
 package runtime
 

src/runtime/gc_globals_precise.go

@@ -1,5 +1,5 @@
 // +build gc.conservative gc.extalloc
-// +build !baremetal
+// +build !baremetal,!wasm
 
 package runtime