gc_leaking: Don't zero out new allocations in some targets (#4302)

Wasm linear memory is always initialized to zero by definition,
so there's no need to waste time zeroing out this allocation. This
is the case for freshly-obtained memory from mmap().

Signed-off-by: L. Pereira <l.pereira@fastly.com>

src/runtime/gc_leaking.go

@@ -44,7 +44,7 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 		runtimePanic("out of memory")
 	}
 	pointer := unsafe.Pointer(addr)
-	memzero(pointer, size)
+	zero_new_alloc(pointer, size)
 	return pointer
 }
 

src/runtime/zero_new_alloc.go

@@ -0,0 +1,12 @@
+//go:build gc.leaking && (baremetal || nintendoswitch)
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+//go:inline
+func zero_new_alloc(ptr unsafe.Pointer, size uintptr) {
+	memzero(ptr, size)
+}

src/runtime/zero_new_alloc_noop.go

@@ -0,0 +1,14 @@
+//go:build gc.leaking && !baremetal && !nintendoswitch
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+//go:inline
+func zero_new_alloc(ptr unsafe.Pointer, size uintptr) {
+	// Wasm linear memory is initialized to zero by default, so
+	// there's no need to do anything.  This is also the case for
+	// fresh-allocated memory from an mmap() system call.
+}