Allow wild pointers in zero-sized memcpy with bulkmemory (#22789)

system/lib/libc/emscripten_memcpy.c

@@ -22,7 +22,7 @@ static void *__memcpy(void *dest, const void *src, size_t n) {
 #elif defined(__wasm_bulk_memory__)
 
 static void *__memcpy(void *restrict dest, const void *restrict src, size_t n) {
-  return _emscripten_memcpy_bulkmem(dest, src, n);
+  return n ? _emscripten_memcpy_bulkmem(dest, src, n) : dest;
 }
 
 #else

test/core/test_memcpy_zero_bytes.c

@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <string.h>
+
+#include <emscripten/heap.h>
+
+int main() {
+  char buf[1024];
+  void* oob_address = (void*)(emscripten_get_heap_size() + 10);
+  memcpy(buf, oob_address, 0);
+  printf("done\n");
+}

test/core/test_memcpy_zero_bytes.out

@@ -0,0 +1 @@
+done

test/test_core.py

@@ -2686,6 +2686,13 @@ def test_copyop(self):
     # (llvm-gcc copies items one by one).
     self.do_core_test('test_copyop.cpp')
 
+  @parameterized({
+    '': ([],),
+    'bulkmem': (['-mbulk-memory'],),
+  })
+  def test_memcpy_zero_bytes(self, args):
+    self.do_core_test('test_memcpy_zero_bytes.c', emcc_args=['-fno-builtin'] + args)
+
   def test_memcpy2(self):
     self.do_core_test('test_memcpy2.c')