[SYCL] Return nullptr when allocation size is zero in usm allocator (#…

…12765)

Currently, usm allocator throws when the allocation size is zero.
However, this behavior is not aligned with that of std::allocator.
Refer KhronosGroup/SYCL-Docs#355 for
discussion regarding this. The spec says that the allocation functions
must succeed when the size is zero. The value returned in this case is
unspecified (it can either be a NULL pointer or a non-NULL pointer)

This PR makes USM allocator return a null pointer when the allocation
size is zero.

sycl/include/sycl/usm/usm_allocator.hpp

@@ -71,6 +71,9 @@ class usm_allocator {
   T *allocate(size_t NumberOfElements, const detail::code_location CodeLoc =
                                            detail::code_location::current()) {
 
+    if (!NumberOfElements)
+      return nullptr;
+
     auto Result = reinterpret_cast<T *>(
         aligned_alloc(getAlignment(), NumberOfElements * sizeof(value_type),
                       MDevice, MContext, AllocKind, MPropList, CodeLoc));

sycl/test-e2e/USM/usm_allocator_zero_allocation.cpp

@@ -0,0 +1,30 @@
+// RUN: %{build} -o %t.out
+// RUN: %{run} %t.out
+
+#include <sycl/sycl.hpp>
+
+using namespace sycl;
+
+template <usm::alloc alloc_kind> void test(queue &q) {
+  sycl::usm_allocator<int, alloc_kind> ua(q);
+  int *p = ua.allocate(0);
+
+  assert(!p && "Our implementation of usm_allocator is expected to return a "
+               "null pointer when allocation size is zero.");
+
+  ua.deallocate(p, 0);
+}
+
+int main() {
+  queue q;
+  auto dev = q.get_device();
+
+  if (dev.has(aspect::usm_host_allocations)) {
+    test<usm::alloc::host>(q);
+  }
+  if (dev.has(aspect::usm_shared_allocations)) {
+    test<usm::alloc::shared>(q);
+  }
+
+  return 0;
+}