[llvm] [SYCL] Add platform enumeration and info query using liboffload (PR #166927)

[Andrei Elovikov via llvm-commits]

================
@@ -15,15 +15,96 @@
 #ifndef _LIBSYCL___IMPL_PLATFORM_HPP
 #define _LIBSYCL___IMPL_PLATFORM_HPP
 
+#include <sycl/__impl/backend.hpp>
 #include <sycl/__impl/detail/config.hpp>
+#include <sycl/__impl/detail/obj_base.hpp>
+#include <sycl/__impl/info/platform.hpp>
+
+#include <memory>
+#include <vector>
 
 _LIBSYCL_BEGIN_NAMESPACE_SYCL
 
-class _LIBSYCL_EXPORT platform {
+namespace detail {
+class platform_impl;
+} // namespace detail
+
+// 4.6.2. Platform class
+class _LIBSYCL_EXPORT platform
+    : public detail::ObjBase<detail::platform_impl, platform> {
 public:
-  /// Constructs a SYCL platform which contains the default device.
-  platform();
+  /// Constructs a platform object that is a copy of the platform which contains
+  /// the device returned by default_selector_v.
+  // platform();
+
+  /// Constructs a platform object that is a copy of the platform which contains
+  /// the device that is selected by selector.
+  /// \param DeviceSelectorInstance is SYCL 2020 Device Selector, a simple
+  /// callable taking a device reference and returning an integer rank.
+  // template <typename DeviceSelector>
+  // explicit platform(const DeviceSelector& DeviceSelectorInstance);
+
+  /// Returns the backend associated with this platform.
+  ///
+  /// \return the backend associated with this platform
+  backend get_backend() const noexcept;
+
+  /// Returns all SYCL devices associated with this platform.
+  ///
+  /// If there are no devices that match given device
+  /// type, resulting vector is empty.
+  ///
+  /// \param DeviceType is a SYCL device type.
+  /// \return a vector of SYCL devices.
+  // std::vector<device>
+  //     get_devices(info::device_type DeviceType = info::device_type::all)
+  //     const;
+
+  /// Queries this SYCL platform for info.
+  ///
+  /// The return type depends on information being queried.
+  template <typename Param>
+  typename detail::is_platform_info_desc<Param>::return_type get_info() const {
+    return get_info_impl<Param>();
+  }
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aelovikov-intel wrote:

And why is `get_info_impl` needed at all? I'd expect it's just the leftover from our downstream `_GLIBCXX_USE_CXX11_ABI == 0` support.

https://github.com/llvm/llvm-project/pull/166927