Make Linker.backend a classmethod (#1910)

* feat(core): convert Linker.backend from property to classmethod

Allows querying the linking backend without constructing a Linker
instance — useful for dispatching on input format (PTX vs. LTOIR)
before linking.

Updates existing call sites (Program init, test_linker) to use the
new invocation form Linker.backend().

* test(core): add tests for Linker.backend classmethod

Covers classmethod invocation (Linker.backend() without an instance),
memoisation flag handling, probe-on-first-use, and non-property
attribute semantics.

* docs(core): add 1.0.0 release notes for Linker.backend change

Breaking change: Linker.backend is now a classmethod, so call sites
must use parens: Linker.backend().

---------

Co-authored-by: Leo Fang <leof@nvidia.com>

Author: cpcloud

cuda_core/cuda/core/_linker.pyx

@@ -168,10 +168,23 @@ cdef class Linker:
         else:
             return as_py(self._culink_handle)
 
-    @property
-    def backend(self) -> CompilerBackendType:
-        """Return this Linker instance's underlying :class:`CompilerBackendType`."""
-        return CompilerBackendType.NVJITLINK if self._use_nvjitlink else CompilerBackendType.DRIVER
+    @classmethod
+    def which_backend(cls) -> CompilerBackendType:
+        """Return which linking backend will be used.
+
+        Returns :attr:`~CompilerBackendType.NVJITLINK` when the nvJitLink
+        library is available and meets the minimum version requirement,
+        otherwise :attr:`~CompilerBackendType.DRIVER`.
+
+        .. note::
+
+            Prefer letting :class:`Linker` decide. Query ``which_backend()``
+            only when you need to dispatch based on input format (for
+            example: choose PTX vs. LTOIR before constructing a
+            ``Linker``). The returned value names an implementation
+            detail whose support matrix may shift across CTK releases.
+        """
+        return CompilerBackendType.DRIVER if _decide_nvjitlink_or_driver() else CompilerBackendType.NVJITLINK
 
 
 # =============================================================================

cuda_core/cuda/core/_program.pyx

@@ -785,7 +785,7 @@ cdef inline int Program_init(Program self, object code, str code_type, object op
         self._linker = Linker(
             ObjectCode._init(code_bytes, code_type), options=_translate_program_options(options)
         )
-        self._backend = str(self._linker.backend)
+        self._backend = str(Linker.which_backend())
 
     elif code_type == "nvvm":
         _get_nvvm_module()  # Validate NVVM availability

cuda_core/docs/source/release/1.0.0-notes.rst

@@ -120,6 +120,12 @@ Breaking changes
     ``CUgraphConditionalHandle`` value. Previously, ``.handle`` had to be
     extracted explicitly.
 
+- :meth:`Linker.which_backend` is now a classmethod instead of the former
+  ``backend`` instance property. Call sites must use ``Linker.which_backend()``
+  (with parentheses) instead of ``linker.backend``. This allows querying the
+  linking backend without constructing a ``Linker`` instance — for example, to
+  choose between PTX and LTOIR input before linking.
+
 - :attr:`DeviceMemoryResource.peer_accessible_by` now returns a
   :class:`collections.abc.MutableSet` of :obj:`~_device.Device` objects instead
   of a sorted ``tuple[int, ...]``. The property setter is unchanged.

cuda_core/tests/test_linker.py

@@ -2,6 +2,8 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
+import inspect
+
 import pytest
 
 from cuda.core import Device, Linker, LinkerOptions, Program, ProgramOptions, _linker
@@ -92,7 +94,7 @@ def test_linker_init(compile_ptx_functions, options):
     linker = Linker(*compile_ptx_functions, options=options)
     object_code = linker.link("cubin")
     assert isinstance(object_code, ObjectCode)
-    assert linker.backend == ("driver" if is_culink_backend else "nvJitLink")
+    assert Linker.which_backend() == ("driver" if is_culink_backend else "nvJitLink")
 
 
 def test_linker_init_invalid_arch(compile_ptx_functions):
@@ -242,3 +244,39 @@ def test_linker_options_nvjitlink_options_as_str():
     assert f"-arch={ARCH}" in options
     assert "-g" in options
     assert "-lineinfo" in options
+
+
+class TestWhichBackendClassmethod:
+    def test_which_backend_returns_nvjitlink(self, monkeypatch):
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", True)
+        assert Linker.which_backend() == "nvJitLink"
+
+    def test_which_backend_returns_driver(self, monkeypatch):
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", False)
+        assert Linker.which_backend() == "driver"
+
+    def test_which_backend_invokes_probe_when_not_memoised(self, monkeypatch):
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", None)
+        called = []
+
+        def fake_decide():
+            called.append(True)
+            return False  # False = not falling back to driver = nvJitLink
+
+        monkeypatch.setattr(_linker, "_decide_nvjitlink_or_driver", fake_decide)
+        result = Linker.which_backend()
+        assert result == "nvJitLink"
+        assert called, "_decide_nvjitlink_or_driver was not called"
+
+    def test_which_backend_is_classmethod(self):
+        attr = inspect.getattr_static(Linker, "which_backend")
+        assert isinstance(attr, classmethod)
+
+    def test_which_backend_is_not_property(self):
+        """which_backend is a classmethod, not a property.
+
+        This is an intentional breaking change from the prior ``backend`` property API.
+        All call sites must use parens: ``Linker.which_backend()``.
+        """
+        attr = inspect.getattr_static(Linker, "which_backend")
+        assert not isinstance(attr, property)