Actually get rid of Windows hacks in cpp_extensions.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

ghstack-source-id: 77b5ed9aa069925703ede06a23b268084347436f
Pull Request resolved: pytorch/pytorch#31161

torch/utils/cpp_extension.py

@@ -446,18 +446,16 @@ def CppExtension(name, sources, *args, **kwargs):
     include_dirs += include_paths()
     kwargs['include_dirs'] = include_dirs
 
-    if IS_WINDOWS:
-        library_dirs = kwargs.get('library_dirs', [])
-        library_dirs += library_paths()
-        kwargs['library_dirs'] = library_dirs
-
-        libraries = kwargs.get('libraries', [])
-        libraries.append('c10')
-        libraries.append('torch')
-        libraries.append('torch_cpu')
-        libraries.append('torch_python')
-        libraries.append('_C')
-        kwargs['libraries'] = libraries
+    library_dirs = kwargs.get('library_dirs', [])
+    library_dirs += library_paths()
+    kwargs['library_dirs'] = library_dirs
+
+    libraries = kwargs.get('libraries', [])
+    libraries.append('c10')
+    libraries.append('torch')
+    libraries.append('torch_cpu')
+    libraries.append('torch_python')
+    kwargs['libraries'] = libraries
 
     kwargs['language'] = 'c++'
     return setuptools.Extension(name, sources, *args, **kwargs)
@@ -497,14 +495,12 @@ def CUDAExtension(name, sources, *args, **kwargs):
 
     libraries = kwargs.get('libraries', [])
     libraries.append('cudart')
-    if IS_WINDOWS:
-        libraries.append('c10')
-        libraries.append('c10_cuda')
-        libraries.append('torch_cpu')
-        libraries.append('torch_cuda')
-        libraries.append('torch')
-        libraries.append('torch_python')
-        libraries.append('_C')
+    libraries.append('c10')
+    libraries.append('c10_cuda')
+    libraries.append('torch')
+    libraries.append('torch_cpu')
+    libraries.append('torch_cuda')
+    libraries.append('torch_python')
     kwargs['libraries'] = libraries
 
     include_dirs = kwargs.get('include_dirs', [])
@@ -561,12 +557,11 @@ def library_paths(cuda=False):
     '''
     paths = []
 
-    if IS_WINDOWS:
-        here = os.path.abspath(__file__)
-        torch_path = os.path.dirname(os.path.dirname(here))
-        lib_path = os.path.join(torch_path, 'lib')
-
-        paths.append(lib_path)
+    # We need to link against libtorch.so
+    here = os.path.abspath(__file__)
+    torch_path = os.path.dirname(os.path.dirname(here))
+    lib_path = os.path.join(torch_path, 'lib')
+    paths.append(lib_path)
 
     if cuda:
         if IS_WINDOWS:
@@ -933,14 +928,14 @@ def verify_ninja_availability():
 
 
 def _prepare_ldflags(extra_ldflags, with_cuda, verbose):
+    here = os.path.abspath(__file__)
+    torch_path = os.path.dirname(os.path.dirname(here))
+    lib_path = os.path.join(torch_path, 'lib')
+
     if IS_WINDOWS:
         python_path = os.path.dirname(sys.executable)
         python_lib_path = os.path.join(python_path, 'libs')
 
-        here = os.path.abspath(__file__)
-        torch_path = os.path.dirname(os.path.dirname(here))
-        lib_path = os.path.join(torch_path, 'lib')
-
         extra_ldflags.append('c10.lib')
         if with_cuda:
             extra_ldflags.append('c10_cuda.lib')
@@ -949,9 +944,13 @@ def _prepare_ldflags(extra_ldflags, with_cuda, verbose):
             extra_ldflags.append('torch_cuda.lib')
         extra_ldflags.append('torch.lib')
         extra_ldflags.append('torch_python.lib')
-        extra_ldflags.append('_C.lib')
         extra_ldflags.append('/LIBPATH:{}'.format(python_lib_path))
         extra_ldflags.append('/LIBPATH:{}'.format(lib_path))
+    else:
+        extra_ldflags.append('-L{}'.format(lib_path))
+        extra_ldflags.append('-lc10')
+        extra_ldflags.append('-ltorch')
+        extra_ldflags.append('-ltorch_python')
 
     if with_cuda:
         if verbose: