[CONTRIB][CC] Enhance cc.cross_compiler (apache#4817)

* [CONTRIB][CC] Enhance cc.cross_compiler

- Enhance cc.cross_compiler to take str argument.
- Remove cc.build_create_shared_func as it is dupilicated with cross_compiler
- Add examples to cc.cross_compiler

* address review comments

python/tvm/contrib/cc.py

@@ -87,38 +87,23 @@ def get_target_triple():
 create_shared.get_target_triple = get_target_by_dump_machine(
     "g++" if sys.platform == "darwin" or sys.platform.startswith("linux") else None)
 
-def build_create_shared_func(options=None, compile_cmd="g++"):
-    """Build create_shared function with particular default options and compile_cmd.
 
-    Parameters
-    ----------
-    options : List[str]
-        The list of additional options string.
+def cross_compiler(compile_func,
+                   options=None,
+                   output_format=None,
+                   get_target_triple=None):
+    """Create a cross compiler function by specializing compile_func with options.
 
-    compile_cmd : Optional[str]
-        The compiler command.
-
-    Returns
-    -------
-    create_shared_wrapper : Callable[[str, str, Optional[str]], None]
-        A compilation function that can be passed to export_library or to autotvm.LocalBuilder.
-    """
-    def create_shared_wrapper(output, objects, options=options, compile_cmd=compile_cmd):
-        create_shared(output, objects, options, compile_cmd)
-    create_shared_wrapper.output_format = create_shared.output_format
-    create_shared_wrapper.get_target_triple = get_target_by_dump_machine(compile_cmd)
-    return create_shared_wrapper
+    This function can be used to construct compile functions that
+    can be passed to AutoTVM measure or export_library.
 
 
-def cross_compiler(compile_func, base_options=None, output_format="so", get_target_triple=None):
-    """Create a cross compiler function.
-
     Parameters
     ----------
-    compile_func : Callable[[str, str, Optional[str]], None]
+    compile_func : Union[str, Callable[[str, str, Optional[str]], None]]
         Function that performs the actual compilation
 
-    base_options : Optional[List[str]]
+    options : Optional[List[str]]
         List of additional optional string.
 
     output_format : Optional[str]
@@ -131,14 +116,44 @@ def cross_compiler(compile_func, base_options=None, output_format="so", get_targ
     -------
     fcompile : Callable[[str, str, Optional[str]], None]
         A compilation function that can be passed to export_library.
+
+    Examples
+    --------
+    .. code-block:: python
+
+       from tvm.contrib import cc, ndk
+       # export using arm gcc
+       mod = build_runtime_module()
+       mod.export_library(path_dso,
+                          cc.cross_compiler("arm-linux-gnueabihf-gcc"))
+       # specialize ndk compilation options.
+       specialized_ndk = cc.cross_compiler(
+           ndk.create_shared,
+           ["--sysroot=/path/to/sysroot", "-shared", "-fPIC", "-lm"])
+       mod.export_library(path_dso, specialized_ndk)
     """
-    if base_options is None:
-        base_options = []
+    base_options = [] if options is None else options
+    kwargs = {}
+
+    # handle case where compile_func is the name of the cc
+    if isinstance(compile_func, str):
+        kwargs = {"cc" : compile_func}
+        compile_func = create_shared
+
+
     def _fcompile(outputs, objects, options=None):
         all_options = base_options
         if options is not None:
             all_options += options
-        compile_func(outputs, objects, options=all_options)
+        compile_func(outputs, objects, options=all_options, **kwargs)
+
+    if not output_format and hasattr(compile_func, "output_format"):
+        output_format = compile_func.output_format
+    output_format = output_format if output_format else "so"
+
+    if not get_target_triple and hasattr(compile_func, "get_target_triple"):
+        get_target_triple = compile_func.get_target_triple
+
     _fcompile.output_format = output_format
     _fcompile.get_target_triple = get_target_triple
     return _fcompile

tests/python/unittest/test_module_load.py

@@ -113,7 +113,8 @@ def check_device(device):
             raise ValueError("Unsupported platform")
 
         path_dso = temp.relpath("dev_lib.so")
-        f.export_library(path_dso)
+        # test cross compiler function
+        f.export_library(path_dso, cc.cross_compiler("g++"))
 
         f1 = tvm.module.load(path_dso)
         a = tvm.nd.array(np.random.uniform(size=1024).astype(A.dtype), ctx)
@@ -134,8 +135,8 @@ def check_stackvm(device):
         name = "myadd_%s" % device
         f = tvm.build(s, [A, B], device, "stackvm", name=name)
         path_dso = temp.relpath("dev_lib.stackvm")
-        #f.export_library(path_dso)
-        #f1 = tvm.module.load(path_dso)
+        f.export_library(path_dso)
+        f1 = tvm.module.load(path_dso)
         a = tvm.nd.array(np.random.uniform(size=1024).astype(A.dtype), ctx)
         b = tvm.nd.array(np.zeros(1024, dtype=A.dtype), ctx)
         f(a, b)