Allow operators to be called outside of the kernel registry

Differential Revision: D67229096

Pull Request resolved: pytorch/executorch#7329

Author: lucylq

shim/xplat/executorch/codegen/codegen.bzl

@@ -367,7 +367,7 @@ def copy_portable_header_files(name):
         default_outs = ["."],
     )
 
-def build_portable_lib(name, oplist_header_name, feature = None):
+def build_portable_lib(name, oplist_header_name, feature = None, expose_operator_symbols = False):
     """Build portable lib from source. We build from source so that the generated header file, 
     selected_op_variants.h, can be used to selectively build the lib for different dtypes.
     """
@@ -389,6 +389,20 @@ def build_portable_lib(name, oplist_header_name, feature = None):
     # Include dtype header.
     portable_header_files["selected_op_variants.h"] = ":{}[selected_op_variants]".format(oplist_header_name)
 
+    # For shared library build, we don't want to expose symbols of
+    # kernel implementation (ex torch::executor::native::tanh_out)
+    # to library users. They should use kernels through registry only.
+    # With visibility=hidden, linker won't expose kernel impl symbols
+    # so it can prune unregistered kernels.
+    # Currently fbcode links all dependent libraries through shared
+    # library, and it blocks users like unit tests to use kernel
+    # implementation directly. So we enable this for xplat only.
+    compiler_flags = ["-Wno-missing-prototypes", "-fvisibility=hidden"]
+    if expose_operator_symbols:
+        # Removing '-fvisibility=hidden' exposes operator symbols.
+        # This allows operators to be called outside of the kernel registry.
+        compiler_flags = ["-Wno-missing-prototypes"]
+
     # Build portable lib.
     runtime.cxx_library(
         name = name,
@@ -398,16 +412,7 @@ def build_portable_lib(name, oplist_header_name, feature = None):
         deps = ["//executorch/kernels/portable/cpu/pattern:all_deps", "//executorch/kernels/portable/cpu/util:all_deps"],
         # header_namespace is only available in xplat. See https://fburl.com/code/we2gvopk
         header_namespace = "executorch/kernels/portable/cpu",
-        compiler_flags = ["-Wno-missing-prototypes"] +
-                         # For shared library build, we don't want to expose symbols of
-                         # kernel implementation (ex torch::executor::native::tanh_out)
-                         # to library users. They should use kernels through registry only.
-                         # With visibility=hidden, linker won't expose kernel impl symbols
-                         # so it can prune unregistered kernels.
-                         # Currently fbcode links all dependent libraries through shared
-                         # library, and it blocks users like unit tests to use kernel
-                         # implementation directly. So we enable this for xplat only.
-                         ["-fvisibility=hidden"],
+        compiler_flags = compiler_flags,
         # WARNING: using a deprecated API to avoid being built into a shared
         # library. In the case of dynamically loading so library we don't want
         # it to depend on other so libraries because that way we have to
@@ -440,7 +445,8 @@ def executorch_generated_lib(
         compiler_flags = [],
         kernel_deps = [],
         dtype_selective_build = False,
-        feature = None):
+        feature = None,
+        expose_operator_symbols = False):
     """Emits 0-3 C++ library targets (in fbcode or xplat) containing code to
     dispatch the operators specified in the provided yaml files.
 
@@ -584,7 +590,7 @@ def executorch_generated_lib(
 
         # Build portable lib.
         portable_lib_name = name + "_portable_lib"
-        build_portable_lib(portable_lib_name, oplist_header_name, feature)
+        build_portable_lib(portable_lib_name, oplist_header_name, feature, expose_operator_symbols)
         portable_lib = [":{}".format(portable_lib_name)]
 
     # Exports headers that declare the function signatures of the C++ functions