From 394211bf88b88c25036429e99894fa7c60eaaace Mon Sep 17 00:00:00 2001
From: Yun Peng <pcloudy@google.com>
Date: Fri, 15 Sep 2017 15:59:14 +0200
Subject: Bazel now can build dynamic library from cc_library

Working towards: https://github.com/bazelbuild/bazel/issues/3311

When building dynamic library on Windows, Bazel builds an import library
and a DLL.

Bazel provides a feature called windows_export_all_symbols, if this
feature is enabled(and no_windows_export_all_symbols is not) for a
cc_library, then Bazel parses object files of that cc_library to generate
a DEF file that will be used during linking time to export symbols from
DLL. This feature can be specified at crosstool, package, target and
command line level.

A few differences from Unix platforms:

1. We don't build the shared library on Windows by default, users have to
specifiy --output_groups=dynamic_library for building dynamic libraries.
This output group is also available on other platforms.

2. By default, cc_test is dynamically linked on Unix, but it will be
statically linked on Windows by default. (meaning the default value of
linkstatic in cc_test is 1 on Windows, and 0 on other platforms)

3. For global data symbols, __declspec(dllimport) must still be used in
source files.

Remaining issues:

1. Extensions for import library and DLL are not correct yet.
2. DLLs are not guaranteed to be available during runtime yet.
3. Diamond problem
If a cc_library A is specified as linkstatic=0, then no dynamic library
will be built for it, so if another cc_library B depends on it, A will
be statically linked into B, and if a cc_binary C depends on B, A will
also be statically linked into C and B will be dynamically linked to C.
This is wrong because A is duplicated in both B and C.
It is essentially a diamond problem describled in C++ Transitive Library.
(https://docs.google.com/document/d/1-tv0_79zGyBoDmaP_pYWaBVUwHUteLpAs90_rUl-VY8/edit?usp=sharing)
Hopefully, we can avoid this by using cc_shared_library rule in future.

Change-Id: I23640d4caf8afe65d60b1522af6368536d7a8408
PiperOrigin-RevId: 168829958
---
 tools/def_parser/BUILD       | 18 ++++++++++++++++++
 tools/def_parser/BUILD.tools | 13 +++++++++++++
 tools/def_parser/no_op.bat   | 17 +++++++++++++++++
 3 files changed, 48 insertions(+)
 create mode 100644 tools/def_parser/BUILD
 create mode 100644 tools/def_parser/BUILD.tools
 create mode 100644 tools/def_parser/no_op.bat

(limited to 'tools/def_parser')

diff --git a/tools/def_parser/BUILD b/tools/def_parser/BUILD
new file mode 100644
index 0000000000..7070bebf00
--- /dev/null
+++ b/tools/def_parser/BUILD
@@ -0,0 +1,18 @@
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+)
+
+# cc_toolchain now implicitly depends on @bazel_tools//tools/def_parser:def_parser
+# We need to make sure @bazel_tools//tools/def_parser:def_parser is not
+# a cc_binary, because otherwise, we'll introduce a cycle in dependency graph:
+# .-> @bazel_tools//tools/def_parser:def_parser (cc_binary)
+# |   cc_toolchain
+# `-- @bazel_tools//tools/def_parser:def_parser (cc_binary)
+
+filegroup(
+    name = "def_parser",
+    srcs = ["no_op.bat"],
+)
diff --git a/tools/def_parser/BUILD.tools b/tools/def_parser/BUILD.tools
new file mode 100644
index 0000000000..bd1b891fa8
--- /dev/null
+++ b/tools/def_parser/BUILD.tools
@@ -0,0 +1,13 @@
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "def_parser",
+    srcs = select({
+      "//src:host_windows": ["def_parser.exe"],
+      "//src:host_windows_msvc": ["def_parser.exe"],
+      "//src:host_windows_msys": ["def_parser.exe"],
+      "//conditions:default": [
+        "no_op.bat",
+      ],
+    }),
+)
diff --git a/tools/def_parser/no_op.bat b/tools/def_parser/no_op.bat
new file mode 100644
index 0000000000..b422585831
--- /dev/null
+++ b/tools/def_parser/no_op.bat
@@ -0,0 +1,17 @@
+:: Copyright 2016 The Bazel Authors. All rights reserved.
+::
+:: Licensed under the Apache License, Version 2.0 (the "License");
+:: you may not use this file except in compliance with the License.
+:: You may obtain a copy of the License at
+::
+::    http://www.apache.org/licenses/LICENSE-2.0
+::
+:: Unless required by applicable law or agreed to in writing, software
+:: distributed under the License is distributed on an "AS IS" BASIS,
+:: WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+:: See the License for the specific language governing permissions and
+:: limitations under the License.
+
+:: Invoke the python script under pydir with the same basename
+@echo OFF
+echo IGNORING: %0 %*
-- 
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