[Bazel] Update README with examples

[chandlerc]

LGTM, two minor suggestions below.

utils/bazel/examples/submodule/WORKSPACE

5

This should be using a git submodule. I would guess?

8

Maybe a comment clarifying that this isn't constrained?

[GMNGeoffrey]

• Address comments

utils/bazel/.gitignore

 # Bazel artifacts
-/bazel-*
+**/bazel-*
 
 # Per-user bazelrc files
 user.bazelrc

utils/bazel/README.md

 The LLVM, MLIR, and Clang subprojects have configurations for Linux (Clang and
 GCC), Mac (Clang and GCC), and Windows (MSVC). Configuration options that are
 platform-specific are selected for in defines. Many are also hardcoded to the
 values currently used by all supported configurations. If there is a
 configuration you'd like to use that isn't supported, please send a patch.
 
 # Usage
 
-To use in dependent projects using Bazel, you can import LLVM (e.g. as a
-submodule or using `http_archive`) and then use the provided configuration rule.
-
-FIXME: This needs to be updated to a commit that exists once such a commit
-exists.
-FIXME: It shouldn't be necessary to configure `http_archive` twice for the same
-archive (though caching means this isn't too expensive).
-
-```starlark
-load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
-load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
-
-LLVM_COMMIT = "0a1f0ee78122fc0642e8a1a18e1b2bc89c813387"
-
-LLVM_SHA256 = "4f59737ccfdad2cfb4587d796ce97c1eb5433de7ea0f57f248554b83e92d81d2"
-
-http_archive(
-    name = "llvm-project-raw",
-    build_file_content = "#empty",
-    sha256 = LLVM_SHA256,
-    strip_prefix = "llvm-project-" + LLVM_COMMIT,
-    urls = ["https://github.com/llvm/llvm-project/archive/{commit}.tar.gz".format(commit = LLVM_COMMIT)],
-)
-
-http_archive(
-    name = "llvm-bazel",
-    sha256 = LLVM_SHA256,
-    strip_prefix = "llvm-project-{}/utils/bazel".format(LLVM_COMMIT),
-    urls = ["https://github.com/llvm/llvm-project/archive/{commit}.tar.gz".format(commit = LLVM_COMMIT)],
-)
-
-load("@llvm-bazel//:configure.bzl", "llvm_configure")
-
-llvm_configure(
-    name = "llvm-project",
-    src_path = ".",
-    src_workspace = "@llvm-project-raw//:WORKSPACE",
-)
-
-load("@llvm-bazel//:terminfo.bzl", "llvm_terminfo_system")
-
-maybe(
-    llvm_terminfo_system,
-    name = "llvm_terminfo",
-)
-
-load("@llvm-bazel//:zlib.bzl", "llvm_zlib_system")
-
-maybe(
-    llvm_zlib_system,
-    name = "llvm_zlib",
-)
-```
+To use in dependent projects using Bazel, you can import LLVM and then use the
+provided configuration rule. See example usage in the `examples/` directory.

utils/bazel/examples/http_archive/WORKSPACE

+# This file is licensed under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+"""An example WORKSPACE for configuring LLVM using http_archive."""
+
+workspace(name = "http_archive_example")
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+# Replace with the LLVM commit you want to use.
+LLVM_COMMIT = "09ac97ce350316b95b8cddb796d52f71b6f68296"
+
+# The easiest way to calculate this for a new commit is to set it to empty and
+# then run a bazel build and it will report the digest necessary to cache the
+# archive and make the build reproducible.
+LLVM_SHA256 = "2fb1aa06d12f8db349a27426cb0ced062987c5c2a75143c69f4284929e2750ff"
+
+# FIXME: It shouldn't be necessary to use http_archive twice here. Caching
+# should mean that this isn't too expensive though.
+
+http_archive(
+    name = "llvm-project-raw",
+    build_file_content = "#empty",
+    sha256 = LLVM_SHA256,
+    strip_prefix = "llvm-project-" + LLVM_COMMIT,
+    urls = ["https://github.com/llvm/llvm-project/archive/{commit}.tar.gz".format(commit = LLVM_COMMIT)],
+)
+
+http_archive(
+    name = "llvm-bazel",
+    sha256 = LLVM_SHA256,
+    strip_prefix = "llvm-project-{}/utils/bazel".format(LLVM_COMMIT),
+    urls = ["https://github.com/llvm/llvm-project/archive/{commit}.tar.gz".format(commit = LLVM_COMMIT)],
+)
+
+load("@llvm-bazel//:configure.bzl", "llvm_configure", "llvm_disable_optional_support_deps")
+
+llvm_configure(
+    name = "llvm-project",
+    src_path = ".",
+    src_workspace = "@llvm-project-raw//:WORKSPACE",
+)
+
+# Disables optional dependencies for Support like zlib and terminfo. You may
+# instead want to configure them using the macros in the corresponding bzl
+# files.
+llvm_disable_optional_support_deps()

utils/bazel/examples/submodule/WORKSPACE

+# This file is licensed under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+"""An example WORKSPACE for configuring LLVM using http_archive."""
+
+workspace(name = "submodule_example")
+
+SUBMODULE_PATH = "third_party/llvm-project"
+
+local_repository(
+    name = "llvm-bazel",
+    path = SUBMODULE_PATH + "/utils/bazel",
+)
+
+llvm_configure(
+    name = "llvm-project",
+    src_path = SUBMODULE_PATH,
+    src_workspace = "@submodule_example//:WORKSPACE",
+)
+
+# Disables optional dependencies for Support like zlib and terminfo. You may
+# instead want to configure them using the macros in the corresponding bzl
+# files.
+llvm_disable_optional_support_deps()