[build-script] Add option to build lld as part of LLVM

utils/build-script-impl

@@ -1833,16 +1833,8 @@ for host in "${ALL_HOSTS[@]}"; do
                     llvm_enable_projects+=("clang-tools-extra")
                 fi
 
-                # On non-Darwin platforms, build lld so we can always have a
-                # linker that is compatible with the swift we are using to
-                # compile the stdlib.
-                #
-                # This makes it easier to build target stdlibs on systems that
-                # have old toolchains without more modern linker features.
-                if [[ "$(uname -s)" != "Darwin" ]] ; then
-                    if [[ ! "${SKIP_BUILD_LLD}" ]]; then
-                        llvm_enable_projects+=("lld")
-                    fi
+                if [[ ! "${SKIP_BUILD_LLD}" ]]; then
+                    llvm_enable_projects+=("lld")
                 fi
 
                 cmake_options+=(

utils/build_swift/build_swift/driver_arguments.py

@@ -721,6 +721,9 @@ def create_argument_parser():
     option(['--build-libparser-only'], toggle_true('build_libparser_only'),
            help='build only libParser for SwiftSyntax')
 
+    option(['--build-lld'], toggle_true('build_lld'),
+           help='build lld as part of llvm')
+
     option('--skip-build-clang-tools-extra',
            toggle_false('build_clang_tools_extra'),
            default=True,

utils/build_swift/tests/expected_options.py

@@ -71,6 +71,7 @@ EXPECTED_DEFAULTS = {
     'build_lldb': False,
     'build_libcxx': False,
     'build_ninja': False,
+    'build_lld': False,
     'build_osx': True,
     'build_playgroundsupport': False,
     'build_runtime_with_host_compiler': False,
@@ -531,6 +532,7 @@ EXPECTED_OPTIONS = [
     EnableOption('--android'),
     EnableOption('--build-external-benchmarks'),
     EnableOption('--build-ninja'),
+    EnableOption('--build-lld'),
     EnableOption('--build-runtime-with-host-compiler'),
     EnableOption('--build-swift-dynamic-sdk-overlay'),
     EnableOption('--build-swift-dynamic-stdlib'),

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -427,6 +427,20 @@ class BuildScriptInvocation(object):
                 "--llvm-install-components=%s" % args.llvm_install_components
             ]
 
+        # On non-Darwin platforms, build lld so we can always have a
+        # linker that is compatible with the swift we are using to
+        # compile the stdlib.
+        #
+        # This makes it easier to build target stdlibs on systems that
+        # have old toolchains without more modern linker features.
+        #
+        # On Darwin, only build lld if explicitly requested using --build-lld.
+        should_build_lld = (platform.system() != 'Darwin' or args.build_lld)
+        if not should_build_lld:
+            impl_args += [
+                "--skip-build-lld"
+            ]
+
         if not args.clean_libdispatch:
             impl_args += [
                 "--skip-clean-libdispatch"

validation-test/BuildSystem/build_lld.test

@@ -0,0 +1,9 @@
+# RUN: %empty-directory(%t)
+# RUN: mkdir -p %t
+# RUN: SKIP_XCODE_VERSION_CHECK=1 SWIFT_BUILD_ROOT=%t %swift_src_root/utils/build-script --dry-run --build-lld 2>&1 | %FileCheck %s
+
+# REQUIRES: standalone_build
+
+# Check that lld is in LLVM_ENABLE_PROJECTS of the llvm-project/llvm build
+
+# CHECK: '-DLLVM_ENABLE_PROJECTS={{[^']*}}lld{{[^']*}}'{{.*}}llvm-project/llvm{{$}}