Add features to enable LTO

swift/internal/compiling.bzl

@@ -60,6 +60,7 @@ load(
     "SWIFT_FEATURE_FASTBUILD",
     "SWIFT_FEATURE_FILE_PREFIX_MAP",
     "SWIFT_FEATURE_FULL_DEBUG_INFO",
+    "SWIFT_FEATURE_FULL_LTO",
     "SWIFT_FEATURE_GLOBAL_MODULE_CACHE_USES_TMPDIR",
     "SWIFT_FEATURE_INDEX_WHILE_BUILDING",
     "SWIFT_FEATURE_LAYERING_CHECK",
@@ -75,6 +76,7 @@ load(
     "SWIFT_FEATURE_SUPPORTS_LIBRARY_EVOLUTION",
     "SWIFT_FEATURE_SUPPORTS_SYSTEM_MODULE_FLAG",
     "SWIFT_FEATURE_SYSTEM_MODULE",
+    "SWIFT_FEATURE_THIN_LTO",
     "SWIFT_FEATURE_TREAT_WARNINGS_AS_ERRORS",
     "SWIFT_FEATURE_USE_C_MODULES",
     "SWIFT_FEATURE_USE_EXPLICIT_SWIFT_MODULE_MAP",
@@ -259,7 +261,11 @@ def compile_action_configs(
             configurators = [
                 swift_toolchain_config.add_arg("-emit-bc"),
             ],
-            features = [SWIFT_FEATURE_EMIT_BC],
+            features = [
+                [SWIFT_FEATURE_EMIT_BC],
+                [SWIFT_FEATURE_FULL_LTO],
+                [SWIFT_FEATURE_THIN_LTO],
+            ],
         ),
 
         # Add the single object file or object file map, whichever is needed.
@@ -399,6 +405,24 @@ def compile_action_configs(
             configurators = [_constant_value_extraction_configurator],
             features = [SWIFT_FEATURE__SUPPORTS_CONST_VALUE_EXTRACTION],
         ),
+
+        # Link Time Optimization (LTO)
+        swift_toolchain_config.action_config(
+            actions = [swift_action_names.COMPILE],
+            configurators = [
+                swift_toolchain_config.add_arg("-lto=llvm-thin"),
+            ],
+            features = [SWIFT_FEATURE_THIN_LTO],
+            not_features = [SWIFT_FEATURE_FULL_LTO],
+        ),
+        swift_toolchain_config.action_config(
+            actions = [swift_action_names.COMPILE],
+            configurators = [
+                swift_toolchain_config.add_arg("-lto=llvm-full"),
+            ],
+            features = [SWIFT_FEATURE_FULL_LTO],
+            not_features = [SWIFT_FEATURE_THIN_LTO],
+        ),
     ]
 
     if generated_header_rewriter:
@@ -3142,10 +3166,22 @@ def _declare_compile_outputs(
 
     # If enabled the compiler will emit LLVM BC files instead of Mach-O object
     # files.
+    # LTO implies emitting LLVM BC files, too
+
+    full_lto_enabled = is_feature_enabled(
+        feature_configuration = feature_configuration,
+        feature_name = SWIFT_FEATURE_FULL_LTO,
+    )
+
+    thin_lto_enabled = is_feature_enabled(
+        feature_configuration = feature_configuration,
+        feature_name = SWIFT_FEATURE_THIN_LTO,
+    )
+
     emits_bc = is_feature_enabled(
         feature_configuration = feature_configuration,
         feature_name = SWIFT_FEATURE_EMIT_BC,
-    )
+    ) or full_lto_enabled or thin_lto_enabled
 
     if not output_nature.emits_multiple_objects:
         # If we're emitting a single object, we don't use an object map; we just

swift/internal/feature_names.bzl

@@ -362,3 +362,9 @@ SWIFT_FEATURE_ADD_TARGET_NAME_TO_OUTPUT = "swift.add_target_name_to_output"
 # extraction (Swift 5.8 and above). Users should never manually enable, disable, or query this
 # feature.
 SWIFT_FEATURE__SUPPORTS_CONST_VALUE_EXTRACTION = "swift._supports_const_value_extraction"
+
+# Enable thin LTO and update output-file-map correctly
+SWIFT_FEATURE_THIN_LTO = "swift.thin_lto"
+
+# Enable full LTO and update output-file-map correctly
+SWIFT_FEATURE_FULL_LTO = "swift.full_lto"

test/compiler_arguments_tests.bzl

@@ -14,6 +14,22 @@ split_test = make_action_command_line_test_rule(
     },
 )
 
+thin_lto_test = make_action_command_line_test_rule(
+    config_settings = {
+        "//command_line_option:features": [
+            "swift.thin_lto",
+        ],
+    },
+)
+
+full_lto_test = make_action_command_line_test_rule(
+    config_settings = {
+        "//command_line_option:features": [
+            "swift.full_lto",
+        ],
+    },
+)
+
 def compiler_arguments_test_suite(name):
     """Test suite for various command line flags passed to Swift compiles.
 
@@ -69,6 +85,22 @@ def compiler_arguments_test_suite(name):
         target_under_test = "@build_bazel_rules_swift//test/fixtures/compiler_arguments:lib_package_name",
     )
 
+    thin_lto_test(
+        name = "{}_thin_lto".format(name),
+        expected_argv = ["-lto=llvm-thin"],
+        mnemonic = "SwiftCompile",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/compiler_arguments:bin",
+    )
+
+    full_lto_test(
+        name = "{}_full_lto".format(name),
+        expected_argv = ["-lto=llvm-full"],
+        mnemonic = "SwiftCompile",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/compiler_arguments:bin",
+    )
+
     native.test_suite(
         name = name,
         tags = [name],

test/fixtures/compiler_arguments/BUILD

@@ -34,3 +34,9 @@ swift_test(
     srcs = ["empty.swift"],
     tags = FIXTURE_TAGS,
 )
+
+swift_binary(
+    name = "bin",
+    srcs = ["empty.swift"],
+    tags = FIXTURE_TAGS,
+)

test/output_file_map_tests.bzl

@@ -71,6 +71,22 @@ output_file_map_embed_target_name_bitcode_wmo_test = make_output_file_map_test_r
     },
 )
 
+output_file_map_thin_lto_test = make_output_file_map_test_rule(
+    config_settings = {
+        "//command_line_option:features": [
+            "swift.thin_lto",
+        ],
+    },
+)
+
+output_file_map_full_lto_test = make_output_file_map_test_rule(
+    config_settings = {
+        "//command_line_option:features": [
+            "swift.full_lto",
+        ],
+    },
+)
+
 def output_file_map_test_suite(name):
     """Test suite for `swift_library` generating output file maps.
 
@@ -147,6 +163,28 @@ def output_file_map_test_suite(name):
         target_under_test = "@build_bazel_rules_swift//test/fixtures/debug_settings:simple",
     )
 
+    output_file_map_thin_lto_test(
+        name = "{}_thin_lto".format(name),
+        expected_mapping = {
+            "llvm-bc": "test/fixtures/debug_settings/simple_objs/Empty.swift.bc",
+        },
+        file_entry = "test/fixtures/debug_settings/Empty.swift",
+        output_file_map = "test/fixtures/debug_settings/simple.output_file_map.json",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/debug_settings:simple",
+    )
+
+    output_file_map_full_lto_test(
+        name = "{}_full_lto".format(name),
+        expected_mapping = {
+            "llvm-bc": "test/fixtures/debug_settings/simple_objs/Empty.swift.bc",
+        },
+        file_entry = "test/fixtures/debug_settings/Empty.swift",
+        output_file_map = "test/fixtures/debug_settings/simple.output_file_map.json",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/debug_settings:simple",
+    )
+
     native.test_suite(
         name = name,
         tags = [name],