[5/5] support C++20 Modules, support two phase compilation

src/main/java/com/google/devtools/build/lib/rules/cpp/CompileBuildVariables.java

@@ -66,6 +66,8 @@ public enum CompileBuildVariables {
   CPP_MODULE_MODMAP_FILE("cpp_module_modmap_file"),
   /** Variable for the c++20 module output file name. */
   CPP_MODULE_OUTPUT_FILE("cpp_module_output_file"),
+  /** Variable for the c++20 module with 2-phase compilation. */
+  CPP_MODULES_WITH_TWO_PHASE_COMPILATION("cpp_modules_with_two_phase_compilation"),
   /** Variable for the module map file name. */
   MODULE_MAP_FILE("module_map_file"),
   /** Variable for the dependent module map file name. */

src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileAction.java

@@ -664,11 +664,23 @@ public NestedSet<Artifact> discoverInputs(ActionExecutionContext actionExecution
         // This is necessary because the inputs that can be pruned by .d file parsing must be
         // returned from discoverInputs() and they cannot be in mandatoryInputs. Thus, even with
         // include scanning turned off, we pretend that we "discover" these headers.
-        additionalInputs =
-            NestedSetBuilder.fromNestedSet(ccCompilationContext.getDeclaredIncludeSrcs())
-                .addTransitive(additionalPrunableHeaders)
-                .addAll(usedCpp20Modules)
-                .build();
+
+        // Support C++20 Modules with two-phase compilation
+        // For compiling C++20 Modules files to object files (.pcm -> .o) 
+        // headers are not needed as the module interface is already compiled into the pcm file.
+        if (actionName.equals(CppActionNames.CPP20_MODULE_CODEGEN)) {
+          additionalInputs =
+              NestedSetBuilder.<Artifact>stableOrder()
+                  .addAll(usedCpp20Modules)
+                  .build();
+        }
+        else {
+          additionalInputs =
+              NestedSetBuilder.fromNestedSet(ccCompilationContext.getDeclaredIncludeSrcs())
+                  .addTransitive(additionalPrunableHeaders)
+                  .addAll(usedCpp20Modules)
+                  .build();
+        }
         if (needsIncludeValidation) {
           verifyActionIncludePaths(systemIncludeDirs, siblingLayout);
         }
@@ -1557,6 +1569,12 @@ public ActionResult execute(ActionExecutionContext actionExecutionContext)
     }
 
     if (getDotdFile() == null) {
+      if (discoversInputs() && isCpp20ModuleCompilationAction(actionName) && usedCpp20Modules != null) {
+        NestedSet<Artifact> discoveredInputs = NestedSetBuilder.<Artifact>stableOrder()
+            .addAll(usedCpp20Modules)
+            .build();
+        updateActionInputs(discoveredInputs);
+      }
       return ActionResult.create(spawnResults);
     }
 

src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileActionBuilder.java

@@ -367,7 +367,11 @@ NestedSet<Artifact> buildMandatoryInputs() throws EvalException {
     if (ccToolchain.getGrepIncludes() != null) {
       realMandatoryInputsBuilder.add(ccToolchain.getGrepIncludes());
     }
-    if (!getShouldScanIncludes() && dotdFile == null && !shouldParseShowIncludes()) {
+    // Support C++20 Modules with two-phase compilation
+    // For compiling C++20 Modules files to object files (.pcm -> .o) 
+    // headers are not needed as the module interface is already compiled into the pcm file.
+    if (!getShouldScanIncludes() && dotdFile == null && !shouldParseShowIncludes()
+        && !actionName.equals(CppActionNames.CPP20_MODULE_CODEGEN)) {
       realMandatoryInputsBuilder.addTransitive(ccCompilationContext.getDeclaredIncludeSrcs());
       realMandatoryInputsBuilder.addTransitive(additionalPrunableHeaders);
     }

src/main/starlark/builtins_bzl/common/cc/compile/compile.bzl

@@ -637,6 +637,233 @@ def _create_gen_modmap_action(
         toolchain = None,
     )
 
+def _create_compile_module_interface_to_module_file_only_action(
+        *,
+        action_construction_context,
+        cc_compilation_context,
+        label,
+        source_label,
+        source_artifact,
+        output_name,
+        cc_toolchain,
+        feature_configuration,
+        configuration,
+        cpp_configuration,
+        language,
+        conlyopts,
+        copts,
+        cxxopts,
+        copts_filter,
+        common_compile_variables,
+        fdo_build_variables,
+        cpp_module_map,
+        enable_coverage,
+        additional_include_scanning_roots,
+        use_pic,
+        output_file,
+        additional_build_variables = {},
+        action_name = None,
+        additional_outputs = [],
+        module_files = depset(),
+        modmap_file = None,
+        modmap_input_file = None):
+    output_pic_nopic_name = output_name
+    if use_pic:
+        output_pic_nopic_name = _cc_internal.get_artifact_name_for_category(
+            cc_toolchain = cc_toolchain,
+            category = artifact_category.PIC_FILE,
+            output_name = output_name,
+        )
+    dotd_file = _maybe_declare_dotd_file(
+        ctx = action_construction_context,
+        label = label,
+        source_artifact = source_artifact,
+        category = artifact_category.CPP_MODULE,
+        output_name = output_pic_nopic_name,
+        cc_toolchain = cc_toolchain,
+        language = language,
+        configuration = configuration,
+        feature_configuration = feature_configuration,
+    )
+    complete_copts = get_copts(
+        language = language,
+        cpp_configuration = cpp_configuration,
+        source_file = source_artifact,
+        copts = copts,
+        conlyopts = conlyopts,
+        cxxopts = cxxopts,
+        label = source_label,
+        # Treat C++20 module interfaces as C++ source files regardless of their extension.
+        override_extension = extensions.CC_SOURCE[0],
+    )
+    compile_variables = get_specific_compile_build_variables(
+        source_file = source_artifact,
+        output_file = output_file,
+        code_coverage_enabled = enable_coverage,
+        gcno_file = None,
+        dwo_file = None,
+        using_fission = False,
+        lto_indexing_file = None,
+        user_compile_flags = complete_copts,
+        dotd_file = dotd_file,
+        diagnostics_file = None,
+        use_pic = use_pic,
+        cpp_module_map = cpp_module_map,
+        feature_configuration = feature_configuration,
+        direct_module_maps = cc_compilation_context._direct_module_maps,
+        fdo_build_variables = fdo_build_variables,
+        additional_build_variables = additional_build_variables,
+    )
+    module_args = {
+        "additional_outputs": additional_outputs,
+        "module_files": module_files,
+        "modmap_file": modmap_file,
+        "modmap_input_file": modmap_input_file,
+    }
+    _cc_internal.create_cc_compile_action(
+        action_construction_context = action_construction_context,
+        cc_compilation_context = cc_compilation_context,
+        cc_toolchain = cc_toolchain,
+        configuration = configuration,
+        copts_filter = copts_filter,
+        feature_configuration = feature_configuration,
+        additional_include_scanning_roots = additional_include_scanning_roots,
+        source = source_artifact,
+        output_file = output_file,
+        diagnostics_file = None,
+        dotd_file = dotd_file,
+        gcno_file = None,
+        dwo_file = None,
+        use_pic = use_pic,
+        lto_indexing_file = None,
+        action_name = action_name,
+        compile_build_variables = _cc_internal.combine_cc_toolchain_variables(
+            common_compile_variables,
+            compile_variables,
+        ),
+        needs_include_validation = _starlark_cc_semantics.needs_include_validation(language),
+        toolchain_type = _starlark_cc_semantics.toolchain,
+        **module_args
+    )
+
+def _create_compile_module_interface_action(
+        *,
+        action_construction_context,
+        cc_compilation_context,
+        label,
+        source_label,
+        source_artifact,
+        output_name,
+        outputs,
+        cc_toolchain,
+        feature_configuration,
+        configuration,
+        cpp_configuration,
+        language,
+        conlyopts,
+        copts,
+        cxxopts,
+        copts_filter,
+        common_compile_variables,
+        fdo_build_variables,
+        output_category,
+        cpp_module_map,
+        add_object,
+        enable_coverage,
+        generate_dwo,
+        bitcode_output,
+        fdo_context,
+        auxiliary_fdo_inputs,
+        additional_compilation_inputs,
+        additional_include_scanning_roots,
+        use_pic,
+        module_files,
+        modmap_file,
+        modmap_input_file,
+        module_file):
+    additional_build_variables = {
+        "cpp_module_modmap_file": modmap_file,
+    }
+    additional_outputs = []
+    current_action_name = ACTION_NAMES.cpp20_module_compile
+    action_source_artifact = source_artifact
+    if feature_configuration.is_enabled("cpp_modules_with_two_phase_compilation"):
+        additional_build_variables["cpp_modules_with_two_phase_compilation"] = "1"
+        additional_outputs = [module_file]
+        _create_compile_module_interface_to_module_file_only_action(
+            action_construction_context = action_construction_context,
+            cc_compilation_context = cc_compilation_context,
+            label = label,
+            source_label = source_label,
+            source_artifact = action_source_artifact,
+            output_name = output_name,
+            cc_toolchain = cc_toolchain,
+            feature_configuration = feature_configuration,
+            configuration = configuration,
+            cpp_configuration = cpp_configuration,
+            language = language,
+            conlyopts = conlyopts,
+            copts = copts,
+            cxxopts = cxxopts,
+            copts_filter = copts_filter,
+            common_compile_variables = common_compile_variables,
+            fdo_build_variables = fdo_build_variables,
+            enable_coverage = enable_coverage,
+            cpp_module_map = cpp_module_map,
+            additional_include_scanning_roots = additional_include_scanning_roots,
+            use_pic = use_pic,
+            additional_build_variables = additional_build_variables,
+            action_name = current_action_name,
+            additional_outputs = additional_outputs,
+            module_files = module_files,
+            modmap_file = modmap_file,
+            modmap_input_file = modmap_input_file,
+            output_file = module_file,
+        )
+        current_action_name = ACTION_NAMES.cpp20_module_codegen
+        action_source_artifact = module_file
+        additional_outputs = []
+    else:
+        additional_build_variables["cpp_module_output_file"] = module_file
+        additional_outputs = [module_file]
+    _create_compile_source_action(
+        action_construction_context = action_construction_context,
+        cc_compilation_context = cc_compilation_context,
+        label = label,
+        source_label = source_label,
+        source_artifact = action_source_artifact,
+        output_name = output_name,
+        outputs = outputs,
+        cc_toolchain = cc_toolchain,
+        feature_configuration = feature_configuration,
+        configuration = configuration,
+        cpp_configuration = cpp_configuration,
+        language = language,
+        conlyopts = conlyopts,
+        copts = copts,
+        cxxopts = cxxopts,
+        copts_filter = copts_filter,
+        common_compile_variables = common_compile_variables,
+        fdo_build_variables = fdo_build_variables,
+        output_category = output_category,
+        cpp_module_map = cpp_module_map,
+        add_object = add_object,
+        enable_coverage = enable_coverage,
+        generate_dwo = generate_dwo,
+        bitcode_output = bitcode_output,
+        fdo_context = fdo_context,
+        auxiliary_fdo_inputs = auxiliary_fdo_inputs,
+        additional_compilation_inputs = additional_compilation_inputs,
+        additional_include_scanning_roots = additional_include_scanning_roots,
+        use_pic = use_pic,
+        additional_build_variables = additional_build_variables,
+        action_name = current_action_name,
+        additional_outputs = additional_outputs,
+        module_files = module_files,
+        modmap_file = modmap_file,
+        modmap_input_file = modmap_input_file,
+    )
+
 def _create_cc_compile_actions_with_cpp20_module_helper(
         *,
         actions,
@@ -813,7 +1040,7 @@ def _create_cc_compile_actions_with_cpp20_module_helper(
         )
         compiled_basenames.add(_basename_without_extension(source_artifact))
 
-        _create_compile_source_action(
+        _create_compile_module_interface_action(
             action_construction_context = action_construction_context,
             cc_compilation_context = cc_compilation_context,
             label = label,
@@ -843,15 +1070,10 @@ def _create_cc_compile_actions_with_cpp20_module_helper(
             additional_compilation_inputs = additional_compilation_inputs,
             additional_include_scanning_roots = additional_include_scanning_roots,
             use_pic = use_pic,
-            additional_build_variables = {
-                "cpp_module_output_file": module_file,
-                "cpp_module_modmap_file": modmap_file,
-            },
-            action_name = ACTION_NAMES.cpp20_module_compile,
-            additional_outputs = [module_file],
             module_files = all_other_module_files,
             modmap_file = modmap_file,
             modmap_input_file = modmap_input_file,
+            module_file = module_file,
         )
 
     all_module_files = depset(direct_module_files, transitive = [transitive_module_files])

src/test/shell/bazel/cc_integration_test.sh

@@ -2291,6 +2291,15 @@ EOF
   bazel clean || fail "Expected clean success"
   bazel build //:main --experimental_cpp_modules --repo_env=CC=clang --copt=-std=c++20 --disk_cache=disk &> $TEST_log || fail "Expected build C++20 Modules success with compiler 'clang'"
   expect_log "17 disk cache hit"
+
+  # Test with two-phase compilation. enable the following tests after rules_cc updated.
+  bazel build //:main --experimental_cpp_modules --repo_env=CC=clang --copt=-std=c++20 --features=cpp_modules_with_two_phase_compilation --disk_cache=disk &> $TEST_log || fail "Expected build C++20 Modules success with compiler 'clang'"
+
+  # Verify that the build can hit the cache without action cycles.
+  bazel clean || fail "Expected clean success"
+  bazel build //:main --experimental_cpp_modules --repo_env=CC=clang --copt=-std=c++20 --features=cpp_modules_with_two_phase_compilation --disk_cache=disk &> $TEST_log || fail "Expected build C++20 Modules success with compiler 'clang'"
+  # 3 module interfaces. 17 + 3 = 20
+  expect_log "20 disk cache hit"
 }
 
 function test_external_repo_lto() {