Also move default_compile_flags to the top of the crosstool

Currently we move legacy_compile_flags to the top of the crosstool, and the same behavior is needed for migrated crosstools in case of default_compile_flags.

#6861
#5883

RELNOTES: None.
PiperOrigin-RevId: 229339668

site/docs/crosstool-reference.md

@@ -1043,4 +1043,75 @@ conditions.
       be enabled, `--force_pic` cannot be used.
    </td>
   </tr>
+  <tr>
+    <td>
+      <strong><code>static_linking_mode | dynamic_linking_mode</code></strong>
+    </td>
+    <td>Enabled by default based on linking mode.</td>
+  </tr>
+  <tr>
+     <td><strong><code>no_legacy_features</code></strong>
+     </td>
+     <td>
+       Prevents Bazel from adding legacy features to
+       the CROSSTOOL configuration when present. See the complete list of
+       features below.
+     </td>
+    </tr>
 </table>
+
+#### Legacy features patching logic
+
+<p>
+  Bazel does following changes to the features and their order to stay backwards
+  compatible:
+
+  <ul>
+    <li>Moves <code>legacy_compile_flags</code> feature to the top of the toolchain</li>
+    <li>Moves <code>default_compile_flags</code> feature to the top of the toolchain</li>
+    <li>Adds <code>dependency_file</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>pic</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>per_object_debug_info</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>preprocessor_defines</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>includes</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>include_paths</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>fdo_instrument</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>fdo_optimize</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>fdo_prefetch_hints</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>autofdo</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>build_interface_libraries</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>dynamic_library_linker_tool</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>symbol_counts</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>shared_flag</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>linkstamps</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>output_execpath_flags</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>runtime_library_search_directories</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>library_search_directories</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>archiver_flags</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>libraries_to_link</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>force_pic_flags</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>user_link_flags</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>legacy_link_flags</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>static_libgcc</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>fission_support</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>strip_debug_symbols</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>coverage</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>llvm_coverage_map_format</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>gcc_coverage_map_format</code> (if not present) feature to the top of the toolchain</li>
+    <li>Adds <code>fully_static_link</code> (if not present) feature to the bottom of the toolchain</li>
+    <li>Adds <code>user_compile_flags</code> (if not present) feature to the bottom of the toolchain</li>
+    <li>Adds <code>sysroot</code> (if not present) feature to the bottom of the toolchain</li>
+    <li>Adds <code>unfiltered_compile_flags</code> (if not present) feature to the bottom of the toolchain</li>
+    <li>Adds <code>linker_param_file</code> (if not present) feature to the bottom of the toolchain</li>
+    <li>Adds <code>compiler_input_flags</code> (if not present) feature to the bottom of the toolchain</li>
+    <li>Adds <code>compiler_output_flags</code> (if not present) feature to the bottom of the toolchain</li>
+  </ul>
+</p>
+
+This is a long list of features. The plan is to get rid of them once
+[Crosstool in Starlark](https://github.com/bazelbuild/bazel/issues/5380) is
+done. For the curious reader see the implementation in
+[CppActionConfigs](https://source.bazel.build/bazel/+/master:src/main/java/com/google/devtools/build/lib/rules/cpp/CppActionConfigs.java?q=cppactionconfigs&ss=bazel),
+and for production toolchains consider adding `no_legacy_features` to make
+the toolchain more standalone.
+

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

@@ -887,11 +887,24 @@ public CcToolchainConfigInfo ccToolchainConfigInfoFromSkylark(
       // compile command line. 'legacy_compile_flags' feature contains all these flags, and so it
       // needs to appear before other features from {@link CppActionConfigs}.
       if (featureNames.contains(CppRuleClasses.LEGACY_COMPILE_FLAGS)) {
-        legacyFeaturesBuilder.add(
+        Feature legacyCompileFlags =
             featureList.stream()
                 .filter(feature -> feature.getName().equals(CppRuleClasses.LEGACY_COMPILE_FLAGS))
                 .findFirst()
-                .get());
+                .get();
+        if (legacyCompileFlags != null) {
+          legacyFeaturesBuilder.add(legacyCompileFlags);
+        }
+      }
+      if (featureNames.contains(CppRuleClasses.DEFAULT_COMPILE_FLAGS)) {
+        Feature defaultCompileFlags =
+            featureList.stream()
+                .filter(feature -> feature.getName().equals(CppRuleClasses.DEFAULT_COMPILE_FLAGS))
+                .findFirst()
+                .get();
+        if (defaultCompileFlags != null) {
+          legacyFeaturesBuilder.add(defaultCompileFlags);
+        }
       }
 
       CppPlatform platform = targetLibc.equals("macos") ? CppPlatform.MAC : CppPlatform.LINUX;
@@ -907,6 +920,7 @@ public CcToolchainConfigInfo ccToolchainConfigInfoFromSkylark(
       legacyFeaturesBuilder.addAll(
           featureList.stream()
               .filter(feature -> !feature.getName().equals(CppRuleClasses.LEGACY_COMPILE_FLAGS))
+              .filter(feature -> !feature.getName().equals(CppRuleClasses.DEFAULT_COMPILE_FLAGS))
               .collect(ImmutableList.toImmutableList()));
       for (CToolchain.Feature feature :
           CppActionConfigs.getFeaturesToAppearLastInFeaturesList(featureNames)) {

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

@@ -223,6 +223,13 @@ public static Label ccToolchainTypeAttribute(RuleDefinitionEnvironment env) {
    */
   public static final String LEGACY_COMPILE_FLAGS = "legacy_compile_flags";
 
+  /**
+   * A string constant for the default_compile_flags feature. If this feature is present in the
+   * toolchain, and the toolchain doesn't specify no_legacy_features, bazel will move
+   * default_compile_flags before other features from {@link CppActionConfigs}.
+   */
+  public static final String DEFAULT_COMPILE_FLAGS = "default_compile_flags";
+
   /**
    * A string constant for the feature that makes us build per-object debug info files.
    */

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

@@ -405,22 +405,32 @@ public static CToolchain addLegacyFeatures(
           }
         }
 
-        // TODO(b/30109612): Remove fragile legacyCompileFlags shuffle once there are no legacy
-        // crosstools.
-        // Existing projects depend on flags from legacy toolchain fields appearing first on the
-        // compile command line. 'legacy_compile_flags' feature contains all these flags, and so it
-        // needs to appear before other features from {@link CppActionConfigs}.
-        CToolchain.Feature legacyCompileFlagsFeature =
-            toolchain
-                .getFeatureList()
-                .stream()
+      // TODO(b/30109612): Remove fragile legacyCompileFlags shuffle once there are no legacy
+      // crosstools.
+      // Existing projects depend on flags from legacy toolchain fields appearing first on the
+      // compile command line. 'legacy_compile_flags' feature contains all these flags, and so it
+      // needs to appear before other features from {@link CppActionConfigs}.
+      if (featureNames.contains(CppRuleClasses.LEGACY_COMPILE_FLAGS)) {
+        CToolchain.Feature legacyCompileFlags =
+            toolchain.getFeatureList().stream()
                 .filter(feature -> feature.getName().equals(CppRuleClasses.LEGACY_COMPILE_FLAGS))
                 .findFirst()
-                .orElse(null);
-        if (legacyCompileFlagsFeature != null) {
-          toolchainBuilder.addFeature(legacyCompileFlagsFeature);
-          toolchain = removeLegacyCompileFlagsFeatureFromToolchain(toolchain);
+                .get();
+        if (legacyCompileFlags != null) {
+          toolchainBuilder.addFeature(legacyCompileFlags);
         }
+      }
+      if (featureNames.contains(CppRuleClasses.DEFAULT_COMPILE_FLAGS)) {
+        CToolchain.Feature defaultCompileFlags =
+            toolchain.getFeatureList().stream()
+                .filter(feature -> feature.getName().equals(CppRuleClasses.DEFAULT_COMPILE_FLAGS))
+                .findFirst()
+                .get();
+        if (defaultCompileFlags != null) {
+          toolchainBuilder.addFeature(defaultCompileFlags);
+        }
+      }
+      toolchain = removeSpecialFeatureFromToolchain(toolchain);
 
       CppPlatform platform =
           toolchain.getTargetLibc().equals("macosx") ? CppPlatform.MAC : CppPlatform.LINUX;
@@ -452,16 +462,15 @@ public static CToolchain addLegacyFeatures(
     return toolchainBuilder.build();
   }
 
-  private static CToolchain removeLegacyCompileFlagsFeatureFromToolchain(CToolchain toolchain) {
+  private static CToolchain removeSpecialFeatureFromToolchain(CToolchain toolchain) {
     FieldDescriptor featuresFieldDescriptor = CToolchain.getDescriptor().findFieldByName("feature");
     return toolchain
         .toBuilder()
         .setField(
             featuresFieldDescriptor,
-            toolchain
-                .getFeatureList()
-                .stream()
+            toolchain.getFeatureList().stream()
                 .filter(feature -> !feature.getName().equals(CppRuleClasses.LEGACY_COMPILE_FLAGS))
+                .filter(feature -> !feature.getName().equals(CppRuleClasses.DEFAULT_COMPILE_FLAGS))
                 .collect(ImmutableList.toImmutableList()))
         .build();
   }

src/test/java/com/google/devtools/build/lib/rules/cpp/SkylarkCcCommonTest.java

@@ -4267,6 +4267,7 @@ public void testCcToolchainInfoFromSkylarkWithLegacyFeatures() throws Exception
         "                    feature(name = 'custom_feature'),",
         "                    feature(name = 'legacy_compile_flags'),",
         "                    feature(name = 'fdo_optimize'),",
+        "                    feature(name = 'default_compile_flags'),",
         "                ],",
         "                action_configs = [action_config(action_name = 'custom-action')],",
         "                artifact_name_patterns = [artifact_name_pattern(",
@@ -4307,10 +4308,11 @@ public void testCcToolchainInfoFromSkylarkWithLegacyFeatures() throws Exception
             .collect(ImmutableSet.toImmutableSet());
     // fdo_optimize should not be re-added to the list of features by legacy behavior
     assertThat(featureNames).containsNoDuplicates();
-    // legacy_compile_flags should appear first in the list of features
-    assertThat(featureNames.get(0)).isEqualTo("legacy_compile_flags");
+    // legacy_compile_flags should appear first in the list of features, followed by
+    // default_compile_flags.
     assertThat(featureNames)
-        .containsAllOf("legacy_compile_flags", "custom_feature", "fdo_optimize")
+        .containsAllOf(
+            "legacy_compile_flags", "default_compile_flags", "custom_feature", "fdo_optimize")
         .inOrder();
     // assemble is one of the action_configs added as a legacy behavior, therefore it needs to be
     // prepended to the action configs defined by the user.