Implement marker passes for JSCompiler stage 2 splitting

This CL implements the proposal to use marker passes for splitting
JSCompiler's Stage 2 optimization phase.

Previously, users adding custom passes could encounter errors if the
target pass for 'addBefore' was only present in one segment of the
split stage 2. This change resolves the issue by:

-   Always returning the complete list of stage 2 passes in
    `DefaultPassConfig.getOptimizations()`, including an empty marker
    pass (`PassNames.OPTIMIZATIONS_HALFWAY_POINT`) to indicate the split.
-   Modifying `Compiler.performTranspilationAndOptimizations()` to
    construct the actual pass list for each stage 2 segment based on
    the marker pass, and then providing this list to `PhaseOptimizer`.

PiperOrigin-RevId: 734309816

Author: rahul-kamat

src/com/google/javascript/jscomp/Compiler.java

@@ -2974,17 +2974,21 @@ boolean endsWith(String suffix) {
   void performTranspilationAndOptimizations(OptimizationPasses optimizationPasses) {
     checkState(options.shouldOptimize());
     // getOptimizations() also includes transpilation passes
-    ImmutableList<PassFactory> optimizations =
+    ImmutableList<PassFactory> allOptimizationPassesToRun =
         getPassConfig().getOptimizations(optimizationPasses).build();
-    if (optimizations.isEmpty()) {
+    if (allOptimizationPassesToRun.isEmpty()) {
       return;
     }
 
+    // Get the subset of optimization passes to run in the current segment of optimizations.
+    List<PassFactory> optimizationPassesToRunInCurrentSegment =
+        getOptimizationPassesToRunInCurrentSegment(optimizationPasses, allOptimizationPassesToRun);
+
     // note whether any script gets transpiled
     markTranspiledFiles();
 
     phaseOptimizer = createPhaseOptimizer();
-    phaseOptimizer.consume(optimizations);
+    phaseOptimizer.consume(optimizationPassesToRunInCurrentSegment);
     phaseOptimizer.process(externsRoot, jsRoot);
     phaseOptimizer = null;
   }
@@ -3015,6 +3019,75 @@ ControlFlowGraph<Node> computeCFG() {
     return cfg;
   }
 
+  /**
+   * Takes the entire list of optimization passes to run in the current build, and returns the
+   * subset of passes that should be run in this current segment of optimizations.
+   *
+   * <p>The 3 cases are:
+   *
+   * <ol>
+   *   <li>If we are running `OptimizationPasses.ALL`, this will return the entire list of
+   *       optimization passes.
+   *   <li>If we are running `OptimizationPasses.FIRST_HALF`, this will return all passes up to the
+   *       half-way point marker pass `OPTIMIZATIONS_HALFWAY_POINT`.
+   *   <li>If we are running `OptimizationPasses.SECOND_HALF`, this will return all passes after the
+   *       half-way point marker pass `OPTIMIZATIONS_HALFWAY_POINT`.
+   * </ol>
+   */
+  private List<PassFactory> getOptimizationPassesToRunInCurrentSegment(
+      OptimizationPasses optimizationPasses, List<PassFactory> allOptimizationPassesToRun) {
+
+    // Create a list of passes based on which segment of optimizations we are running.
+    List<PassFactory> optimizationPassList = new ArrayList<>();
+
+    switch (optimizationPasses) {
+      case ALL:
+        optimizationPassList = allOptimizationPassesToRun;
+        break;
+      case FIRST_HALF:
+        optimizationPassList =
+            passListUntil(allOptimizationPassesToRun, PassNames.OPTIMIZATIONS_HALFWAY_POINT);
+        break;
+      case SECOND_HALF:
+        optimizationPassList =
+            passListAfter(allOptimizationPassesToRun, PassNames.OPTIMIZATIONS_HALFWAY_POINT);
+        break;
+    }
+
+    return optimizationPassList;
+  }
+
+  /**
+   * Returns a subset of passes from the given list of passes, up to but not including the given
+   * pass name.
+   */
+  private List<PassFactory> passListUntil(List<PassFactory> passes, String passName) {
+    List<PassFactory> passList = new ArrayList<>();
+    for (PassFactory pass : passes) {
+      if (pass.getName().equals(passName)) {
+        return passList;
+      }
+      passList.add(pass);
+    }
+    return passList;
+  }
+
+  /**
+   * Returns a subset of passes from the given list of passes, starting right after the given pass
+   * name.
+   */
+  private List<PassFactory> passListAfter(List<PassFactory> passes, String passName) {
+    List<PassFactory> passList = new ArrayList<>();
+    for (PassFactory pass : passes) {
+      if (pass.getName().equals(passName)) {
+        passList.clear();
+        continue;
+      }
+      passList.add(pass);
+    }
+    return passList;
+  }
+
   private static final String SYNTHETIC_FILE_NAME_PREFIX = " [synthetic:";
 
   private static final InputId SYNTHETIC_CODE_INPUT_ID =

src/com/google/javascript/jscomp/DefaultPassConfig.java

@@ -524,18 +524,9 @@ protected PassListBuilder getOptimizations(OptimizationPasses optimizationPasses
       return passes;
     }
 
-    switch (optimizationPasses) {
-      case FIRST_HALF:
-        passes.addAll(getEarlyOptimizationPasses());
-        break;
-      case SECOND_HALF:
-        passes.addAll(getLateOptimizationPasses());
-        break;
-      case ALL:
-        passes.addAll(getEarlyOptimizationPasses());
-        passes.addAll(getLateOptimizationPasses());
-        break;
-    }
+    passes.addAll(getEarlyOptimizationPasses());
+    passes.maybeAdd(createEmptyPass(PassNames.OPTIMIZATIONS_HALFWAY_POINT));
+    passes.addAll(getLateOptimizationPasses());
 
     return passes;
   }

src/com/google/javascript/jscomp/PassNames.java

@@ -77,6 +77,7 @@ public final class PassNames {
   public static final String LINT_CHECKS = "lintChecks";
   public static final String MARK_UNNORMALIZED = "markUnnormalized";
   public static final String NORMALIZE = "normalize";
+  public static final String OPTIMIZATIONS_HALFWAY_POINT = "optimizationsHalfwayPoint";
   public static final String OPTIMIZE_CALLS = "optimizeCalls";
   public static final String PARSE_INPUTS = "parseInputs";
   public static final String PEEPHOLE_OPTIMIZATIONS = "peepholeOptimizations";