Remove unnecessary sorting from CppCompileAction.computeKey().

This showed up as a hotspot (3% of CPU time) in profiles of large C++ builds.

--
MOS_MIGRATED_REVID=121943041

Author: Googler

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

@@ -1093,19 +1093,6 @@ public NestedSet<Artifact> getDeclaredIncludeSrcs() {
     return context.getDeclaredIncludeSrcs();
   }
 
-  /**
-   * Return explicit header files (i.e., header files explicitly listed) in an order
-   * that is stable between builds.
-   */
-  protected final List<PathFragment> getDeclaredIncludeSrcsInStableOrder() {
-    List<PathFragment> paths = new ArrayList<>();
-    for (Artifact declaredIncludeSrc : context.getDeclaredIncludeSrcs()) {
-      paths.add(declaredIncludeSrc.getExecPath());
-    }
-    Collections.sort(paths); // Order is not important, but stability is.
-    return paths;
-  }
-
   @Override
   public ResourceSet estimateResourceConsumption(Executor executor) {
     return executor.getContext(actionContext).estimateResourceConsumption(this);
@@ -1142,8 +1129,13 @@ public String computeKey() {
      */
     f.addPaths(context.getDeclaredIncludeDirs());
     f.addPaths(context.getDeclaredIncludeWarnDirs());
-    f.addPaths(getDeclaredIncludeSrcsInStableOrder());
-    f.addPaths(Artifact.asSortedPathFragments(getMandatoryInputs()));
+    for (Artifact declaredIncludeSrc : context.getDeclaredIncludeSrcs()) {
+      f.addPath(declaredIncludeSrc.getExecPath());
+    }
+    f.addInt(0);  // mark the boundary between input types
+    for (Artifact input : getMandatoryInputs()) {
+      f.addPath(input.getExecPath());
+    }
     return f.hexDigestAndReset();
   }
 
@@ -1252,9 +1244,9 @@ public String describeKey() {
       message.append('\n');
     }
 
-    for (PathFragment path : getDeclaredIncludeSrcsInStableOrder()) {
+    for (Artifact src : getDeclaredIncludeSrcs()) {
       message.append("  Declared include source: ");
-      message.append(ShellEscaper.escapeString(path.getPathString()));
+      message.append(ShellEscaper.escapeString(src.getExecPathString()));
       message.append('\n');
     }