Improvements inspired by review

Author: David Ungar

include/swift/Driver/Action.h

@@ -173,20 +173,17 @@ class CompileJobAction : public JobAction {
   /// if 0 or >1 such inputs exist, return nullptr.
   const InputAction *findSingleSwiftInput() const {
     auto Inputs = getInputs();
-    const InputAction *IA = nullptr;
-    for (auto const *I : Inputs) {
-      if (auto const *S = dyn_cast<InputAction>(I)) {
-        if (S->getType() == file_types::TY_Swift) {
-          if (IA == nullptr) {
-            IA = S;
-          } else {
-            // Already found one, two is too many.
-            return nullptr;
-          }
-        }
-      }
-    }
-    return IA;
+    auto isSwiftInput = [](const Action *A) -> const InputAction* {
+      if (auto const *S = dyn_cast<InputAction>(A))
+        return S->getType() == file_types::TY_Swift ? S : nullptr;
+      return nullptr;
+    };
+    const auto loc1 = std::find_if(Inputs.begin(), Inputs.end(), isSwiftInput);
+    if (loc1 == Inputs.end())
+      return nullptr; // none found
+    // Ensure uniqueness
+    const auto loc2 = std::find_if(loc1 + 1, Inputs.end(), isSwiftInput);
+    return loc2 == Inputs.end() ? dyn_cast<InputAction>(*loc1) : nullptr;
   }
 };
 

include/swift/Driver/FineGrainedDependencyDriverGraph.h

@@ -487,7 +487,7 @@ class ModuleDepGraph {
     return swiftDepsOfJobsThatNeedRunning.insert(swiftDeps).second;
   }
 
-  /// For debugging, write out the graph to a dot file.
+  /// For debugging and visualization, write out the graph to a dot file.
   /// \p diags may be null if no diagnostics are needed.
   void emitDotFileForJob(DiagnosticEngine &, const driver::Job *);
   void emitDotFile(DiagnosticEngine &, StringRef baseName);

lib/AST/FineGrainedDependencies.cpp

@@ -90,10 +90,19 @@ SourceFileDepGraph::findExistingNodePairOrCreateAndAddIfNew(
           DependencyKey(k, DeclAspect::implementation, context, name),
           fingerprint, true /* = isProvides */)};
   // if interface changes, have to rebuild implementation.
+  // This dependency used to be represented by
+  // addArc(nodePair.getInterface(), nodePair.getImplementation());
+  // However, recall that the dependency scheme as of 1/2020 chunks
+  // declarations together by base name.
+  // So if the arc were added, a dirtying of a same-based-named interface
+  // in a different file would dirty the implementation in this file,
+  // causing the needless recompilation of this file.
   // But, if an arc is added for this, then *any* change that causes
   // a same-named interface to be dirty will dirty this implementation,
   // even if that interface is in another file.
-  // So, make the interface->implementation arc implicit.
+  // Therefor no such arc is added here, and any dirtying of either
+  // the interface or implementation of this declaration will cause
+  // the driver to recompile this source file.
   return nodePair;
 }
 

lib/Driver/FineGrainedDependencyDriverGraph.cpp

@@ -428,8 +428,9 @@ void ModuleDepGraph::traceDeparture(size_t pathLengthAfterArrival) {
   currentPath.pop_back();
 }
 
-// Emitting Dot file for ModuleDepGraph
-// ===========================================
+// =============================================================================
+// MARK: Emitting Dot file for ModuleDepGraph
+// =============================================================================
 
 void ModuleDepGraph::emitDotFileForJob(DiagnosticEngine &diags,
                                        const Job *job) {