[clangd] Pass Context implicitly using TLS.

Summary:
Instead of passing Context explicitly around, we now have a thread-local
Context object `Context::current()` which is an implicit argument to
every function.
Most manipulation of this should use the WithContextValue helper, which
augments the current Context to add a single KV pair, and restores the
old context on destruction.

Advantages are:
- less boilerplate in functions that just propagate contexts
- reading most code doesn't require understanding context at all, and
  using context as values in fewer places still
- fewer options to pass the "wrong" context when it changes within a
  scope (e.g. when using Span)
- contexts pass through interfaces we can't modify, such as VFS
- propagating contexts across threads was slightly tricky (e.g.
  copy vs move, no move-init in lambdas), and is now encapsulated in
  the threadpool

Disadvantages are all the usual TLS stuff - hidden magic, and
potential for higher memory usage on threads that don't use the
context. (In practice, it's just one pointer)

Reviewers: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D42517

1 files changed, 23 insertions, 30 deletions

diff --git a/clang-tools-extra/clangd/ClangdUnit.cpp b/clang-tools-extra/clangd/ClangdUnit.cpp
index b2276edc983..917294499fb 100644
--- a/clang-tools-extra/clangd/ClangdUnit.cpp
+++ b/clang-tools-extra/clangd/ClangdUnit.cpp
@@ -229,8 +229,7 @@ void clangd::dumpAST(ParsedAST &AST, llvm::raw_ostream &OS) {
 }
 
 llvm::Optional<ParsedAST>
-ParsedAST::Build(const Context &Ctx,
-                 std::unique_ptr<clang::CompilerInvocation> CI,
+ParsedAST::Build(std::unique_ptr<clang::CompilerInvocation> CI,
                  std::shared_ptr<const PreambleData> Preamble,
                  std::unique_ptr<llvm::MemoryBuffer> Buffer,
                  std::shared_ptr<PCHContainerOperations> PCHs,
@@ -254,12 +253,12 @@ ParsedAST::Build(const Context &Ctx,
   auto Action = llvm::make_unique<ClangdFrontendAction>();
   const FrontendInputFile &MainInput = Clang->getFrontendOpts().Inputs[0];
   if (!Action->BeginSourceFile(*Clang, MainInput)) {
-    log(Ctx, "BeginSourceFile() failed when building AST for " +
-                 MainInput.getFile());
+    log("BeginSourceFile() failed when building AST for " +
+        MainInput.getFile());
     return llvm::None;
   }
   if (!Action->Execute())
-    log(Ctx, "Execute() failed when building AST for " + MainInput.getFile());
+    log("Execute() failed when building AST for " + MainInput.getFile());
 
   // UnitDiagsConsumer is local, we can not store it in CompilerInstance that
   // has a longer lifetime.
@@ -387,8 +386,7 @@ CppFile::CppFile(PathRef FileName, bool StorePreamblesInMemory,
       RebuildCounter(0), RebuildInProgress(false), ASTMemUsage(0),
       PreambleMemUsage(0), PCHs(std::move(PCHs)),
       ASTCallback(std::move(ASTCallback)) {
-  // FIXME(ibiryukov): we should pass a proper Context here.
-  log(Context::empty(), "Created CppFile for " + FileName);
+  log("Created CppFile for " + FileName);
 
   std::lock_guard<std::mutex> Lock(Mutex);
   LatestAvailablePreamble = nullptr;
@@ -442,11 +440,11 @@ UniqueFunction<void()> CppFile::deferCancelRebuild() {
 }
 
 llvm::Optional<std::vector<DiagWithFixIts>>
-CppFile::rebuild(const Context &Ctx, ParseInputs &&Inputs) {
-  return deferRebuild(std::move(Inputs))(Ctx);
+CppFile::rebuild(ParseInputs &&Inputs) {
+  return deferRebuild(std::move(Inputs))();
 }
 
-UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>(const Context &)>
+UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>()>
 CppFile::deferRebuild(ParseInputs &&Inputs) {
   std::shared_ptr<const PreambleData> OldPreamble;
   std::shared_ptr<PCHContainerOperations> PCHs;
@@ -483,13 +481,11 @@ CppFile::deferRebuild(ParseInputs &&Inputs) {
   std::shared_ptr<CppFile> That = shared_from_this();
   auto FinishRebuild =
       [OldPreamble, RequestRebuildCounter, PCHs,
-       That](ParseInputs Inputs,
-             const Context &Ctx) mutable /* to allow changing OldPreamble. */
+       That](ParseInputs Inputs) mutable /* to allow changing OldPreamble. */
       -> llvm::Optional<std::vector<DiagWithFixIts>> {
-    log(Context::empty(),
-        "Rebuilding file " + That->FileName + " with command [" +
-            Inputs.CompileCommand.Directory + "] " +
-            llvm::join(Inputs.CompileCommand.CommandLine, " "));
+    log("Rebuilding file " + That->FileName + " with command [" +
+        Inputs.CompileCommand.Directory + "] " +
+        llvm::join(Inputs.CompileCommand.CommandLine, " "));
 
     // Only one execution of this method is possible at a time.
     // RebuildGuard will wait for any ongoing rebuilds to finish and will put us
@@ -532,16 +528,16 @@ CppFile::deferRebuild(ParseInputs &&Inputs) {
       if (OldPreamble &&
           OldPreamble->Preamble.CanReuse(*CI, ContentsBuffer.get(), Bounds,
                                          Inputs.FS.get())) {
-        log(Ctx, "Reusing preamble for file " + Twine(That->FileName));
+        log("Reusing preamble for file " + Twine(That->FileName));
         return OldPreamble;
       }
-      log(Ctx, "Premble for file " + Twine(That->FileName) +
-                   " cannot be reused. Attempting to rebuild it.");
+      log("Premble for file " + Twine(That->FileName) +
+          " cannot be reused. Attempting to rebuild it.");
       // We won't need the OldPreamble anymore, release it so it can be
       // deleted (if there are no other references to it).
       OldPreamble.reset();
 
-      trace::Span Tracer(Ctx, "Preamble");
+      trace::Span Tracer("Preamble");
       SPAN_ATTACH(Tracer, "File", That->FileName);
       std::vector<DiagWithFixIts> PreambleDiags;
       StoreDiagsConsumer PreambleDiagnosticsConsumer(/*ref*/ PreambleDiags);
@@ -566,17 +562,15 @@ CppFile::deferRebuild(ParseInputs &&Inputs) {
       CI->getFrontendOpts().SkipFunctionBodies = false;
 
       if (BuiltPreamble) {
-        log(Tracer.Ctx, "Built preamble of size " +
-                            Twine(BuiltPreamble->getSize()) + " for file " +
-                            Twine(That->FileName));
+        log("Built preamble of size " + Twine(BuiltPreamble->getSize()) +
+            " for file " + Twine(That->FileName));
 
         return std::make_shared<PreambleData>(
             std::move(*BuiltPreamble),
             SerializedDeclsCollector.takeTopLevelDeclIDs(),
             std::move(PreambleDiags));
       } else {
-        log(Tracer.Ctx,
-            "Could not build a preamble for file " + Twine(That->FileName));
+        log("Could not build a preamble for file " + Twine(That->FileName));
         return nullptr;
       }
     };
@@ -606,18 +600,17 @@ CppFile::deferRebuild(ParseInputs &&Inputs) {
     // Compute updated AST.
     llvm::Optional<ParsedAST> NewAST;
     {
-      trace::Span Tracer(Ctx, "Build");
+      trace::Span Tracer("Build");
       SPAN_ATTACH(Tracer, "File", That->FileName);
-      NewAST =
-          ParsedAST::Build(Tracer.Ctx, std::move(CI), std::move(NewPreamble),
-                           std::move(ContentsBuffer), PCHs, Inputs.FS);
+      NewAST = ParsedAST::Build(std::move(CI), std::move(NewPreamble),
+                                std::move(ContentsBuffer), PCHs, Inputs.FS);
     }
 
     if (NewAST) {
       Diagnostics.insert(Diagnostics.end(), NewAST->getDiagnostics().begin(),
                          NewAST->getDiagnostics().end());
       if (That->ASTCallback)
-        That->ASTCallback(Ctx, That->FileName, NewAST.getPointer());
+        That->ASTCallback(That->FileName, NewAST.getPointer());
     } else {
       // Don't report even Preamble diagnostics if we coulnd't build AST.
       Diagnostics.clear();