[clangd] Limit completion results.

Summary:
* truncate symbols from static/dynamic index to the limited number
(which would save lots of cost in constructing the merged symbols).
* add an CLI option allowing to limit the number of returned completion results.
(default to 100)

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D42484

llvm-svn: 323408

4 files changed, 23 insertions, 1 deletions

diff --git a/clang-tools-extra/clangd/CodeComplete.cpp b/clang-tools-extra/clangd/CodeComplete.cpp
index e070831ed2e..d0916aa30fd 100644
--- a/clang-tools-extra/clangd/CodeComplete.cpp
+++ b/clang-tools-extra/clangd/CodeComplete.cpp
@@ -864,6 +864,8 @@ private:
     SymbolSlab::Builder ResultsBuilder;
     // Build the query.
     FuzzyFindRequest Req;
+    if (Opts.Limit)
+      Req.MaxCandidateCount = Opts.Limit;
     Req.Query = Filter->pattern();
     Req.Scopes =
         getQueryScopes(Recorder.CCContext, Recorder.CCSema->getSourceManager());
diff --git a/clang-tools-extra/clangd/index/Merge.cpp b/clang-tools-extra/clangd/index/Merge.cpp
index 8264dcc2b9b..61e3024bced 100644
--- a/clang-tools-extra/clangd/index/Merge.cpp
+++ b/clang-tools-extra/clangd/index/Merge.cpp
@@ -50,7 +50,7 @@ class MergedIndex : public SymbolIndex {
      for (const Symbol &S : Dyn)
        if (!SeenDynamicSymbols.count(S.ID))
          Callback(S);
-     return More;
+     return !More; // returning true indicates the result is complete.
   }
 
 private:
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
index 69f41ce8ac9..2d965198f89 100644
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -86,6 +86,12 @@ static llvm::cl::opt<PCHStorageFlag> PCHStorage(
         clEnumValN(PCHStorageFlag::Memory, "memory", "store PCHs in memory")),
     llvm::cl::init(PCHStorageFlag::Disk));
 
+static llvm::cl::opt<int> LimitCompletionResult(
+    "limit-completion",
+    llvm::cl::desc("Limit the number of completion results returned by clangd. "
+                   "0 means no limit."),
+    llvm::cl::init(0));
+
 static llvm::cl::opt<bool> RunSynchronously(
     "run-synchronously",
     llvm::cl::desc("Parse on main thread. If set, -j is ignored"),
@@ -215,6 +221,7 @@ int main(int argc, char *argv[]) {
   clangd::CodeCompleteOptions CCOpts;
   CCOpts.EnableSnippets = EnableSnippets;
   CCOpts.IncludeIneligibleResults = IncludeIneligibleResults;
+  CCOpts.Limit = LimitCompletionResult;
   // Initialize and run ClangdLSPServer.
   ClangdLSPServer LSPServer(Out, WorkerThreadsCount, StorePreamblesInMemory,
                             CCOpts, ResourceDirRef, CompileCommandsDirPath,
diff --git a/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp b/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
index 729c2bb098c..7c166cc197b 100644
--- a/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
+++ b/clang-tools-extra/unittests/clangd/CodeCompleteTests.cpp
@@ -529,6 +529,19 @@ TEST(CompletionTest, SemaIndexMerge) {
       UnorderedElementsAre(Named("local"), Named("Index"), Named("both")));
 }
 
+TEST(CompletionTest, SemaIndexMergeWithLimit) {
+  clangd::CodeCompleteOptions Opts;
+  Opts.Limit = 1;
+  auto Results = completions(
+      R"cpp(
+          namespace ns { int local; void both(); }
+          void f() { ::ns::^ }
+      )cpp",
+      {func("ns::both"), cls("ns::Index")}, Opts);
+  EXPECT_EQ(Results.items.size(), Opts.Limit);
+  EXPECT_TRUE(Results.isIncomplete);
+}
+
 TEST(CompletionTest, IndexSuppressesPreambleCompletions) {
   MockFSProvider FS;
   MockCompilationDatabase CDB;