[Preamble] Reuse preamble even if an unsaved file does not exist

When a preamble is created an unsaved file not existing on disk is
already part of PrecompiledPreamble::FilesInPreamble. However, when
checking whether the preamble can be re-used, a failed stat of such an
unsaved file invalidated the preamble, which led to pointless and time
consuming preamble regenerations on subsequent reparses.

Do not require anymore that unsaved files should exist on disk.

This avoids costly preamble invalidations depending on timing issues for
the cases where the file on disk might be removed just to be regenerated
a bit later.

It also allows an IDE to provide in-memory files that might not exist on
disk, e.g. because the build system hasn't generated those yet.

Differential Revision: https://reviews.llvm.org/D41005

llvm-svn: 361226

4 files changed, 109 insertions, 22 deletions

diff --git a/clang/include/clang/Frontend/ASTUnit.h b/clang/include/clang/Frontend/ASTUnit.h
index 750f6c8577b..a81c57e40a8 100644
--- a/clang/include/clang/Frontend/ASTUnit.h
+++ b/clang/include/clang/Frontend/ASTUnit.h
@@ -205,7 +205,10 @@ private:
   /// we'll attempt to rebuild the precompiled header. This way, if
   /// building the precompiled preamble fails, we won't try again for
   /// some number of calls.
-  unsigned PreambleRebuildCounter = 0;
+  unsigned PreambleRebuildCountdown = 0;
+
+  /// Counter indicating how often the preamble was build in total.
+  unsigned PreambleCounter = 0;
 
   /// Cache pairs "filename - source location"
   ///
@@ -574,6 +577,8 @@ public:
                        mapLocationToPreamble(R.getEnd()));
   }
 
+  unsigned getPreambleCounterForTests() const { return PreambleCounter; }
+
   // Retrieve the diagnostics associated with this AST
   using stored_diag_iterator = StoredDiagnostic *;
   using stored_diag_const_iterator = const StoredDiagnostic *;
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 221bf97c350..95d1bf27d7b 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -1304,22 +1304,22 @@ ASTUnit::getMainBufferWithPrecompiledPreamble(
                             PreambleInvocationIn.getDiagnosticOpts());
       getDiagnostics().setNumWarnings(NumWarningsInPreamble);
 
-      PreambleRebuildCounter = 1;
+      PreambleRebuildCountdown = 1;
       return MainFileBuffer;
     } else {
       Preamble.reset();
       PreambleDiagnostics.clear();
       TopLevelDeclsInPreamble.clear();
       PreambleSrcLocCache.clear();
-      PreambleRebuildCounter = 1;
+      PreambleRebuildCountdown = 1;
     }
   }
 
   // If the preamble rebuild counter > 1, it's because we previously
   // failed to build a preamble and we're not yet ready to try
   // again. Decrement the counter and return a failure.
-  if (PreambleRebuildCounter > 1) {
-    --PreambleRebuildCounter;
+  if (PreambleRebuildCountdown > 1) {
+    --PreambleRebuildCountdown;
     return nullptr;
   }
 
@@ -1329,6 +1329,8 @@ ASTUnit::getMainBufferWithPrecompiledPreamble(
   if (!AllowRebuild)
     return nullptr;
 
+  ++PreambleCounter;
+
   SmallVector<StandaloneDiagnostic, 4> NewPreambleDiagsStandalone;
   SmallVector<StoredDiagnostic, 4> NewPreambleDiags;
   ASTUnitPreambleCallbacks Callbacks;
@@ -1356,18 +1358,18 @@ ASTUnit::getMainBufferWithPrecompiledPreamble(
 
     if (NewPreamble) {
       Preamble = std::move(*NewPreamble);
-      PreambleRebuildCounter = 1;
+      PreambleRebuildCountdown = 1;
     } else {
       switch (static_cast<BuildPreambleError>(NewPreamble.getError().value())) {
       case BuildPreambleError::CouldntCreateTempFile:
         // Try again next time.
-        PreambleRebuildCounter = 1;
+        PreambleRebuildCountdown = 1;
         return nullptr;
       case BuildPreambleError::CouldntCreateTargetInfo:
       case BuildPreambleError::BeginSourceFileFailed:
       case BuildPreambleError::CouldntEmitPCH:
         // These erros are more likely to repeat, retry after some period.
-        PreambleRebuildCounter = DefaultPreambleRebuildInterval;
+        PreambleRebuildCountdown = DefaultPreambleRebuildInterval;
         return nullptr;
       }
       llvm_unreachable("unexpected BuildPreambleError");
@@ -1507,7 +1509,7 @@ ASTUnit *ASTUnit::LoadFromCompilerInvocationAction(
   AST->OnlyLocalDecls = OnlyLocalDecls;
   AST->CaptureDiagnostics = CaptureDiagnostics;
   if (PrecompilePreambleAfterNParses > 0)
-    AST->PreambleRebuildCounter = PrecompilePreambleAfterNParses;
+    AST->PreambleRebuildCountdown = PrecompilePreambleAfterNParses;
   AST->TUKind = Action ? Action->getTranslationUnitKind() : TU_Complete;
   AST->ShouldCacheCodeCompletionResults = CacheCodeCompletionResults;
   AST->IncludeBriefCommentsInCodeCompletion
@@ -1641,7 +1643,7 @@ bool ASTUnit::LoadFromCompilerInvocation(
 
   std::unique_ptr<llvm::MemoryBuffer> OverrideMainBuffer;
   if (PrecompilePreambleAfterNParses > 0) {
-    PreambleRebuildCounter = PrecompilePreambleAfterNParses;
+    PreambleRebuildCountdown = PrecompilePreambleAfterNParses;
     OverrideMainBuffer =
         getMainBufferWithPrecompiledPreamble(PCHContainerOps, *Invocation, VFS);
     getDiagnostics().Reset();
@@ -1819,7 +1821,7 @@ bool ASTUnit::Reparse(std::shared_ptr<PCHContainerOperations> PCHContainerOps,
   // If we have a preamble file lying around, or if we might try to
   // build a precompiled preamble, do so now.
   std::unique_ptr<llvm::MemoryBuffer> OverrideMainBuffer;
-  if (Preamble || PreambleRebuildCounter > 0)
+  if (Preamble || PreambleRebuildCountdown > 0)
     OverrideMainBuffer =
         getMainBufferWithPrecompiledPreamble(PCHContainerOps, *Invocation, VFS);
 
diff --git a/clang/lib/Frontend/PrecompiledPreamble.cpp b/clang/lib/Frontend/PrecompiledPreamble.cpp
index 59467c3c41b..25f128fe11a 100644
--- a/clang/lib/Frontend/PrecompiledPreamble.cpp
+++ b/clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -454,20 +454,33 @@ bool PrecompiledPreamble::CanReuse(const CompilerInvocation &Invocation,
         Status.getSize(), llvm::sys::toTimeT(Status.getLastModificationTime()));
   }
 
+  // OverridenFileBuffers tracks only the files not found in VFS.
+  llvm::StringMap<PreambleFileHash> OverridenFileBuffers;
   for (const auto &RB : PreprocessorOpts.RemappedFileBuffers) {
-    llvm::vfs::Status Status;
-    if (!moveOnNoError(VFS->status(RB.first), Status))
-      return false;
-
-    OverriddenFiles[Status.getUniqueID()] =
+    const PrecompiledPreamble::PreambleFileHash PreambleHash =
         PreambleFileHash::createForMemoryBuffer(RB.second);
+    llvm::vfs::Status Status;
+    if (moveOnNoError(VFS->status(RB.first), Status))
+      OverriddenFiles[Status.getUniqueID()] = PreambleHash;
+    else
+      OverridenFileBuffers[RB.first] = PreambleHash;
   }
 
   // Check whether anything has changed.
   for (const auto &F : FilesInPreamble) {
+    auto OverridenFileBuffer = OverridenFileBuffers.find(F.first());
+    if (OverridenFileBuffer != OverridenFileBuffers.end()) {
+      // The file's buffer was remapped and the file was not found in VFS.
+      // Check whether it matches up with the previous mapping.
+      if (OverridenFileBuffer->second != F.second)
+        return false;
+      continue;
+    }
+
     llvm::vfs::Status Status;
     if (!moveOnNoError(VFS->status(F.first()), Status)) {
-      // If we can't stat the file, assume that something horrible happened.
+      // If the file's buffer is not remapped and we can't stat it,
+      // assume that something horrible happened.
       return false;
     }
 
@@ -481,7 +494,8 @@ bool PrecompiledPreamble::CanReuse(const CompilerInvocation &Invocation,
       continue;
     }
 
-    // The file was not remapped; check whether it has changed on disk.
+    // Neither the file's buffer nor the file itself was remapped;
+    // check whether it has changed on disk.
     if (Status.getSize() != uint64_t(F.second.Size) ||
         llvm::sys::toTimeT(Status.getLastModificationTime()) !=
             F.second.ModTime)
diff --git a/clang/unittests/Frontend/PCHPreambleTest.cpp b/clang/unittests/Frontend/PCHPreambleTest.cpp
index 70567405f10..0dc567efaab 100644
--- a/clang/unittests/Frontend/PCHPreambleTest.cpp
+++ b/clang/unittests/Frontend/PCHPreambleTest.cpp
@@ -52,7 +52,10 @@ class PCHPreambleTest : public ::testing::Test {
   FileSystemOptions FSOpts;
 
 public:
-  void SetUp() override {
+  void SetUp() override { ResetVFS(); }
+  void TearDown() override {}
+
+  void ResetVFS() {
     VFS = new ReadCountingInMemoryFileSystem();
     // We need the working directory to be set to something absolute,
     // otherwise it ends up being inadvertently set to the current
@@ -63,9 +66,6 @@ public:
     VFS->setCurrentWorkingDirectory("//./");
   }
 
-  void TearDown() override {
-  }
-
   void AddFile(const std::string &Filename, const std::string &Contents) {
     ::time_t now;
     ::time(&now);
@@ -123,6 +123,72 @@ private:
   }
 };
 
+TEST_F(PCHPreambleTest, ReparseReusesPreambleWithUnsavedFileNotExistingOnDisk) {
+  std::string Header1 = "//./header1.h";
+  std::string MainName = "//./main.cpp";
+  AddFile(MainName, R"cpp(
+#include "//./header1.h"
+int main() { return ZERO; }
+)cpp");
+  RemapFile(Header1, "#define ZERO 0\n");
+
+  // Parse with header file provided as unsaved file, which does not exist on
+  // disk.
+  std::unique_ptr<ASTUnit> AST(ParseAST(MainName));
+  ASSERT_TRUE(AST.get());
+  ASSERT_FALSE(AST->getDiagnostics().hasErrorOccurred());
+
+  // Reparse and check that the preamble was reused.
+  ASSERT_TRUE(ReparseAST(AST));
+  ASSERT_EQ(AST->getPreambleCounterForTests(), 1U);
+}
+
+TEST_F(PCHPreambleTest, ReparseReusesPreambleAfterUnsavedFileWasCreatedOnDisk) {
+  std::string Header1 = "//./header1.h";
+  std::string MainName = "//./main.cpp";
+  AddFile(MainName, R"cpp(
+#include "//./header1.h"
+int main() { return ZERO; }
+)cpp");
+  RemapFile(Header1, "#define ZERO 0\n");
+
+  // Parse with header file provided as unsaved file, which does not exist on
+  // disk.
+  std::unique_ptr<ASTUnit> AST(ParseAST(MainName));
+  ASSERT_TRUE(AST.get());
+  ASSERT_FALSE(AST->getDiagnostics().hasErrorOccurred());
+
+  // Create the unsaved file also on disk and check that preamble was reused.
+  AddFile(Header1, "#define ZERO 0\n");
+  ASSERT_TRUE(ReparseAST(AST));
+  ASSERT_EQ(AST->getPreambleCounterForTests(), 1U);
+}
+
+TEST_F(PCHPreambleTest,
+       ReparseReusesPreambleAfterUnsavedFileWasRemovedFromDisk) {
+  std::string Header1 = "//./foo/header1.h";
+  std::string MainName = "//./main.cpp";
+  std::string MainFileContent = R"cpp(
+#include "//./foo/header1.h"
+int main() { return ZERO; }
+)cpp";
+  AddFile(MainName, MainFileContent);
+  AddFile(Header1, "#define ZERO 0\n");
+  RemapFile(Header1, "#define ZERO 0\n");
+
+  // Parse with header file provided as unsaved file, which exists on disk.
+  std::unique_ptr<ASTUnit> AST(ParseAST(MainName));
+  ASSERT_TRUE(AST.get());
+  ASSERT_FALSE(AST->getDiagnostics().hasErrorOccurred());
+  ASSERT_EQ(AST->getPreambleCounterForTests(), 1U);
+
+  // Remove the unsaved file from disk and check that the preamble was reused.
+  ResetVFS();
+  AddFile(MainName, MainFileContent);
+  ASSERT_TRUE(ReparseAST(AST));
+  ASSERT_EQ(AST->getPreambleCounterForTests(), 1U);
+}
+
 TEST_F(PCHPreambleTest, ReparseWithOverriddenFileDoesNotInvalidatePreamble) {
   std::string Header1 = "//./header1.h";
   std::string Header2 = "//./header2.h";