[CachedDiagnostics] Optimize in case no diagnostics is emitted

When no diagnostics is emitted from the compilation, just use an empty
object as cached diagnostics. That is an optimization for storage to
avoid emitting some supporting file to recreate source manager when not
needed, also save a bit time from emitting YAML file.

rdar://128426132

Author: cachemeifyoucan

lib/Frontend/CachedDiagnostics.cpp

@@ -629,6 +629,11 @@ llvm::Error DiagnosticSerializer::deserializeDiagnosticInfo(
 
 llvm::Error
 DiagnosticSerializer::serializeEmittedDiagnostics(llvm::raw_ostream &os) {
+  // If no diagnostics is produced, do not write anything so output is smaller
+  // without referencing any files.
+  if (DiagInfos.empty())
+    return llvm::Error::success();
+
   // Convert all file backed source file into CASIDs.
   for (auto &File : Files) {
     if (!File.Content.empty() || !File.ContentCASID.empty())
@@ -822,12 +827,14 @@ CachingDiagnosticsProcessor::CachingDiagnosticsProcessor(
 
     // compress the YAML file.
     llvm::SmallVector<uint8_t, 512> Compression;
-    if (llvm::compression::zstd::isAvailable())
-      llvm::compression::zstd::compress(arrayRefFromStringRef(Output),
-                                        Compression);
-    else if (llvm::compression::zlib::isAvailable())
-      llvm::compression::zlib::compress(arrayRefFromStringRef(Output),
-                                        Compression);
+    if (!Output.empty()) {
+      if (llvm::compression::zstd::isAvailable())
+        llvm::compression::zstd::compress(arrayRefFromStringRef(Output),
+                                          Compression);
+      else if (llvm::compression::zlib::isAvailable())
+        llvm::compression::zlib::compress(arrayRefFromStringRef(Output),
+                                          Compression);
+    }
 
     // Write the uncompressed size in the end.
     if (!Compression.empty()) {
@@ -872,6 +879,10 @@ llvm::Error CachingDiagnosticsProcessor::serializeEmittedDiagnostics(
 
 llvm::Error
 CachingDiagnosticsProcessor::replayCachedDiagnostics(llvm::StringRef Buffer) {
+  // If empty buffer, no diagnostics to replay.
+  if (Buffer.empty())
+    return llvm::Error::success();
+
   SmallVector<uint8_t, 512> Uncompressed;
   if (llvm::compression::zstd::isAvailable() ||
       llvm::compression::zlib::isAvailable()) {