[libSyntax] Improve performance for commonly requested SyntaxArena.containsPointer calls

In practice SyntaxArena.containsPointer is almost always called with a
pointer from the SyntaxArena's source buffer. To avoid walking through
all of the bump allocator's slabs until we find the one containing the
source buffer, add a hot use memory region (which lives inside the bump
allocator) that is checked first before consulting the bump allocator.

Author: ahoppen

include/swift/Syntax/SyntaxArena.h

@@ -31,17 +31,34 @@ class SyntaxArena : public llvm::ThreadSafeRefCountedBase<SyntaxArena> {
 
   llvm::BumpPtrAllocator Allocator;
 
+  /// The start (inclusive) and end (exclusive) pointers of a memory region that
+  /// is frequently requested using \c containsPointer. Must be inside \c
+  /// Allocator but \c containsPointer will check this region first and will
+  /// thus return more quickly for pointers that lie within this region.
+  const void *HotUseMemoryRegionStart = nullptr;
+  const void *HotUseMemoryRegionEnd = nullptr;
+
 public:
   SyntaxArena() {}
 
   static RC<SyntaxArena> make() { return RC<SyntaxArena>(new SyntaxArena()); }
 
+  void setHotUseMemoryRegion(const void *Start, const void *End) {
+    assert(containsPointer(Start) &&
+           "The hot use memory region should be in the Arena's bump allocator");
+    HotUseMemoryRegionStart = Start;
+    HotUseMemoryRegionEnd = End;
+  }
+
   llvm::BumpPtrAllocator &getAllocator() { return Allocator; }
   void *Allocate(size_t size, size_t alignment) {
     return Allocator.Allocate(size, alignment);
   }
 
   bool containsPointer(const void *Ptr) {
+    if (HotUseMemoryRegionStart <= Ptr && Ptr < HotUseMemoryRegionEnd) {
+      return true;
+    }
     return getAllocator().identifyObject(Ptr) != llvm::None;
   }
 };

lib/SyntaxParse/SyntaxTreeCreator.cpp

@@ -47,6 +47,8 @@ SyntaxTreeCreator::SyntaxTreeCreator(SourceManager &SM, unsigned bufferID,
   std::uninitialized_copy(BufferContent.begin(), BufferContent.end(), Data);
   ArenaSourceBuffer = StringRef(Data, BufferContent.size());
   assert(ArenaSourceBuffer == BufferContent);
+  Arena->setHotUseMemoryRegion(ArenaSourceBuffer.begin(),
+                               ArenaSourceBuffer.end());
 }
 
 SyntaxTreeCreator::~SyntaxTreeCreator() = default;