[XRay][compiler-rt] Generational Buffer Management

Summary:
This change updates the buffer queue implementation to support using a
generation number to identify the lifetime of buffers. This first part
introduces the notion of the generation number, without changing the way
we handle the buffers yet.

What's missing here is the cleanup of the buffers. Ideally we'll keep
the two most recent generations. We need to ensure that before we do any
writes to the buffers, that we check the generation number(s) first.

Those changes will follow-on from this change.

Depends on D52588.

Reviewers: mboerger, eizan

Subscribers: llvm-commits, jfb

Differential Revision: https://reviews.llvm.org/D52974

llvm-svn: 344881

1 files changed, 21 insertions, 0 deletions

diff --git a/compiler-rt/lib/xray/xray_buffer_queue.h b/compiler-rt/lib/xray/xray_buffer_queue.h
index c1fa9fab768..cbd42835f8a 100644
--- a/compiler-rt/lib/xray/xray_buffer_queue.h
+++ b/compiler-rt/lib/xray/xray_buffer_queue.h
@@ -33,6 +33,7 @@ class BufferQueue {
 public:
   struct Buffer {
     atomic_uint64_t Extents{0};
+    uint64_t Generation{0};
     void *Data = nullptr;
     size_t Size = 0;
   };
@@ -130,6 +131,10 @@ private:
   // Count of buffers that have been handed out through 'getBuffer'.
   size_t LiveBuffers;
 
+  // We use a generation number to identify buffers and which generation they're
+  // associated with.
+  atomic_uint64_t Generation;
+
 public:
   enum class ErrorCode : unsigned {
     Ok,
@@ -137,6 +142,7 @@ public:
     QueueFinalizing,
     UnrecognizedBuffer,
     AlreadyFinalized,
+    AlreadyInitialized,
   };
 
   static const char *getErrorString(ErrorCode E) {
@@ -151,6 +157,8 @@ public:
       return "buffer being returned not owned by buffer queue";
     case ErrorCode::AlreadyFinalized:
       return "queue already finalized";
+    case ErrorCode::AlreadyInitialized:
+      return "queue already initialized";
     }
     return "unknown error";
   }
@@ -181,10 +189,23 @@ public:
   ///     the buffer being released.
   ErrorCode releaseBuffer(Buffer &Buf);
 
+  /// Initializes the buffer queue, starting a new generation. We can re-set the
+  /// size of buffers with |BS| along with the buffer count with |BC|.
+  ///
+  /// Returns:
+  ///   - ErrorCode::Ok when we successfully initialize the buffer. This
+  ///   requires that the buffer queue is previously finalized.
+  ///   - ErrorCode::AlreadyInitialized when the buffer queue is not finalized.
+  ErrorCode init(size_t BS, size_t BC);
+
   bool finalizing() const {
     return atomic_load(&Finalizing, memory_order_acquire);
   }
 
+  uint64_t generation() const {
+    return atomic_load(&Generation, memory_order_acquire);
+  }
+
   /// Returns the configured size of the buffers in the buffer queue.
   size_t ConfiguredBufferSize() const { return BufferSize; }