Add a RingBuffer class to sanitizer_common

Summary: a constrained RingBuffer optimized for fast push

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: kubamracek, mgorny, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D51196

llvm-svn: 340884

1 files changed, 80 insertions, 0 deletions

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_ring_buffer.h b/compiler-rt/lib/sanitizer_common/sanitizer_ring_buffer.h
new file mode 100644
index 00000000000..f3535b9cb4f
--- /dev/null
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_ring_buffer.h
@@ -0,0 +1,80 @@
+//===-- sanitizer_ring_buffer.h ---------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Simple ring buffer.
+//
+//===----------------------------------------------------------------------===//
+#ifndef SANITIZER_RING_BUFFER_H
+#define SANITIZER_RING_BUFFER_H
+
+#include "sanitizer_common.h"
+
+namespace __sanitizer {
+// RingBuffer<T>: fixed-size ring buffer optimized for speed of push().
+// T should be a POD type and sizeof(T) should be divisible by sizeof(void*).
+// At creation, all elements are zero.
+template<class T>
+class RingBuffer {
+ public:
+  static RingBuffer *New(uptr Size) {
+    void *Ptr = MmapOrDie(SizeInBytes(Size), "RingBuffer");
+    RingBuffer *RB = reinterpret_cast<RingBuffer*>(Ptr);
+    uptr End = reinterpret_cast<uptr>(Ptr) + SizeInBytes(Size);
+    RB->last_ = RB->next_ = reinterpret_cast<T*>(End - sizeof(T));
+    CHECK_EQ(sizeof(T) % sizeof(void*), 0U);
+    return RB;
+  }
+  void Delete() {
+    UnmapOrDie(this, SizeInBytes(size()));
+  }
+  uptr size() const {
+    return last_ + 1 -
+           reinterpret_cast<T *>(reinterpret_cast<uptr>(this) +
+                                 2 * sizeof(T *));
+  }
+  void push(T t) {
+    *next_ = t;
+    next_--;
+    // The condition below works only if sizeof(T) is divisible by sizeof(T*).
+    if (next_ <= reinterpret_cast<T*>(&next_))
+      next_ = last_;
+  }
+
+  T operator[](uptr Idx) const {
+    CHECK_LT(Idx, size());
+    sptr IdxNext = Idx + 1;
+    if (IdxNext > last_ - next_)
+      IdxNext -= size();
+    return next_[IdxNext];
+  }
+
+ private:
+  RingBuffer() {}
+  ~RingBuffer() {}
+  RingBuffer(const RingBuffer&) = delete;
+
+  static uptr SizeInBytes(uptr Size) {
+    return Size * sizeof(T) + 2 * sizeof(T*);
+  }
+
+  // Data layout:
+  // LNDDDDDDDD
+  // D: data elements.
+  // L: last_, always points to the last data element.
+  // N: next_, initially equals to last_, is decremented on every push,
+  //    wraps around if it's less or equal than its own address.
+
+  T *last_;
+  T *next_;
+  T data_[1];  // flexible array.
+};
+
+}  // namespace __sanitizer
+
+#endif  // SANITIZER_RING_BUFFER_H