Added Region::maybeGrow and Region::maybeShrink helper functions to be shared by multiple Allocators

include/region.h

@@ -49,6 +49,10 @@ public:
   size_t getMaxSize() const;
   std::byte* getAddress() const;
 
+  // Helper functions for allocators
+  bool maybeGrow(size_t minSize);
+  void maybeShrink(size_t targetSize);
+
 private:
   std::byte* m_data;
   size_t m_committedSize;

src/arena.cpp

@@ -77,17 +77,9 @@ size_t Arena::getMaxSize() const
 std::byte* Arena::alloc(size_t size)
 {
   size_t neededSize = m_usedSize + size;
-  if (neededSize > m_region.getSize())
+  if (!m_region.maybeGrow(std::max(neededSize, m_minSize))) {
-  {
-    // Increase the size exponentially when we run out
-    size_t newSize = std::bit_ceil(neededSize) << 1;
-    if (newSize < m_minSize) {
-      newSize = m_minSize;
-    }
-    if (!m_region.resize(newSize)) {
     return nullptr;
   }
-  }
 
   std::byte* ret = m_region.getAddress() + m_usedSize;
   m_usedSize = neededSize;
@@ -128,13 +120,7 @@ void Arena::reset()
   m_watermark[kWatermarkLen - 1] = m_usedSize;
 
   size_t targetSize = std::bit_ceil(highWatermark) << 1;
-  if (targetSize < m_minSize) {
+  m_region.maybeShrink(std::max(m_minSize, targetSize));
-    targetSize = m_minSize;
-  }
-  size_t thresholdSize = targetSize << 1; // The threshold for shrinking is greater than the target to implement hysteresis
-  if (m_region.getSize() > thresholdSize) {
-    m_region.resize(targetSize);
-  }
 
   m_usedSize = 0;
 }

src/region.cpp

@@ -154,4 +154,32 @@ std::byte* Region::getAddress() const
   return m_data;
 }
 
-};
+bool Region::maybeGrow(size_t minSize)
+{
+  if (minSize < m_committedSize) {
+    // Already big enough
+    return true;
+  }
+
+  if (minSize > m_maxSize) {
+    // We can't grow this big
+    return false;
+  }
+
+  // We need to grow.
+  // Increase the size exponentially, capped to m_maxSize
+  size_t newSize = std::bit_ceil(minSize) << 1;
+  newSize = std::min(m_maxSize, newSize);
+  
+  return resize(newSize);
+}
+
+void Region::maybeShrink(size_t targetSize)
+{
+  size_t thresholdSize = targetSize << 1; // The threshold for shrinking is greater than the target to implement hysteresis
+  if (getSize() > thresholdSize) {
+    resize(targetSize);
+  }
+}
+
+} // namespace mimir