Add a new llvm::SmallVector template, which is similar to the vector class, but

contains optimizations to avoid heap allocation if the vector size is smaller
than some threshold.  This can significantly improve the performance of code
that allocates many small vectors by eliminating tons of small malloc/free's.

llvm-svn: 29281

include/llvm/ADT/SmallVector.h

@@ -0,0 +1,196 @@
+//===- llvm/ADT/SmallVector.h - 'Normally small' vectors --------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by Chris Lattner and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the SmallVector class.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_SMALLVECTOR_H
+#define LLVM_ADT_SMALLVECTOR_H
+
+#include <cassert>
+#include <memory>
+
+namespace llvm {
+
+/// SmallVector - This is a 'vector' (really, a variable-sized array), optimized
+/// for the case when the array is small.  It contains some number of elements
+/// in-place, which allows it to avoid heap allocation when the actual number of
+/// elements is below that threshold.  This allows normal "small" cases to be
+/// fast without losing generality for large inputs.
+///
+/// Note that this does not attempt to be exception safe.
+///
+template <typename T, unsigned N>
+class SmallVector {
+  // Allocate raw space for N elements of type T.  If T has a ctor or dtor, we
+  // don't want it to be automatically run, so we need to represent the space as
+  // something else.  An array of char would work great, but might not be
+  // aligned sufficiently.  Instead, we either use GCC extensions, or some
+  // number of union instances for the space, which guarantee maximal alignment.
+  union U {
+    double D;
+    long double LD;
+    long long L;
+    void *P;
+  };
+  
+  /// InlineElts - These are the 'N' elements that are stored inline in the body
+  /// of the vector
+  U InlineElts[(sizeof(T)*N+sizeof(U)-1)/sizeof(U)];
+  T *Begin, *End, *Capacity;
+public:
+  // Default ctor - Initialize to empty.
+  SmallVector() : Begin((T*)InlineElts), End(Begin), Capacity(Begin+N) {
+  }
+  
+  SmallVector(const SmallVector &RHS) {
+    unsigned RHSSize = RHS.size();
+    Begin = (T*)InlineElts;
+
+    // Doesn't fit in the small case?  Allocate space.
+    if (RHSSize > N) {
+      End = Capacity = Begin;
+      grow(RHSSize);
+    }
+    End = Begin+RHSSize;
+    Capacity = Begin+N;
+    std::uninitialized_copy(RHS.begin(), RHS.end(), Begin);
+  }
+  ~SmallVector() {
+    // If this wasn't grown from the inline copy, deallocate the old space.
+    if ((void*)Begin != (void*)InlineElts)
+      delete[] (char*)Begin;
+  }
+  
+  typedef size_t size_type;
+  typedef T* iterator;
+  typedef const T* const_iterator;
+  typedef T& reference;
+  typedef const T& const_reference;
+
+  bool empty() const { return Begin == End; }
+  size_type size() const { return End-Begin; }
+  
+  iterator begin() { return Begin; }
+  const_iterator begin() const { return Begin; }
+
+  iterator end() { return End; }
+  const_iterator end() const { return End; }
+  
+  reference operator[](unsigned idx) {
+    assert(idx < size() && "out of range reference!");
+    return Begin[idx];
+  }
+  const_reference operator[](unsigned idx) const {
+    assert(idx < size() && "out of range reference!");
+    return Begin[idx];
+  }
+  
+  reference back() {
+    assert(!empty() && "SmallVector is empty!");
+    return end()[-1];
+  }
+  const_reference back() const {
+    assert(!empty() && "SmallVector is empty!");
+    return end()[-1];
+  }
+  
+  void push_back(const_reference Elt) {
+    if (End < Capacity) {
+  Retry:
+      new (End) T(Elt);
+      ++End;
+      return;
+    }
+    grow();
+    goto Retry;
+  }
+  
+  const SmallVector &operator=(const SmallVector &RHS) {
+    // Avoid self-assignment.
+    if (this == &RHS) return *this;
+    
+    // If we already have sufficient space, assign the common elements, then
+    // destroy any excess.
+    unsigned RHSSize = RHS.size();
+    unsigned CurSize = size();
+    if (CurSize >= RHSSize) {
+      // Assign common elements.
+      for (unsigned i = 0; i != RHSSize; ++i)
+        Begin[i] = RHS.Begin[i];
+      
+      // Destroy excess elements.
+      for (unsigned i = RHSSize; i != CurSize; ++i)
+        Begin[i].~T();
+      
+      // Trim.
+      End = Begin + RHSSize;
+      return *this;
+    }
+    
+    // If we have to grow to have enough elements, destroy the current elements.
+    // This allows us to avoid copying them during the grow.
+    if (Capacity-Begin < RHSSize) {
+      // Destroy current elements.
+      for (T *I = Begin, E = End; I != E; ++I)
+        I->~T();
+      End = Begin;
+      CurSize = 0;
+      grow(RHSSize);
+    } else {
+      // Otherwise, use assignment for the already-constructed elements.
+      for (unsigned i = 0; i != CurSize; ++i)
+        Begin[i] = RHS.Begin[i];
+    }
+    
+    // Copy construct the new elements in place.
+    std::uninitialized_copy(RHS.Begin+CurSize, RHS.End, Begin+CurSize);
+    
+    // Set end.
+    End = Begin+RHSSize;
+  }
+  
+private:
+  /// isSmall - Return true if this is a smallvector which has not had dynamic
+  /// memory allocated for it.
+  bool isSmall() const {
+    return (void*)Begin == (void*)InlineElts;
+  }
+
+  /// grow - double the size of the allocated memory, guaranteeing space for at
+  /// least one more element or MinSize if specified.
+  void grow(unsigned MinSize = 0) {
+    unsigned CurCapacity = Capacity-Begin;
+    unsigned CurSize = size();
+    unsigned NewCapacity = 2*CurCapacity;
+    if (NewCapacity < MinSize)
+      NewCapacity = MinSize;
+    T *NewElts = reinterpret_cast<T*>(new char[NewCapacity*sizeof(T)]);
+
+    // Copy the elements over.
+    std::uninitialized_copy(Begin, End, NewElts);
+    
+    // Destroy the original elements.
+    for (T *I = Begin, *E = End; I != E; ++I)
+      I->~T();
+    
+    // If this wasn't grown from the inline copy, deallocate the old space.
+    if (!isSmall())
+      delete[] (char*)Begin;
+    
+    Begin = NewElts;
+    End = NewElts+CurSize;
+    Capacity = Begin+NewCapacity*2;
+  }
+};
+
+} // End llvm namespace
+
+#endif