Lazily allocate DenseMaps.

This makes lookup slightly more expensive but it's worth it, unused
DenseMaps are common in LLVM code apparently.

1% speedup on clang -O3 bzip2.c
4% speedup on clang -O3 oggenc.c (Release build of clang on i386/linux)

llvm-svn: 127088

include/llvm/ADT/DenseMap.h

@@ -53,13 +53,13 @@ public:
     CopyFrom(other);
   }
 
-  explicit DenseMap(unsigned NumInitBuckets = 64) {
+  explicit DenseMap(unsigned NumInitBuckets = 0) {
     init(NumInitBuckets);
   }
 
   template<typename InputIt>
   DenseMap(const InputIt &I, const InputIt &E) {
-    init(64);
+    init(NextPowerOf2(std::distance(I, E)));
     insert(I, E);
   }
   
@@ -98,7 +98,10 @@ public:
   unsigned size() const { return NumEntries; }
 
   /// Grow the densemap so that it has at least Size buckets. Does not shrink
-  void resize(size_t Size) { grow(Size); }
+  void resize(size_t Size) {
+    if (Size > NumBuckets)
+      grow(Size);
+  }
 
   void clear() {
     if (NumEntries == 0 && NumTombstones == 0) return;
@@ -313,6 +316,11 @@ private:
     unsigned ProbeAmt = 1;
     BucketT *BucketsPtr = Buckets;
 
+    if (NumBuckets == 0) {
+      FoundBucket = 0;
+      return false;
+    }
+
     // FoundTombstone - Keep track of whether we find a tombstone while probing.
     BucketT *FoundTombstone = 0;
     const KeyT EmptyKey = getEmptyKey();
@@ -354,6 +362,12 @@ private:
     NumEntries = 0;
     NumTombstones = 0;
     NumBuckets = InitBuckets;
+
+    if (InitBuckets == 0) {
+      Buckets = 0;
+      return;
+    }
+
     assert(InitBuckets && (InitBuckets & (InitBuckets-1)) == 0 &&
            "# initial buckets must be a power of two!");
     Buckets = static_cast<BucketT*>(operator new(sizeof(BucketT)*InitBuckets));
@@ -367,6 +381,9 @@ private:
     unsigned OldNumBuckets = NumBuckets;
     BucketT *OldBuckets = Buckets;
 
+    if (NumBuckets < 64)
+      NumBuckets = 64;
+
     // Double the number of buckets.
     while (NumBuckets < AtLeast)
       NumBuckets <<= 1;