[PATCH] Prevent infinite growth of DenseMap/DenseSet

[Howard Hinnant]

Please review:

Rehash but don't grow when full of tombstones.

This problem was found and fixed by José Fonseca in March 2011 for SmallPtrSet, committed r128566.  But as far as I can tell, all other llvm hash tables retain the same problem:  the bucket count can grow without bound while size() remains near constant by repeated insert/erase cycles that tend to fill the container with tombstones.  Here is a demo that has been reduced to a trivial case:

int
main()
{
    llvm::DenseSet<unsigned> d;
    for (unsigned i = 0; i < 0xFFFFFFF; ++i)
    {
        d.insert(i);
        d.erase(i);
    }
}

While the container size() never grows above 1, the bucket count grows like this:

nb = 64
nb = 128
nb = 256
nb = 512
nb = 1024
nb = 2048
nb = 4096
nb = 8192
nb = 16384
nb = 32768
nb = 65536
nb = 131072
nb = 262144
nb = 524288
nb = 1048576
nb = 2097152
nb = 4194304
nb = 8388608
nb = 16777216
nb = 33554432
nb = 67108864
nb = 134217728
nb = 268435456

The above program currently consumes a few GB ram.  Here is a patch that keeps the bucket count commensurate with size():

Index: include/llvm/ADT/DenseMap.h
===================================================================
--- include/llvm/ADT/DenseMap.h	(revision 193642)
+++ include/llvm/ADT/DenseMap.h	(working copy)
@@ -439,8 +439,8 @@
       LookupBucketFor(Key, TheBucket);
       NumBuckets = getNumBuckets();
     }
-    if (NumBuckets-(NewNumEntries+getNumTombstones()) <= NumBuckets/8) {
-      this->grow(NumBuckets * 2);
+    else if (NumBuckets-(NewNumEntries+getNumTombstones()) <= NumBuckets/8) {
+      this->grow(NumBuckets);
       LookupBucketFor(Key, TheBucket);
     }
     assert(TheBucket);

Howard