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diff --git a/src/jit/blockset.h b/src/jit/blockset.h
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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+//
+// This include file determines how BlockSet is implemented.
+//
+#ifndef _BLOCKSET_INCLUDED_
+#define _BLOCKSET_INCLUDED_ 1
+
+// A BlockSet is a set of BasicBlocks, represented by the BasicBlock number (bbNum).
+// Unlike VARSET_TP, we only support a single implementation: the bitset "shortlong"
+// implementation.
+//
+// Note that BasicBlocks in the JIT are numbered starting at 1. We always just waste the
+// 0th bit to avoid having to do "bbNum - 1" calculations everywhere (at the BlockSet call
+// sites). This makes reading the code easier, and avoids potential problems of forgetting
+// to do a "- 1" somewhere.
+//
+// Basic blocks can be renumbered during compilation, so it is important to not mix
+// BlockSets created before and after a renumbering. Every time the blocks are renumbered
+// creates a different "epoch", during which the basic block numbers are stable.
+
+#include "bitset.h"
+#include "compilerbitsettraits.h"
+#include "bitsetasshortlong.h"
+
+class BlockSetOps : public BitSetOps</*BitSetType*/ BitSetShortLongRep,
+                                     /*Brand*/ BSShortLong,
+                                     /*Env*/ Compiler*,
+                                     /*BitSetTraits*/ BasicBlockBitSetTraits>
+{
+public:
+    // Specialize BlockSetOps::MakeFull(). Since we number basic blocks from one, we remove bit zero from
+    // the block set. Otherwise, IsEmpty() would never return true.
+    static BitSetShortLongRep MakeFull(Compiler* env)
+    {
+        BitSetShortLongRep retval;
+
+        // First, make a full set using the BitSetOps::MakeFull
+
+        retval = BitSetOps</*BitSetType*/ BitSetShortLongRep,
+                           /*Brand*/ BSShortLong,
+                           /*Env*/ Compiler*,
+                           /*BitSetTraits*/ BasicBlockBitSetTraits>::MakeFull(env);
+
+        // Now, remove element zero, since we number basic blocks starting at one, and index the set with the
+        // basic block number. If we left this, then IsEmpty() would never return true.
+        BlockSetOps::RemoveElemD(env, retval, 0);
+
+        return retval;
+    }
+};
+
+typedef BitSetShortLongRep BlockSet;
+
+// These types should be used as the types for BlockSet arguments and return values, respectively.
+typedef BlockSetOps::ValArgType BlockSet_ValArg_T;
+typedef BlockSetOps::RetValType BlockSet_ValRet_T;
+
+// Initialize "_varName" to "_initVal."  Copies contents, not references; if "_varName" is uninitialized, allocates a
+// var set for it (using "_comp" for any necessary allocation), and copies the contents of "_initVal" into it.
+#define BLOCKSET_INIT(_comp, _varName, _initVal) _varName(BlockSetOps::MakeCopy(_comp, _initVal))
+
+// Initializes "_varName" to "_initVal", without copying: if "_initVal" is an indirect representation, copies its
+// pointer into "_varName".
+#define BLOCKSET_INIT_NOCOPY(_varName, _initVal) _varName(_initVal)
+
+// The iterator pattern.
+
+// Use this to initialize an iterator "_iterName" to iterate over a BlockSet "_blockSet".
+// "_blockNum" will be an unsigned variable to which we assign the elements of "_blockSet".
+#define BLOCKSET_ITER_INIT(_comp, _iterName, _blockSet, _blockNum)                                                     \
+    unsigned          _blockNum = 0;                                                                                   \
+    BlockSetOps::Iter _iterName(_comp, _blockSet)
+
+#endif // _BLOCKSET_INCLUDED_