pred_iterator got a lot slower since use-diet landed. This is a

really simple cache class for these queries.  Hopefully this can
be removed if pred_iterator speeds back up.

llvm-svn: 60742

llvm/include/llvm/Support/PredIteratorCache.h

@@ -0,0 +1,56 @@
+//===- llvm/Support/PredIteratorCache.h - pred_iterator Cache ---*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the PredIteratorCache class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/Allocator.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallVector.h"
+
+namespace llvm {
+
+  /// PredIteratorCache - This class is an extremely trivial cache for
+  /// predecessor iterator queries.  This is useful for code that repeatedly
+  /// wants the predecessor list for the same blocks.
+  class PredIteratorCache {
+    /// BlockToPredsMap - Pointer to null-terminated list.
+    DenseMap<BasicBlock*, BasicBlock**> BlockToPredsMap;
+
+    /// Memory - This is the space that holds cached preds.
+    BumpPtrAllocator Memory;
+  public:
+    
+    /// GetPreds - Get a cached list for the null-terminated predecessor list of
+    /// the specified block.  This can be used in a loop like this:
+    ///   for (BasicBlock **PI = PredCache->GetPreds(BB); *PI; ++PI)
+    ///      use(*PI);
+    /// instead of:
+    /// for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
+    BasicBlock **GetPreds(BasicBlock *BB) {
+      BasicBlock **&Entry = BlockToPredsMap[BB];
+      if (Entry) return Entry;
+      
+      SmallVector<BasicBlock*, 32> PredCache(pred_begin(BB), pred_end(BB));
+      PredCache.push_back(0); // null terminator.
+      
+      Entry = Memory.Allocate<BasicBlock*>(PredCache.size());
+      std::copy(PredCache.begin(), PredCache.end(), Entry);
+      return Entry;
+    }
+    
+    /// clear - Remove all information.
+    void clear() {
+      BlockToPredsMap.clear();
+      Memory.Reset();
+    }
+  };
+} // end namespace llvm