New BranchProbabilityInfo analysis. Patch by Jakub Staszak!

BranchProbabilityInfo provides an interface for IR passes to query the
likelihood that control follows a CFG edge. This patch provides an
initial implementation of static branch predication that will populate
BranchProbabilityInfo for branches with no external profile
information using very simple heuristics. It currently isn't hooked up
to any external profile data, so static prediction does all the work.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132613 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 68 insertions, 0 deletions

diff --git a/include/llvm/Analysis/BranchProbabilityInfo.h b/include/llvm/Analysis/BranchProbabilityInfo.h
new file mode 100644
index 0000000000..bf2a6a765f
--- /dev/null
+++ b/include/llvm/Analysis/BranchProbabilityInfo.h
@@ -0,0 +1,68 @@
+//===--- BranchProbabilityInfo.h - Branch Probability Analysis --*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass is used to evaluate branch probabilties.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
+#define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H
+
+#include "llvm/InitializePasses.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Support/Debug.h"
+
+namespace llvm {
+
+class BranchProbabilityInfo : public FunctionPass {
+
+  // Default weight value. Used when we don't have information about the edge.
+  static const unsigned int DEFAULT_WEIGHT = 16;
+
+  typedef std::pair<BasicBlock *, BasicBlock *> Edge;
+
+  DenseMap<Edge, unsigned> Weights;
+
+public:
+  static char ID;
+
+  BranchProbabilityInfo() : FunctionPass(ID) {
+    initializeBranchProbabilityInfoPass(*PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const {
+    AU.addRequired<LoopInfo>();
+    AU.setPreservesAll();
+  }
+
+  bool runOnFunction(Function &F);
+
+  // Returned value is between 1 and UINT_MAX. Look at BranchProbabilityInfo.cpp
+  // for details.
+  unsigned getEdgeWeight(BasicBlock *Src, BasicBlock *Dst) const;
+
+  // Look at BranchProbabilityInfo.cpp for details. Use it with caution!
+  void setEdgeWeight(BasicBlock *Src, BasicBlock *Dst, unsigned Weight);
+
+  // A 'Hot' edge is an edge which probability is >= 80%.
+  bool isEdgeHot(BasicBlock *Src, BasicBlock *Dst) const;
+
+  // Return a hot successor for the block BB or null if there isn't one.
+  BasicBlock *getHotSucc(BasicBlock *BB) const;
+
+  // Print value between 0 (0% probability) and 1 (100% probability),
+  // however the value is never equal to 0, and can be 1 only iff SRC block
+  // has only one successor.
+  raw_ostream &printEdgeProbability(raw_ostream &OS, BasicBlock *Src,
+                                   BasicBlock *Dst) const;
+};
+
+}
+
+#endif