[llvm-commits] [llvm] r142789 - /llvm/trunk/include/llvm/Analysis/BranchProbabilityInfo.h

[Chandler Carruth]

Author: chandlerc
Date: Mon Oct 24 00:55:58 2011
New Revision: 142789

URL: http://llvm.org/viewvc/llvm-project?rev=142789&view=rev
Log:
Doxygen-ify the comments on the public interface for BPI. Also, move the
two more subtle routines to the bottom and expand on their cautionary
comments a bit. No functionality or actual interface change here.

Modified:
    llvm/trunk/include/llvm/Analysis/BranchProbabilityInfo.h

Modified: llvm/trunk/include/llvm/Analysis/BranchProbabilityInfo.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/BranchProbabilityInfo.h?rev=142789&r1=142788&r2=142789&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/BranchProbabilityInfo.h (original)
+++ llvm/trunk/include/llvm/Analysis/BranchProbabilityInfo.h Mon Oct 24 00:55:58 2011
@@ -23,6 +23,15 @@
 class LoopInfo;
 class raw_ostream;
 
+/// \brief Analysis pass providing branch probability information.
+///
+/// This is a function analysis pass which provides information on the relative
+/// probabilities of each "edge" in the function's CFG where such an edge is
+/// defined by a pair of basic blocks. The probability for a given block and
+/// a successor block are always relative to the probabilities of the other
+/// successor blocks. Another way of looking at it is that the probabilities
+/// for a given block B and each of its successors should sum to exactly
+/// one (100%).
 class BranchProbabilityInfo : public FunctionPass {
 public:
   static char ID;
@@ -35,32 +44,52 @@
   bool runOnFunction(Function &F);
   void print(raw_ostream &OS, const Module *M = 0) const;
 
-  // Returned value is between 1 and UINT32_MAX. Look at
-  // BranchProbabilityInfo.cpp for details.
-  uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
-
-  // Look at BranchProbabilityInfo.cpp for details. Use it with caution!
-  void setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst,
-                     uint32_t Weight);
+  /// \brief Get an edge's probability, relative to other out-edges of the Src.
+  ///
+  /// This routine provides access to the fractional probability between zero
+  /// (0%) and one (100%) of this edge executing, relative to other edges
+  /// leaving the 'Src' block. The returned probability is never zero, and can
+  /// only be one if the source block has only one successor.
+  BranchProbability getEdgeProbability(const BasicBlock *Src,
+                                       const BasicBlock *Dst) const;
 
-  // A 'Hot' edge is an edge which probability is >= 80%.
+  /// \brief Test if an edge is hot relative to other out-edges of the Src.
+  ///
+  /// Check whether this edge out of the source block is 'hot'. We define hot
+  /// as having a relative probability >= 80%.
   bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;
 
-  // Return a hot successor for the block BB or null if there isn't one.
+  /// \brief Retrieve the hot successor of a block if one exists.
+  ///
+  /// Given a basic block, look through its successors and if one exists for
+  /// which \see isEdgeHot would return true, return that successor block.
   BasicBlock *getHotSucc(BasicBlock *BB) const;
 
-  // Return a probability as a fraction 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.
-  BranchProbability getEdgeProbability(const BasicBlock *Src,
-                                       const BasicBlock *Dst) 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.
+  /// \brief Print an edge's probability.
+  ///
+  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but
+  /// then prints that probability to the provided stream. That stream is then
+  /// returned.
   raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,
                                     const BasicBlock *Dst) const;
 
+  /// \brief Get the raw edge weight calculated for the block pair.
+  ///
+  /// This returns the raw edge weight. It is guaranteed to fall between 1 and
+  /// UINT32_MAX. Note that the raw edge weight is not meaningful in isolation.
+  /// This interface should be very carefully, and primarily by routines that
+  /// are updating the analysis by later calling setEdgeWeight.
+  uint32_t getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const;
+
+  /// \brief Set the raw edge weight for the block pair.
+  ///
+  /// This allows a pass to explicitly set the edge weight for a block. It can
+  /// be used when updating the CFG to update and preserve the branch
+  /// probability information. Read the implementation of how these edge
+  /// weights are calculated carefully before using!
+  void setEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst,
+                     uint32_t Weight);
+
 private:
   typedef std::pair<const BasicBlock *, const BasicBlock *> Edge;