lib/Transforms/Scalar/BasicBlockPlacement.cpp

   1 //===-- BasicBlockPlacement.cpp - Basic Block Code Layout optimization ----===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements a very simple profile guided basic block placement
  11 // algorithm.  The idea is to put frequently executed blocks together at the
  12 // start of the function, and hopefully increase the number of fall-through
  13 // conditional branches.  If there is no profile information for a particular
  14 // function, this pass basically orders blocks in depth-first order
  15 //
  16 // The algorithm implemented here is basically "Algo1" from "Profile Guided Code
  17 // Positioning" by Pettis and Hansen, except that it uses basic block counts
  18 // instead of edge counts.  This should be improved in many ways, but is very
  19 // simple for now.
  20 //
  21 // Basically we "place" the entry block, then loop over all successors in a DFO,
  22 // placing the most frequently executed successor until we run out of blocks.  I
  23 // told you this was _extremely_ simplistic. :) This is also much slower than it
  24 // could be.  When it becomes important, this pass will be rewritten to use a
  25 // better algorithm, and then we can worry about efficiency.
  26 //
  27 //===----------------------------------------------------------------------===//
  28
  29 #define DEBUG_TYPE "block-placement"
  30 #include "llvm/Analysis/ProfileInfo.h"
  31 #include "llvm/Function.h"
  32 #include "llvm/Pass.h"
  33 #include "llvm/Support/CFG.h"
  34 #include "llvm/Support/Compiler.h"
  35 #include "llvm/ADT/Statistic.h"
  36 #include "llvm/Transforms/Scalar.h"
  37 #include <set>
  38 using namespace llvm;
  39
  40 STATISTIC(NumMoved, "Number of basic blocks moved");
  41
  42 namespace {
  43   struct VISIBILITY_HIDDEN BlockPlacement : public FunctionPass {
  44     static char ID; // Pass identification, replacement for typeid
  45     BlockPlacement() : FunctionPass(&ID) {}
  46
  47     virtual bool runOnFunction(Function &F);
  48
  49     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  50       AU.setPreservesCFG();
  51       AU.addRequired<ProfileInfo>();
  52       //AU.addPreserved<ProfileInfo>();  // Does this work?
  53     }
  54   private:
  55     /// PI - The profile information that is guiding us.
  56     ///
  57     ProfileInfo *PI;
  58
  59     /// NumMovedBlocks - Every time we move a block, increment this counter.
  60     ///
  61     unsigned NumMovedBlocks;
  62
  63     /// PlacedBlocks - Every time we place a block, remember it so we don't get
  64     /// into infinite loops.
  65     std::set<BasicBlock*> PlacedBlocks;
  66
  67     /// InsertPos - This an iterator to the next place we want to insert a
  68     /// block.
  69     Function::iterator InsertPos;
  70
  71     /// PlaceBlocks - Recursively place the specified blocks and any unplaced
  72     /// successors.
  73     void PlaceBlocks(BasicBlock *BB);
  74   };
  75 }
  76
  77 char BlockPlacement::ID = 0;
  78 static RegisterPass<BlockPlacement>
  79 X("block-placement", "Profile Guided Basic Block Placement");
  80
  81 FunctionPass *llvm::createBlockPlacementPass() { return new BlockPlacement(); }
  82
  83 bool BlockPlacement::runOnFunction(Function &F) {
  84   PI = &getAnalysis<ProfileInfo>();
  85
  86   NumMovedBlocks = 0;
  87   InsertPos = F.begin();
  88
  89   // Recursively place all blocks.
  90   PlaceBlocks(F.begin());
  91
  92   PlacedBlocks.clear();
  93   NumMoved += NumMovedBlocks;
  94   return NumMovedBlocks != 0;
  95 }
  96
  97
  98 /// PlaceBlocks - Recursively place the specified blocks and any unplaced
  99 /// successors.
 100 void BlockPlacement::PlaceBlocks(BasicBlock *BB) {
 101   assert(!PlacedBlocks.count(BB) && "Already placed this block!");
 102   PlacedBlocks.insert(BB);
 103
 104   // Place the specified block.
 105   if (&*InsertPos != BB) {
 106     // Use splice to move the block into the right place.  This avoids having to
 107     // remove the block from the function then readd it, which causes a bunch of
 108     // symbol table traffic that is entirely pointless.
 109     Function::BasicBlockListType &Blocks = BB->getParent()->getBasicBlockList();
 110     Blocks.splice(InsertPos, Blocks, BB);
 111
 112     ++NumMovedBlocks;
 113   } else {
 114     // This block is already in the right place, we don't have to do anything.
 115     ++InsertPos;
 116   }
 117
 118   // Keep placing successors until we run out of ones to place.  Note that this
 119   // loop is very inefficient (N^2) for blocks with many successors, like switch
 120   // statements.  FIXME!
 121   while (1) {
 122     // Okay, now place any unplaced successors.
 123     succ_iterator SI = succ_begin(BB), E = succ_end(BB);
 124
 125     // Scan for the first unplaced successor.
 126     for (; SI != E && PlacedBlocks.count(*SI); ++SI)
 127       /*empty*/;
 128     if (SI == E) return;  // No more successors to place.
 129
 130     unsigned MaxExecutionCount = PI->getExecutionCount(*SI);
 131     BasicBlock *MaxSuccessor = *SI;
 132
 133     // Scan for more frequently executed successors
 134     for (; SI != E; ++SI)
 135       if (!PlacedBlocks.count(*SI)) {
 136         unsigned Count = PI->getExecutionCount(*SI);
 137         if (Count > MaxExecutionCount ||
 138             // Prefer to not disturb the code.
 139             (Count == MaxExecutionCount && *SI == &*InsertPos)) {
 140           MaxExecutionCount = Count;
 141           MaxSuccessor = *SI;
 142         }
 143       }
 144
 145     // Now that we picked the maximally executed successor, place it.
 146     PlaceBlocks(MaxSuccessor);
 147   }
 148 }