LLVM API Documentation
00001 //===-- BasicBlockPlacement.cpp - Basic Block Code Layout optimization ----===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This file implements a very simple profile guided basic block placement 00011 // algorithm. The idea is to put frequently executed blocks together at the 00012 // start of the function, and hopefully increase the number of fall-through 00013 // conditional branches. If there is no profile information for a particular 00014 // function, this pass basically orders blocks in depth-first order 00015 // 00016 // The algorithm implemented here is basically "Algo1" from "Profile Guided Code 00017 // Positioning" by Pettis and Hansen, except that it uses basic block counts 00018 // instead of edge counts. This should be improved in many ways, but is very 00019 // simple for now. 00020 // 00021 // Basically we "place" the entry block, then loop over all successors in a DFO, 00022 // placing the most frequently executed successor until we run out of blocks. I 00023 // told you this was _extremely_ simplistic. :) This is also much slower than it 00024 // could be. When it becomes important, this pass will be rewritten to use a 00025 // better algorithm, and then we can worry about efficiency. 00026 // 00027 //===----------------------------------------------------------------------===// 00028 00029 #include "llvm/Analysis/ProfileInfo.h" 00030 #include "llvm/Function.h" 00031 #include "llvm/Pass.h" 00032 #include "llvm/Support/CFG.h" 00033 #include "llvm/ADT/Statistic.h" 00034 #include <set> 00035 using namespace llvm; 00036 00037 namespace { 00038 Statistic<> NumMoved("block-placement", "Number of basic blocks moved"); 00039 00040 struct BlockPlacement : public FunctionPass { 00041 virtual bool runOnFunction(Function &F); 00042 00043 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 00044 AU.setPreservesCFG(); 00045 AU.addRequired<ProfileInfo>(); 00046 //AU.addPreserved<ProfileInfo>(); // Does this work? 00047 } 00048 private: 00049 /// PI - The profile information that is guiding us. 00050 /// 00051 ProfileInfo *PI; 00052 00053 /// NumMovedBlocks - Every time we move a block, increment this counter. 00054 /// 00055 unsigned NumMovedBlocks; 00056 00057 /// PlacedBlocks - Every time we place a block, remember it so we don't get 00058 /// into infinite loops. 00059 std::set<BasicBlock*> PlacedBlocks; 00060 00061 /// InsertPos - This an iterator to the next place we want to insert a 00062 /// block. 00063 Function::iterator InsertPos; 00064 00065 /// PlaceBlocks - Recursively place the specified blocks and any unplaced 00066 /// successors. 00067 void PlaceBlocks(BasicBlock *BB); 00068 }; 00069 00070 RegisterOpt<BlockPlacement> X("block-placement", 00071 "Profile Guided Basic Block Placement"); 00072 } 00073 00074 bool BlockPlacement::runOnFunction(Function &F) { 00075 PI = &getAnalysis<ProfileInfo>(); 00076 00077 NumMovedBlocks = 0; 00078 InsertPos = F.begin(); 00079 00080 // Recursively place all blocks. 00081 PlaceBlocks(F.begin()); 00082 00083 PlacedBlocks.clear(); 00084 NumMoved += NumMovedBlocks; 00085 return NumMovedBlocks != 0; 00086 } 00087 00088 00089 /// PlaceBlocks - Recursively place the specified blocks and any unplaced 00090 /// successors. 00091 void BlockPlacement::PlaceBlocks(BasicBlock *BB) { 00092 assert(!PlacedBlocks.count(BB) && "Already placed this block!"); 00093 PlacedBlocks.insert(BB); 00094 00095 // Place the specified block. 00096 if (&*InsertPos != BB) { 00097 // Use splice to move the block into the right place. This avoids having to 00098 // remove the block from the function then readd it, which causes a bunch of 00099 // symbol table traffic that is entirely pointless. 00100 Function::BasicBlockListType &Blocks = BB->getParent()->getBasicBlockList(); 00101 Blocks.splice(InsertPos, Blocks, BB); 00102 00103 ++NumMovedBlocks; 00104 } else { 00105 // This block is already in the right place, we don't have to do anything. 00106 ++InsertPos; 00107 } 00108 00109 // Keep placing successors until we run out of ones to place. Note that this 00110 // loop is very inefficient (N^2) for blocks with many successors, like switch 00111 // statements. FIXME! 00112 while (1) { 00113 // Okay, now place any unplaced successors. 00114 succ_iterator SI = succ_begin(BB), E = succ_end(BB); 00115 00116 // Scan for the first unplaced successor. 00117 for (; SI != E && PlacedBlocks.count(*SI); ++SI) 00118 /*empty*/; 00119 if (SI == E) return; // No more successors to place. 00120 00121 unsigned MaxExecutionCount = PI->getExecutionCount(*SI); 00122 BasicBlock *MaxSuccessor = *SI; 00123 00124 // Scan for more frequently executed successors 00125 for (; SI != E; ++SI) 00126 if (!PlacedBlocks.count(*SI)) { 00127 unsigned Count = PI->getExecutionCount(*SI); 00128 if (Count > MaxExecutionCount || 00129 // Prefer to not disturb the code. 00130 (Count == MaxExecutionCount && *SI == &*InsertPos)) { 00131 MaxExecutionCount = Count; 00132 MaxSuccessor = *SI; 00133 } 00134 } 00135 00136 // Now that we picked the maximally executed successor, place it. 00137 PlaceBlocks(MaxSuccessor); 00138 } 00139 }