1 //===-- BasicBlockPlacement.cpp - Basic Block Code Layout optimization ----===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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
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
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.
27 //===----------------------------------------------------------------------===//
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/ADT/Statistic.h"
35 #include "llvm/Transforms/Scalar.h"
39 STATISTIC(NumMoved, "Number of basic blocks moved");
42 struct BlockPlacement : public FunctionPass {
43 static char ID; // Pass identification, replacement for typeid
44 BlockPlacement() : FunctionPass(ID) {}
46 virtual bool runOnFunction(Function &F);
48 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
50 AU.addRequired<ProfileInfo>();
51 //AU.addPreserved<ProfileInfo>(); // Does this work?
54 /// PI - The profile information that is guiding us.
58 /// NumMovedBlocks - Every time we move a block, increment this counter.
60 unsigned NumMovedBlocks;
62 /// PlacedBlocks - Every time we place a block, remember it so we don't get
63 /// into infinite loops.
64 std::set<BasicBlock*> PlacedBlocks;
66 /// InsertPos - This an iterator to the next place we want to insert a
68 Function::iterator InsertPos;
70 /// PlaceBlocks - Recursively place the specified blocks and any unplaced
72 void PlaceBlocks(BasicBlock *BB);
76 char BlockPlacement::ID = 0;
77 INITIALIZE_PASS_BEGIN(BlockPlacement, "block-placement",
78 "Profile Guided Basic Block Placement", false, false)
79 INITIALIZE_AG_DEPENDENCY(ProfileInfo)
80 INITIALIZE_PASS_END(BlockPlacement, "block-placement",
81 "Profile Guided Basic Block Placement", false, false)
83 FunctionPass *llvm::createBlockPlacementPass() { return new BlockPlacement(); }
85 bool BlockPlacement::runOnFunction(Function &F) {
86 PI = &getAnalysis<ProfileInfo>();
89 InsertPos = F.begin();
91 // Recursively place all blocks.
92 PlaceBlocks(F.begin());
95 NumMoved += NumMovedBlocks;
96 return NumMovedBlocks != 0;
100 /// PlaceBlocks - Recursively place the specified blocks and any unplaced
102 void BlockPlacement::PlaceBlocks(BasicBlock *BB) {
103 assert(!PlacedBlocks.count(BB) && "Already placed this block!");
104 PlacedBlocks.insert(BB);
106 // Place the specified block.
107 if (&*InsertPos != BB) {
108 // Use splice to move the block into the right place. This avoids having to
109 // remove the block from the function then readd it, which causes a bunch of
110 // symbol table traffic that is entirely pointless.
111 Function::BasicBlockListType &Blocks = BB->getParent()->getBasicBlockList();
112 Blocks.splice(InsertPos, Blocks, BB);
116 // This block is already in the right place, we don't have to do anything.
120 // Keep placing successors until we run out of ones to place. Note that this
121 // loop is very inefficient (N^2) for blocks with many successors, like switch
122 // statements. FIXME!
124 // Okay, now place any unplaced successors.
125 succ_iterator SI = succ_begin(BB), E = succ_end(BB);
127 // Scan for the first unplaced successor.
128 for (; SI != E && PlacedBlocks.count(*SI); ++SI)
130 if (SI == E) return; // No more successors to place.
132 double MaxExecutionCount = PI->getExecutionCount(*SI);
133 BasicBlock *MaxSuccessor = *SI;
135 // Scan for more frequently executed successors
136 for (; SI != E; ++SI)
137 if (!PlacedBlocks.count(*SI)) {
138 double Count = PI->getExecutionCount(*SI);
139 if (Count > MaxExecutionCount ||
140 // Prefer to not disturb the code.
141 (Count == MaxExecutionCount && *SI == &*InsertPos)) {
142 MaxExecutionCount = Count;
147 // Now that we picked the maximally executed successor, place it.
148 PlaceBlocks(MaxSuccessor);