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/ADT/Statistic.h"
35 #include "llvm/Transforms/Scalar.h"
36 #include <set>
37 using namespace llvm;
38
39 STATISTIC(NumMoved, "Number of basic blocks moved");
40
41 namespace {
42 struct BlockPlacement : public FunctionPass {
43 static char ID; // Pass identification, replacement for typeid
BlockPlacement__anona080e0260111::BlockPlacement44 BlockPlacement() : FunctionPass(ID) {
45 initializeBlockPlacementPass(*PassRegistry::getPassRegistry());
46 }
47
48 virtual bool runOnFunction(Function &F);
49
getAnalysisUsage__anona080e0260111::BlockPlacement50 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
51 AU.setPreservesCFG();
52 AU.addRequired<ProfileInfo>();
53 //AU.addPreserved<ProfileInfo>(); // Does this work?
54 }
55 private:
56 /// PI - The profile information that is guiding us.
57 ///
58 ProfileInfo *PI;
59
60 /// NumMovedBlocks - Every time we move a block, increment this counter.
61 ///
62 unsigned NumMovedBlocks;
63
64 /// PlacedBlocks - Every time we place a block, remember it so we don't get
65 /// into infinite loops.
66 std::set<BasicBlock*> PlacedBlocks;
67
68 /// InsertPos - This an iterator to the next place we want to insert a
69 /// block.
70 Function::iterator InsertPos;
71
72 /// PlaceBlocks - Recursively place the specified blocks and any unplaced
73 /// successors.
74 void PlaceBlocks(BasicBlock *BB);
75 };
76 }
77
78 char BlockPlacement::ID = 0;
79 INITIALIZE_PASS_BEGIN(BlockPlacement, "block-placement",
80 "Profile Guided Basic Block Placement", false, false)
INITIALIZE_AG_DEPENDENCY(ProfileInfo)81 INITIALIZE_AG_DEPENDENCY(ProfileInfo)
82 INITIALIZE_PASS_END(BlockPlacement, "block-placement",
83 "Profile Guided Basic Block Placement", false, false)
84
85 FunctionPass *llvm::createBlockPlacementPass() { return new BlockPlacement(); }
86
runOnFunction(Function & F)87 bool BlockPlacement::runOnFunction(Function &F) {
88 PI = &getAnalysis<ProfileInfo>();
89
90 NumMovedBlocks = 0;
91 InsertPos = F.begin();
92
93 // Recursively place all blocks.
94 PlaceBlocks(F.begin());
95
96 PlacedBlocks.clear();
97 NumMoved += NumMovedBlocks;
98 return NumMovedBlocks != 0;
99 }
100
101
102 /// PlaceBlocks - Recursively place the specified blocks and any unplaced
103 /// successors.
PlaceBlocks(BasicBlock * BB)104 void BlockPlacement::PlaceBlocks(BasicBlock *BB) {
105 assert(!PlacedBlocks.count(BB) && "Already placed this block!");
106 PlacedBlocks.insert(BB);
107
108 // Place the specified block.
109 if (&*InsertPos != BB) {
110 // Use splice to move the block into the right place. This avoids having to
111 // remove the block from the function then readd it, which causes a bunch of
112 // symbol table traffic that is entirely pointless.
113 Function::BasicBlockListType &Blocks = BB->getParent()->getBasicBlockList();
114 Blocks.splice(InsertPos, Blocks, BB);
115
116 ++NumMovedBlocks;
117 } else {
118 // This block is already in the right place, we don't have to do anything.
119 ++InsertPos;
120 }
121
122 // Keep placing successors until we run out of ones to place. Note that this
123 // loop is very inefficient (N^2) for blocks with many successors, like switch
124 // statements. FIXME!
125 while (1) {
126 // Okay, now place any unplaced successors.
127 succ_iterator SI = succ_begin(BB), E = succ_end(BB);
128
129 // Scan for the first unplaced successor.
130 for (; SI != E && PlacedBlocks.count(*SI); ++SI)
131 /*empty*/;
132 if (SI == E) return; // No more successors to place.
133
134 double MaxExecutionCount = PI->getExecutionCount(*SI);
135 BasicBlock *MaxSuccessor = *SI;
136
137 // Scan for more frequently executed successors
138 for (; SI != E; ++SI)
139 if (!PlacedBlocks.count(*SI)) {
140 double Count = PI->getExecutionCount(*SI);
141 if (Count > MaxExecutionCount ||
142 // Prefer to not disturb the code.
143 (Count == MaxExecutionCount && *SI == &*InsertPos)) {
144 MaxExecutionCount = Count;
145 MaxSuccessor = *SI;
146 }
147 }
148
149 // Now that we picked the maximally executed successor, place it.
150 PlaceBlocks(MaxSuccessor);
151 }
152 }
153