1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
|
//===-- BasicBlockPlacement.cpp - Basic Block Code Layout optimization ----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a very simple profile guided basic block placement
// algorithm. The idea is to put frequently executed blocks together at the
// start of the function, and hopefully increase the number of fall-through
// conditional branches. If there is no profile information for a particular
// function, this pass basically orders blocks in depth-first order
//
// The algorithm implemented here is basically "Algo1" from "Profile Guided Code
// Positioning" by Pettis and Hansen, except that it uses basic block counts
// instead of edge counts. This should be improved in many ways, but is very
// simple for now.
//
// Basically we "place" the entry block, then loop over all successors in a DFO,
// placing the most frequently executed successor until we run out of blocks. I
// told you this was _extremely_ simplistic. :) This is also much slower than it
// could be. When it becomes important, this pass will be rewritten to use a
// better algorithm, and then we can worry about efficiency.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "block-placement"
#include "llvm/Analysis/ProfileInfo.h"
#include "llvm/Function.h"
#include "llvm/Pass.h"
#include "llvm/Support/CFG.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Transforms/Scalar.h"
#include <set>
using namespace llvm;
STATISTIC(NumMoved, "Number of basic blocks moved");
namespace {
struct BlockPlacement : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
BlockPlacement() : FunctionPass(ID) {
initializeBlockPlacementPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnFunction(Function &F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<ProfileInfo>();
//AU.addPreserved<ProfileInfo>(); // Does this work?
}
private:
/// PI - The profile information that is guiding us.
///
ProfileInfo *PI;
/// NumMovedBlocks - Every time we move a block, increment this counter.
///
unsigned NumMovedBlocks;
/// PlacedBlocks - Every time we place a block, remember it so we don't get
/// into infinite loops.
std::set<BasicBlock*> PlacedBlocks;
/// InsertPos - This an iterator to the next place we want to insert a
/// block.
Function::iterator InsertPos;
/// PlaceBlocks - Recursively place the specified blocks and any unplaced
/// successors.
void PlaceBlocks(BasicBlock *BB);
};
}
char BlockPlacement::ID = 0;
INITIALIZE_PASS_BEGIN(BlockPlacement, "block-placement",
"Profile Guided Basic Block Placement", false, false)
INITIALIZE_AG_DEPENDENCY(ProfileInfo)
INITIALIZE_PASS_END(BlockPlacement, "block-placement",
"Profile Guided Basic Block Placement", false, false)
FunctionPass *llvm::createBlockPlacementPass() { return new BlockPlacement(); }
bool BlockPlacement::runOnFunction(Function &F) {
PI = &getAnalysis<ProfileInfo>();
NumMovedBlocks = 0;
InsertPos = F.begin();
// Recursively place all blocks.
PlaceBlocks(F.begin());
PlacedBlocks.clear();
NumMoved += NumMovedBlocks;
return NumMovedBlocks != 0;
}
/// PlaceBlocks - Recursively place the specified blocks and any unplaced
/// successors.
void BlockPlacement::PlaceBlocks(BasicBlock *BB) {
assert(!PlacedBlocks.count(BB) && "Already placed this block!");
PlacedBlocks.insert(BB);
// Place the specified block.
if (&*InsertPos != BB) {
// Use splice to move the block into the right place. This avoids having to
// remove the block from the function then readd it, which causes a bunch of
// symbol table traffic that is entirely pointless.
Function::BasicBlockListType &Blocks = BB->getParent()->getBasicBlockList();
Blocks.splice(InsertPos, Blocks, BB);
++NumMovedBlocks;
} else {
// This block is already in the right place, we don't have to do anything.
++InsertPos;
}
// Keep placing successors until we run out of ones to place. Note that this
// loop is very inefficient (N^2) for blocks with many successors, like switch
// statements. FIXME!
while (1) {
// Okay, now place any unplaced successors.
succ_iterator SI = succ_begin(BB), E = succ_end(BB);
// Scan for the first unplaced successor.
for (; SI != E && PlacedBlocks.count(*SI); ++SI)
/*empty*/;
if (SI == E) return; // No more successors to place.
double MaxExecutionCount = PI->getExecutionCount(*SI);
BasicBlock *MaxSuccessor = *SI;
// Scan for more frequently executed successors
for (; SI != E; ++SI)
if (!PlacedBlocks.count(*SI)) {
double Count = PI->getExecutionCount(*SI);
if (Count > MaxExecutionCount ||
// Prefer to not disturb the code.
(Count == MaxExecutionCount && *SI == &*InsertPos)) {
MaxExecutionCount = Count;
MaxSuccessor = *SI;
}
}
// Now that we picked the maximally executed successor, place it.
PlaceBlocks(MaxSuccessor);
}
}
|
