1 //===-- SpillPlacement.cpp - Optimal Spill Code Placement -----------------===//
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 the spill code placement analysis.
11 //
12 // Each edge bundle corresponds to a node in a Hopfield network. Constraints on
13 // basic blocks are weighted by the block frequency and added to become the node
14 // bias.
15 //
16 // Transparent basic blocks have the variable live through, but don't care if it
17 // is spilled or in a register. These blocks become connections in the Hopfield
18 // network, again weighted by block frequency.
19 //
20 // The Hopfield network minimizes (possibly locally) its energy function:
21 //
22 // E = -sum_n V_n * ( B_n + sum_{n, m linked by b} V_m * F_b )
23 //
24 // The energy function represents the expected spill code execution frequency,
25 // or the cost of spilling. This is a Lyapunov function which never increases
26 // when a node is updated. It is guaranteed to converge to a local minimum.
27 //
28 //===----------------------------------------------------------------------===//
29
30 #define DEBUG_TYPE "spillplacement"
31 #include "SpillPlacement.h"
32 #include "llvm/CodeGen/EdgeBundles.h"
33 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
34 #include "llvm/CodeGen/MachineBasicBlock.h"
35 #include "llvm/CodeGen/MachineFunction.h"
36 #include "llvm/CodeGen/MachineLoopInfo.h"
37 #include "llvm/CodeGen/Passes.h"
38 #include "llvm/Support/Debug.h"
39 #include "llvm/Support/Format.h"
40
41 using namespace llvm;
42
43 char SpillPlacement::ID = 0;
44 INITIALIZE_PASS_BEGIN(SpillPlacement, "spill-code-placement",
45 "Spill Code Placement Analysis", true, true)
46 INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
47 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
48 INITIALIZE_PASS_END(SpillPlacement, "spill-code-placement",
49 "Spill Code Placement Analysis", true, true)
50
51 char &llvm::SpillPlacementID = SpillPlacement::ID;
52
getAnalysisUsage(AnalysisUsage & AU) const53 void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
54 AU.setPreservesAll();
55 AU.addRequiredTransitive<EdgeBundles>();
56 AU.addRequiredTransitive<MachineLoopInfo>();
57 MachineFunctionPass::getAnalysisUsage(AU);
58 }
59
60 /// Node - Each edge bundle corresponds to a Hopfield node.
61 ///
62 /// The node contains precomputed frequency data that only depends on the CFG,
63 /// but Bias and Links are computed each time placeSpills is called.
64 ///
65 /// The node Value is positive when the variable should be in a register. The
66 /// value can change when linked nodes change, but convergence is very fast
67 /// because all weights are positive.
68 ///
69 struct SpillPlacement::Node {
70 /// Scale - Inverse block frequency feeding into[0] or out of[1] the bundle.
71 /// Ideally, these two numbers should be identical, but inaccuracies in the
72 /// block frequency estimates means that we need to normalize ingoing and
73 /// outgoing frequencies separately so they are commensurate.
74 float Scale[2];
75
76 /// Bias - Normalized contributions from non-transparent blocks.
77 /// A bundle connected to a MustSpill block has a huge negative bias,
78 /// otherwise it is a number in the range [-2;2].
79 float Bias;
80
81 /// Value - Output value of this node computed from the Bias and links.
82 /// This is always in the range [-1;1]. A positive number means the variable
83 /// should go in a register through this bundle.
84 float Value;
85
86 typedef SmallVector<std::pair<float, unsigned>, 4> LinkVector;
87
88 /// Links - (Weight, BundleNo) for all transparent blocks connecting to other
89 /// bundles. The weights are all positive and add up to at most 2, weights
90 /// from ingoing and outgoing nodes separately add up to a most 1. The weight
91 /// sum can be less than 2 when the variable is not live into / out of some
92 /// connected basic blocks.
93 LinkVector Links;
94
95 /// preferReg - Return true when this node prefers to be in a register.
preferRegSpillPlacement::Node96 bool preferReg() const {
97 // Undecided nodes (Value==0) go on the stack.
98 return Value > 0;
99 }
100
101 /// mustSpill - Return True if this node is so biased that it must spill.
mustSpillSpillPlacement::Node102 bool mustSpill() const {
103 // Actually, we must spill if Bias < sum(weights).
104 // It may be worth it to compute the weight sum here?
105 return Bias < -2.0f;
106 }
107
108 /// Node - Create a blank Node.
NodeSpillPlacement::Node109 Node() {
110 Scale[0] = Scale[1] = 0;
111 }
112
113 /// clear - Reset per-query data, but preserve frequencies that only depend on
114 // the CFG.
clearSpillPlacement::Node115 void clear() {
116 Bias = Value = 0;
117 Links.clear();
118 }
119
120 /// addLink - Add a link to bundle b with weight w.
121 /// out=0 for an ingoing link, and 1 for an outgoing link.
addLinkSpillPlacement::Node122 void addLink(unsigned b, float w, bool out) {
123 // Normalize w relative to all connected blocks from that direction.
124 w *= Scale[out];
125
126 // There can be multiple links to the same bundle, add them up.
127 for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
128 if (I->second == b) {
129 I->first += w;
130 return;
131 }
132 // This must be the first link to b.
133 Links.push_back(std::make_pair(w, b));
134 }
135
136 /// addBias - Bias this node from an ingoing[0] or outgoing[1] link.
137 /// Return the change to the total number of positive biases.
addBiasSpillPlacement::Node138 void addBias(float w, bool out) {
139 // Normalize w relative to all connected blocks from that direction.
140 w *= Scale[out];
141 Bias += w;
142 }
143
144 /// update - Recompute Value from Bias and Links. Return true when node
145 /// preference changes.
updateSpillPlacement::Node146 bool update(const Node nodes[]) {
147 // Compute the weighted sum of inputs.
148 float Sum = Bias;
149 for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
150 Sum += I->first * nodes[I->second].Value;
151
152 // The weighted sum is going to be in the range [-2;2]. Ideally, we should
153 // simply set Value = sign(Sum), but we will add a dead zone around 0 for
154 // two reasons:
155 // 1. It avoids arbitrary bias when all links are 0 as is possible during
156 // initial iterations.
157 // 2. It helps tame rounding errors when the links nominally sum to 0.
158 const float Thres = 1e-4f;
159 bool Before = preferReg();
160 if (Sum < -Thres)
161 Value = -1;
162 else if (Sum > Thres)
163 Value = 1;
164 else
165 Value = 0;
166 return Before != preferReg();
167 }
168 };
169
runOnMachineFunction(MachineFunction & mf)170 bool SpillPlacement::runOnMachineFunction(MachineFunction &mf) {
171 MF = &mf;
172 bundles = &getAnalysis<EdgeBundles>();
173 loops = &getAnalysis<MachineLoopInfo>();
174
175 assert(!nodes && "Leaking node array");
176 nodes = new Node[bundles->getNumBundles()];
177
178 // Compute total ingoing and outgoing block frequencies for all bundles.
179 BlockFrequency.resize(mf.getNumBlockIDs());
180 for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
181 float Freq = LiveIntervals::getSpillWeight(true, false,
182 loops->getLoopDepth(I));
183 unsigned Num = I->getNumber();
184 BlockFrequency[Num] = Freq;
185 nodes[bundles->getBundle(Num, 1)].Scale[0] += Freq;
186 nodes[bundles->getBundle(Num, 0)].Scale[1] += Freq;
187 }
188
189 // Scales are reciprocal frequencies.
190 for (unsigned i = 0, e = bundles->getNumBundles(); i != e; ++i)
191 for (unsigned d = 0; d != 2; ++d)
192 if (nodes[i].Scale[d] > 0)
193 nodes[i].Scale[d] = 1 / nodes[i].Scale[d];
194
195 // We never change the function.
196 return false;
197 }
198
releaseMemory()199 void SpillPlacement::releaseMemory() {
200 delete[] nodes;
201 nodes = 0;
202 }
203
204 /// activate - mark node n as active if it wasn't already.
activate(unsigned n)205 void SpillPlacement::activate(unsigned n) {
206 if (ActiveNodes->test(n))
207 return;
208 ActiveNodes->set(n);
209 nodes[n].clear();
210 }
211
212
213 /// addConstraints - Compute node biases and weights from a set of constraints.
214 /// Set a bit in NodeMask for each active node.
addConstraints(ArrayRef<BlockConstraint> LiveBlocks)215 void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
216 for (ArrayRef<BlockConstraint>::iterator I = LiveBlocks.begin(),
217 E = LiveBlocks.end(); I != E; ++I) {
218 float Freq = getBlockFrequency(I->Number);
219 const float Bias[] = {
220 0, // DontCare,
221 1, // PrefReg,
222 -1, // PrefSpill
223 -HUGE_VALF // MustSpill
224 };
225
226 // Live-in to block?
227 if (I->Entry != DontCare) {
228 unsigned ib = bundles->getBundle(I->Number, 0);
229 activate(ib);
230 nodes[ib].addBias(Freq * Bias[I->Entry], 1);
231 }
232
233 // Live-out from block?
234 if (I->Exit != DontCare) {
235 unsigned ob = bundles->getBundle(I->Number, 1);
236 activate(ob);
237 nodes[ob].addBias(Freq * Bias[I->Exit], 0);
238 }
239 }
240 }
241
addLinks(ArrayRef<unsigned> Links)242 void SpillPlacement::addLinks(ArrayRef<unsigned> Links) {
243 for (ArrayRef<unsigned>::iterator I = Links.begin(), E = Links.end(); I != E;
244 ++I) {
245 unsigned Number = *I;
246 unsigned ib = bundles->getBundle(Number, 0);
247 unsigned ob = bundles->getBundle(Number, 1);
248
249 // Ignore self-loops.
250 if (ib == ob)
251 continue;
252 activate(ib);
253 activate(ob);
254 if (nodes[ib].Links.empty() && !nodes[ib].mustSpill())
255 Linked.push_back(ib);
256 if (nodes[ob].Links.empty() && !nodes[ob].mustSpill())
257 Linked.push_back(ob);
258 float Freq = getBlockFrequency(Number);
259 nodes[ib].addLink(ob, Freq, 1);
260 nodes[ob].addLink(ib, Freq, 0);
261 }
262 }
263
scanActiveBundles()264 bool SpillPlacement::scanActiveBundles() {
265 Linked.clear();
266 RecentPositive.clear();
267 for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n)) {
268 nodes[n].update(nodes);
269 // A node that must spill, or a node without any links is not going to
270 // change its value ever again, so exclude it from iterations.
271 if (nodes[n].mustSpill())
272 continue;
273 if (!nodes[n].Links.empty())
274 Linked.push_back(n);
275 if (nodes[n].preferReg())
276 RecentPositive.push_back(n);
277 }
278 return !RecentPositive.empty();
279 }
280
281 /// iterate - Repeatedly update the Hopfield nodes until stability or the
282 /// maximum number of iterations is reached.
283 /// @param Linked - Numbers of linked nodes that need updating.
iterate()284 void SpillPlacement::iterate() {
285 // First update the recently positive nodes. They have likely received new
286 // negative bias that will turn them off.
287 while (!RecentPositive.empty())
288 nodes[RecentPositive.pop_back_val()].update(nodes);
289
290 if (Linked.empty())
291 return;
292
293 // Run up to 10 iterations. The edge bundle numbering is closely related to
294 // basic block numbering, so there is a strong tendency towards chains of
295 // linked nodes with sequential numbers. By scanning the linked nodes
296 // backwards and forwards, we make it very likely that a single node can
297 // affect the entire network in a single iteration. That means very fast
298 // convergence, usually in a single iteration.
299 for (unsigned iteration = 0; iteration != 10; ++iteration) {
300 // Scan backwards, skipping the last node which was just updated.
301 bool Changed = false;
302 for (SmallVectorImpl<unsigned>::const_reverse_iterator I =
303 llvm::next(Linked.rbegin()), E = Linked.rend(); I != E; ++I) {
304 unsigned n = *I;
305 if (nodes[n].update(nodes)) {
306 Changed = true;
307 if (nodes[n].preferReg())
308 RecentPositive.push_back(n);
309 }
310 }
311 if (!Changed || !RecentPositive.empty())
312 return;
313
314 // Scan forwards, skipping the first node which was just updated.
315 Changed = false;
316 for (SmallVectorImpl<unsigned>::const_iterator I =
317 llvm::next(Linked.begin()), E = Linked.end(); I != E; ++I) {
318 unsigned n = *I;
319 if (nodes[n].update(nodes)) {
320 Changed = true;
321 if (nodes[n].preferReg())
322 RecentPositive.push_back(n);
323 }
324 }
325 if (!Changed || !RecentPositive.empty())
326 return;
327 }
328 }
329
prepare(BitVector & RegBundles)330 void SpillPlacement::prepare(BitVector &RegBundles) {
331 Linked.clear();
332 RecentPositive.clear();
333 // Reuse RegBundles as our ActiveNodes vector.
334 ActiveNodes = &RegBundles;
335 ActiveNodes->clear();
336 ActiveNodes->resize(bundles->getNumBundles());
337 }
338
339 bool
finish()340 SpillPlacement::finish() {
341 assert(ActiveNodes && "Call prepare() first");
342
343 // Write preferences back to ActiveNodes.
344 bool Perfect = true;
345 for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n))
346 if (!nodes[n].preferReg()) {
347 ActiveNodes->reset(n);
348 Perfect = false;
349 }
350 ActiveNodes = 0;
351 return Perfect;
352 }
353