1 //===-- SpillPlacement.cpp - Optimal Spill Code Placement -----------------===//
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 the spill code placement analysis.
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
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.
20 // The Hopfield network minimizes (possibly locally) its energy function:
22 // E = -sum_n V_n * ( B_n + sum_{n, m linked by b} V_m * F_b )
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.
28 //===----------------------------------------------------------------------===//
30 #define DEBUG_TYPE "spillplacement"
31 #include "SpillPlacement.h"
32 #include "llvm/ADT/BitVector.h"
33 #include "llvm/CodeGen/EdgeBundles.h"
34 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
35 #include "llvm/CodeGen/MachineBasicBlock.h"
36 #include "llvm/CodeGen/MachineFunction.h"
37 #include "llvm/CodeGen/MachineLoopInfo.h"
38 #include "llvm/CodeGen/Passes.h"
39 #include "llvm/Support/Debug.h"
40 #include "llvm/Support/Format.h"
44 char SpillPlacement::ID = 0;
45 INITIALIZE_PASS_BEGIN(SpillPlacement, "spill-code-placement",
46 "Spill Code Placement Analysis", true, true)
47 INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
48 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
49 INITIALIZE_PASS_END(SpillPlacement, "spill-code-placement",
50 "Spill Code Placement Analysis", true, true)
52 char &llvm::SpillPlacementID = SpillPlacement::ID;
54 void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
56 AU.addRequiredTransitive<EdgeBundles>();
57 AU.addRequiredTransitive<MachineLoopInfo>();
58 MachineFunctionPass::getAnalysisUsage(AU);
61 /// Node - Each edge bundle corresponds to a Hopfield node.
63 /// The node contains precomputed frequency data that only depends on the CFG,
64 /// but Bias and Links are computed each time placeSpills is called.
66 /// The node Value is positive when the variable should be in a register. The
67 /// value can change when linked nodes change, but convergence is very fast
68 /// because all weights are positive.
70 struct SpillPlacement::Node {
71 /// Scale - Inverse block frequency feeding into[0] or out of[1] the bundle.
72 /// Ideally, these two numbers should be identical, but inaccuracies in the
73 /// block frequency estimates means that we need to normalize ingoing and
74 /// outgoing frequencies separately so they are commensurate.
77 /// Bias - Normalized contributions from non-transparent blocks.
78 /// A bundle connected to a MustSpill block has a huge negative bias,
79 /// otherwise it is a number in the range [-2;2].
82 /// Value - Output value of this node computed from the Bias and links.
83 /// This is always in the range [-1;1]. A positive number means the variable
84 /// should go in a register through this bundle.
87 typedef SmallVector<std::pair<float, unsigned>, 4> LinkVector;
89 /// Links - (Weight, BundleNo) for all transparent blocks connecting to other
90 /// bundles. The weights are all positive and add up to at most 2, weights
91 /// from ingoing and outgoing nodes separately add up to a most 1. The weight
92 /// sum can be less than 2 when the variable is not live into / out of some
93 /// connected basic blocks.
96 /// preferReg - Return true when this node prefers to be in a register.
97 bool preferReg() const {
98 // Undecided nodes (Value==0) go on the stack.
102 /// mustSpill - Return True if this node is so biased that it must spill.
103 bool mustSpill() const {
104 // Actually, we must spill if Bias < sum(weights).
105 // It may be worth it to compute the weight sum here?
109 /// Node - Create a blank Node.
111 Scale[0] = Scale[1] = 0;
114 /// clear - Reset per-query data, but preserve frequencies that only depend on
121 /// addLink - Add a link to bundle b with weight w.
122 /// out=0 for an ingoing link, and 1 for an outgoing link.
123 void addLink(unsigned b, float w, bool out) {
124 // Normalize w relative to all connected blocks from that direction.
127 // There can be multiple links to the same bundle, add them up.
128 for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
129 if (I->second == b) {
133 // This must be the first link to b.
134 Links.push_back(std::make_pair(w, b));
137 /// addBias - Bias this node from an ingoing[0] or outgoing[1] link.
138 /// Return the change to the total number of positive biases.
139 void addBias(float w, bool out) {
140 // Normalize w relative to all connected blocks from that direction.
145 /// update - Recompute Value from Bias and Links. Return true when node
146 /// preference changes.
147 bool update(const Node nodes[]) {
148 // Compute the weighted sum of inputs.
150 for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
151 Sum += I->first * nodes[I->second].Value;
153 // The weighted sum is going to be in the range [-2;2]. Ideally, we should
154 // simply set Value = sign(Sum), but we will add a dead zone around 0 for
156 // 1. It avoids arbitrary bias when all links are 0 as is possible during
157 // initial iterations.
158 // 2. It helps tame rounding errors when the links nominally sum to 0.
159 const float Thres = 1e-4f;
160 bool Before = preferReg();
163 else if (Sum > Thres)
167 return Before != preferReg();
171 bool SpillPlacement::runOnMachineFunction(MachineFunction &mf) {
173 bundles = &getAnalysis<EdgeBundles>();
174 loops = &getAnalysis<MachineLoopInfo>();
176 assert(!nodes && "Leaking node array");
177 nodes = new Node[bundles->getNumBundles()];
179 // Compute total ingoing and outgoing block frequencies for all bundles.
180 BlockFrequency.resize(mf.getNumBlockIDs());
181 for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
182 float Freq = LiveIntervals::getSpillWeight(true, false,
183 loops->getLoopDepth(I));
184 unsigned Num = I->getNumber();
185 BlockFrequency[Num] = Freq;
186 nodes[bundles->getBundle(Num, 1)].Scale[0] += Freq;
187 nodes[bundles->getBundle(Num, 0)].Scale[1] += Freq;
190 // Scales are reciprocal frequencies.
191 for (unsigned i = 0, e = bundles->getNumBundles(); i != e; ++i)
192 for (unsigned d = 0; d != 2; ++d)
193 if (nodes[i].Scale[d] > 0)
194 nodes[i].Scale[d] = 1 / nodes[i].Scale[d];
196 // We never change the function.
200 void SpillPlacement::releaseMemory() {
205 /// activate - mark node n as active if it wasn't already.
206 void SpillPlacement::activate(unsigned n) {
207 if (ActiveNodes->test(n))
212 // Very large bundles usually come from big switches, indirect branches,
213 // landing pads, or loops with many 'continue' statements. It is difficult to
214 // allocate registers when so many different blocks are involved.
216 // Give a small negative bias to large bundles such that 1/32 of the
217 // connected blocks need to be interested before we consider expanding the
218 // region through the bundle. This helps compile time by limiting the number
219 // of blocks visited and the number of links in the Hopfield network.
220 if (bundles->getBlocks(n).size() > 100)
221 nodes[n].Bias = -0.0625f;
225 /// addConstraints - Compute node biases and weights from a set of constraints.
226 /// Set a bit in NodeMask for each active node.
227 void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
228 for (ArrayRef<BlockConstraint>::iterator I = LiveBlocks.begin(),
229 E = LiveBlocks.end(); I != E; ++I) {
230 float Freq = getBlockFrequency(I->Number);
231 const float Bias[] = {
236 -HUGE_VALF // MustSpill
240 if (I->Entry != DontCare) {
241 unsigned ib = bundles->getBundle(I->Number, 0);
243 nodes[ib].addBias(Freq * Bias[I->Entry], 1);
246 // Live-out from block?
247 if (I->Exit != DontCare) {
248 unsigned ob = bundles->getBundle(I->Number, 1);
250 nodes[ob].addBias(Freq * Bias[I->Exit], 0);
255 /// addPrefSpill - Same as addConstraints(PrefSpill)
256 void SpillPlacement::addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong) {
257 for (ArrayRef<unsigned>::iterator I = Blocks.begin(), E = Blocks.end();
259 float Freq = getBlockFrequency(*I);
262 unsigned ib = bundles->getBundle(*I, 0);
263 unsigned ob = bundles->getBundle(*I, 1);
266 nodes[ib].addBias(-Freq, 1);
267 nodes[ob].addBias(-Freq, 0);
271 void SpillPlacement::addLinks(ArrayRef<unsigned> Links) {
272 for (ArrayRef<unsigned>::iterator I = Links.begin(), E = Links.end(); I != E;
274 unsigned Number = *I;
275 unsigned ib = bundles->getBundle(Number, 0);
276 unsigned ob = bundles->getBundle(Number, 1);
278 // Ignore self-loops.
283 if (nodes[ib].Links.empty() && !nodes[ib].mustSpill())
284 Linked.push_back(ib);
285 if (nodes[ob].Links.empty() && !nodes[ob].mustSpill())
286 Linked.push_back(ob);
287 float Freq = getBlockFrequency(Number);
288 nodes[ib].addLink(ob, Freq, 1);
289 nodes[ob].addLink(ib, Freq, 0);
293 bool SpillPlacement::scanActiveBundles() {
295 RecentPositive.clear();
296 for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n)) {
297 nodes[n].update(nodes);
298 // A node that must spill, or a node without any links is not going to
299 // change its value ever again, so exclude it from iterations.
300 if (nodes[n].mustSpill())
302 if (!nodes[n].Links.empty())
304 if (nodes[n].preferReg())
305 RecentPositive.push_back(n);
307 return !RecentPositive.empty();
310 /// iterate - Repeatedly update the Hopfield nodes until stability or the
311 /// maximum number of iterations is reached.
312 /// @param Linked - Numbers of linked nodes that need updating.
313 void SpillPlacement::iterate() {
314 // First update the recently positive nodes. They have likely received new
315 // negative bias that will turn them off.
316 while (!RecentPositive.empty())
317 nodes[RecentPositive.pop_back_val()].update(nodes);
322 // Run up to 10 iterations. The edge bundle numbering is closely related to
323 // basic block numbering, so there is a strong tendency towards chains of
324 // linked nodes with sequential numbers. By scanning the linked nodes
325 // backwards and forwards, we make it very likely that a single node can
326 // affect the entire network in a single iteration. That means very fast
327 // convergence, usually in a single iteration.
328 for (unsigned iteration = 0; iteration != 10; ++iteration) {
329 // Scan backwards, skipping the last node which was just updated.
330 bool Changed = false;
331 for (SmallVectorImpl<unsigned>::const_reverse_iterator I =
332 llvm::next(Linked.rbegin()), E = Linked.rend(); I != E; ++I) {
334 if (nodes[n].update(nodes)) {
336 if (nodes[n].preferReg())
337 RecentPositive.push_back(n);
340 if (!Changed || !RecentPositive.empty())
343 // Scan forwards, skipping the first node which was just updated.
345 for (SmallVectorImpl<unsigned>::const_iterator I =
346 llvm::next(Linked.begin()), E = Linked.end(); I != E; ++I) {
348 if (nodes[n].update(nodes)) {
350 if (nodes[n].preferReg())
351 RecentPositive.push_back(n);
354 if (!Changed || !RecentPositive.empty())
359 void SpillPlacement::prepare(BitVector &RegBundles) {
361 RecentPositive.clear();
362 // Reuse RegBundles as our ActiveNodes vector.
363 ActiveNodes = &RegBundles;
364 ActiveNodes->clear();
365 ActiveNodes->resize(bundles->getNumBundles());
369 SpillPlacement::finish() {
370 assert(ActiveNodes && "Call prepare() first");
372 // Write preferences back to ActiveNodes.
374 for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n))
375 if (!nodes[n].preferReg()) {
376 ActiveNodes->reset(n);