1//===- SwitchLoweringUtils.cpp - Switch Lowering --------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file contains switch inst lowering optimizations and utilities for
10// codegen, so that it can be used for both SelectionDAG and GlobalISel.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/CodeGen/SwitchLoweringUtils.h"
15#include "llvm/CodeGen/FunctionLoweringInfo.h"
16#include "llvm/CodeGen/MachineJumpTableInfo.h"
17#include "llvm/CodeGen/TargetLowering.h"
18#include "llvm/Target/TargetMachine.h"
19
20using namespace llvm;
21using namespace SwitchCG;
22
23uint64_t SwitchCG::getJumpTableRange(const CaseClusterVector &Clusters,
24 unsigned First, unsigned Last) {
25 assert(Last >= First);
26 const APInt &LowCase = Clusters[First].Low->getValue();
27 const APInt &HighCase = Clusters[Last].High->getValue();
28 assert(LowCase.getBitWidth() == HighCase.getBitWidth());
29
30 // FIXME: A range of consecutive cases has 100% density, but only requires one
31 // comparison to lower. We should discriminate against such consecutive ranges
32 // in jump tables.
33 return (HighCase - LowCase).getLimitedValue(Limit: (UINT64_MAX - 1) / 100) + 1;
34}
35
36uint64_t
37SwitchCG::getJumpTableNumCases(const SmallVectorImpl<unsigned> &TotalCases,
38 unsigned First, unsigned Last) {
39 assert(Last >= First);
40 assert(TotalCases[Last] >= TotalCases[First]);
41 uint64_t NumCases =
42 TotalCases[Last] - (First == 0 ? 0 : TotalCases[First - 1]);
43 return NumCases;
44}
45
46void SwitchCG::SwitchLowering::findJumpTables(CaseClusterVector &Clusters,
47 const SwitchInst *SI,
48 std::optional<SDLoc> SL,
49 MachineBasicBlock *DefaultMBB,
50 ProfileSummaryInfo *PSI,
51 BlockFrequencyInfo *BFI) {
52#ifndef NDEBUG
53 // Clusters must be non-empty, sorted, and only contain Range clusters.
54 assert(!Clusters.empty());
55 for (CaseCluster &C : Clusters)
56 assert(C.Kind == CC_Range);
57 for (unsigned i = 1, e = Clusters.size(); i < e; ++i)
58 assert(Clusters[i - 1].High->getValue().slt(Clusters[i].Low->getValue()));
59#endif
60
61 assert(TLI && "TLI not set!");
62 if (!TLI->areJTsAllowed(Fn: SI->getParent()->getParent()))
63 return;
64
65 const unsigned MinJumpTableEntries = TLI->getMinimumJumpTableEntries();
66 const unsigned SmallNumberOfEntries = MinJumpTableEntries / 2;
67
68 // Bail if not enough cases.
69 const int64_t N = Clusters.size();
70 if (N < 2 || N < MinJumpTableEntries)
71 return;
72
73 // Accumulated number of cases in each cluster and those prior to it.
74 SmallVector<unsigned, 8> TotalCases(N);
75 for (unsigned i = 0; i < N; ++i) {
76 const APInt &Hi = Clusters[i].High->getValue();
77 const APInt &Lo = Clusters[i].Low->getValue();
78 TotalCases[i] = (Hi - Lo).getLimitedValue() + 1;
79 if (i != 0)
80 TotalCases[i] += TotalCases[i - 1];
81 }
82
83 uint64_t Range = getJumpTableRange(Clusters,First: 0, Last: N - 1);
84 uint64_t NumCases = getJumpTableNumCases(TotalCases, First: 0, Last: N - 1);
85 assert(NumCases < UINT64_MAX / 100);
86 assert(Range >= NumCases);
87
88 // Cheap case: the whole range may be suitable for jump table.
89 if (TLI->isSuitableForJumpTable(SI, NumCases, Range, PSI, BFI)) {
90 CaseCluster JTCluster;
91 if (buildJumpTable(Clusters, First: 0, Last: N - 1, SI, SL, DefaultMBB, JTCluster)) {
92 Clusters[0] = JTCluster;
93 Clusters.resize(new_size: 1);
94 return;
95 }
96 }
97
98 // The algorithm below is not suitable for -O0.
99 if (TM->getOptLevel() == CodeGenOptLevel::None)
100 return;
101
102 // Split Clusters into minimum number of dense partitions. The algorithm uses
103 // the same idea as Kannan & Proebsting "Correction to 'Producing Good Code
104 // for the Case Statement'" (1994), but builds the MinPartitions array in
105 // reverse order to make it easier to reconstruct the partitions in ascending
106 // order. In the choice between two optimal partitionings, it picks the one
107 // which yields more jump tables. The algorithm is described in
108 // https://arxiv.org/pdf/1910.02351v2
109
110 // MinPartitions[i] is the minimum nbr of partitions of Clusters[i..N-1].
111 SmallVector<unsigned, 8> MinPartitions(N);
112 // LastElement[i] is the last element of the partition starting at i.
113 SmallVector<unsigned, 8> LastElement(N);
114 // PartitionsScore[i] is used to break ties when choosing between two
115 // partitionings resulting in the same number of partitions.
116 SmallVector<unsigned, 8> PartitionsScore(N);
117 // For PartitionsScore, a small number of comparisons is considered as good as
118 // a jump table and a single comparison is considered better than a jump
119 // table.
120 enum PartitionScores : unsigned {
121 NoTable = 0,
122 Table = 1,
123 FewCases = 1,
124 SingleCase = 2
125 };
126
127 // Base case: There is only one way to partition Clusters[N-1].
128 MinPartitions[N - 1] = 1;
129 LastElement[N - 1] = N - 1;
130 PartitionsScore[N - 1] = PartitionScores::SingleCase;
131
132 // Note: loop indexes are signed to avoid underflow.
133 for (int64_t i = N - 2; i >= 0; i--) {
134 // Find optimal partitioning of Clusters[i..N-1].
135 // Baseline: Put Clusters[i] into a partition on its own.
136 MinPartitions[i] = MinPartitions[i + 1] + 1;
137 LastElement[i] = i;
138 PartitionsScore[i] = PartitionsScore[i + 1] + PartitionScores::SingleCase;
139
140 // Search for a solution that results in fewer partitions.
141 for (int64_t j = N - 1; j > i; j--) {
142 // Try building a partition from Clusters[i..j].
143 Range = getJumpTableRange(Clusters, First: i, Last: j);
144 NumCases = getJumpTableNumCases(TotalCases, First: i, Last: j);
145 assert(NumCases < UINT64_MAX / 100);
146 assert(Range >= NumCases);
147
148 if (TLI->isSuitableForJumpTable(SI, NumCases, Range, PSI, BFI)) {
149 unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
150 unsigned Score = j == N - 1 ? 0 : PartitionsScore[j + 1];
151 int64_t NumEntries = j - i + 1;
152
153 if (NumEntries == 1)
154 Score += PartitionScores::SingleCase;
155 else if (NumEntries <= SmallNumberOfEntries)
156 Score += PartitionScores::FewCases;
157 else if (NumEntries >= MinJumpTableEntries)
158 Score += PartitionScores::Table;
159
160 // If this leads to fewer partitions, or to the same number of
161 // partitions with better score, it is a better partitioning.
162 if (NumPartitions < MinPartitions[i] ||
163 (NumPartitions == MinPartitions[i] && Score > PartitionsScore[i])) {
164 MinPartitions[i] = NumPartitions;
165 LastElement[i] = j;
166 PartitionsScore[i] = Score;
167 }
168 }
169 }
170 }
171
172 // Iterate over the partitions, replacing some with jump tables in-place.
173 unsigned DstIndex = 0;
174 for (unsigned First = 0, Last; First < N; First = Last + 1) {
175 Last = LastElement[First];
176 assert(Last >= First);
177 assert(DstIndex <= First);
178 unsigned NumClusters = Last - First + 1;
179
180 CaseCluster JTCluster;
181 if (NumClusters >= MinJumpTableEntries &&
182 buildJumpTable(Clusters, First, Last, SI, SL, DefaultMBB, JTCluster)) {
183 Clusters[DstIndex++] = JTCluster;
184 } else {
185 for (unsigned I = First; I <= Last; ++I)
186 std::memmove(dest: &Clusters[DstIndex++], src: &Clusters[I], n: sizeof(Clusters[I]));
187 }
188 }
189 Clusters.resize(new_size: DstIndex);
190}
191
192bool SwitchCG::SwitchLowering::buildJumpTable(const CaseClusterVector &Clusters,
193 unsigned First, unsigned Last,
194 const SwitchInst *SI,
195 const std::optional<SDLoc> &SL,
196 MachineBasicBlock *DefaultMBB,
197 CaseCluster &JTCluster) {
198 assert(First <= Last);
199
200 auto Prob = BranchProbability::getZero();
201 unsigned NumCmps = 0;
202 std::vector<MachineBasicBlock*> Table;
203 DenseMap<MachineBasicBlock*, BranchProbability> JTProbs;
204
205 // Initialize probabilities in JTProbs.
206 for (unsigned I = First; I <= Last; ++I)
207 JTProbs[Clusters[I].MBB] = BranchProbability::getZero();
208
209 for (unsigned I = First; I <= Last; ++I) {
210 assert(Clusters[I].Kind == CC_Range);
211 Prob += Clusters[I].Prob;
212 const APInt &Low = Clusters[I].Low->getValue();
213 const APInt &High = Clusters[I].High->getValue();
214 NumCmps += (Low == High) ? 1 : 2;
215 if (I != First) {
216 // Fill the gap between this and the previous cluster.
217 const APInt &PreviousHigh = Clusters[I - 1].High->getValue();
218 assert(PreviousHigh.slt(Low));
219 uint64_t Gap = (Low - PreviousHigh).getLimitedValue() - 1;
220 for (uint64_t J = 0; J < Gap; J++)
221 Table.push_back(x: DefaultMBB);
222 }
223 uint64_t ClusterSize = (High - Low).getLimitedValue() + 1;
224 for (uint64_t J = 0; J < ClusterSize; ++J)
225 Table.push_back(x: Clusters[I].MBB);
226 JTProbs[Clusters[I].MBB] += Clusters[I].Prob;
227 }
228
229 unsigned NumDests = JTProbs.size();
230 if (TLI->isSuitableForBitTests(NumDests, NumCmps,
231 Low: Clusters[First].Low->getValue(),
232 High: Clusters[Last].High->getValue(), DL: *DL)) {
233 // Clusters[First..Last] should be lowered as bit tests instead.
234 return false;
235 }
236
237 // Create the MBB that will load from and jump through the table.
238 // Note: We create it here, but it's not inserted into the function yet.
239 MachineFunction *CurMF = FuncInfo.MF;
240 MachineBasicBlock *JumpTableMBB =
241 CurMF->CreateMachineBasicBlock(BB: SI->getParent());
242
243 // Add successors. Note: use table order for determinism.
244 SmallPtrSet<MachineBasicBlock *, 8> Done;
245 for (MachineBasicBlock *Succ : Table) {
246 if (Done.count(Ptr: Succ))
247 continue;
248 addSuccessorWithProb(Src: JumpTableMBB, Dst: Succ, Prob: JTProbs[Succ]);
249 Done.insert(Ptr: Succ);
250 }
251 JumpTableMBB->normalizeSuccProbs();
252
253 unsigned JTI = CurMF->getOrCreateJumpTableInfo(JTEntryKind: TLI->getJumpTableEncoding())
254 ->createJumpTableIndex(DestBBs: Table);
255
256 // Set up the jump table info.
257 JumpTable JT(Register(), JTI, JumpTableMBB, nullptr, SL);
258 JumpTableHeader JTH(Clusters[First].Low->getValue(),
259 Clusters[Last].High->getValue(), SI->getCondition(),
260 nullptr, false);
261 JTCases.emplace_back(args: std::move(JTH), args: std::move(JT));
262
263 JTCluster = CaseCluster::jumpTable(Low: Clusters[First].Low, High: Clusters[Last].High,
264 JTCasesIndex: JTCases.size() - 1, Prob);
265 return true;
266}
267
268void SwitchCG::SwitchLowering::findBitTestClusters(CaseClusterVector &Clusters,
269 const SwitchInst *SI) {
270 // Partition Clusters into as few subsets as possible, where each subset has a
271 // range that fits in a machine word and has <= 3 unique destinations.
272
273#ifndef NDEBUG
274 // Clusters must be sorted and contain Range or JumpTable clusters.
275 assert(!Clusters.empty());
276 assert(Clusters[0].Kind == CC_Range || Clusters[0].Kind == CC_JumpTable);
277 for (const CaseCluster &C : Clusters)
278 assert(C.Kind == CC_Range || C.Kind == CC_JumpTable);
279 for (unsigned i = 1; i < Clusters.size(); ++i)
280 assert(Clusters[i-1].High->getValue().slt(Clusters[i].Low->getValue()));
281#endif
282
283 // The algorithm below is not suitable for -O0.
284 if (TM->getOptLevel() == CodeGenOptLevel::None)
285 return;
286
287 // If target does not have legal shift left, do not emit bit tests at all.
288 EVT PTy = TLI->getPointerTy(DL: *DL);
289 if (!TLI->isOperationLegal(Op: ISD::SHL, VT: PTy))
290 return;
291
292 int BitWidth = PTy.getSizeInBits();
293 const int64_t N = Clusters.size();
294
295 // MinPartitions[i] is the minimum nbr of partitions of Clusters[i..N-1].
296 SmallVector<unsigned, 8> MinPartitions(N);
297 // LastElement[i] is the last element of the partition starting at i.
298 SmallVector<unsigned, 8> LastElement(N);
299
300 // FIXME: This might not be the best algorithm for finding bit test clusters.
301
302 // Base case: There is only one way to partition Clusters[N-1].
303 MinPartitions[N - 1] = 1;
304 LastElement[N - 1] = N - 1;
305
306 // Note: loop indexes are signed to avoid underflow.
307 for (int64_t i = N - 2; i >= 0; --i) {
308 // Find optimal partitioning of Clusters[i..N-1].
309 // Baseline: Put Clusters[i] into a partition on its own.
310 MinPartitions[i] = MinPartitions[i + 1] + 1;
311 LastElement[i] = i;
312
313 // Search for a solution that results in fewer partitions.
314 // Note: the search is limited by BitWidth, reducing time complexity.
315 for (int64_t j = std::min(a: N - 1, b: i + BitWidth - 1); j > i; --j) {
316 // Try building a partition from Clusters[i..j].
317
318 // Check the range.
319 if (!TLI->rangeFitsInWord(Low: Clusters[i].Low->getValue(),
320 High: Clusters[j].High->getValue(), DL: *DL))
321 continue;
322
323 // Check nbr of destinations and cluster types.
324 // FIXME: This works, but doesn't seem very efficient.
325 bool RangesOnly = true;
326 BitVector Dests(FuncInfo.MF->getNumBlockIDs());
327 for (int64_t k = i; k <= j; k++) {
328 if (Clusters[k].Kind != CC_Range) {
329 RangesOnly = false;
330 break;
331 }
332 Dests.set(Clusters[k].MBB->getNumber());
333 }
334 if (!RangesOnly || Dests.count() > 3)
335 break;
336
337 // Check if it's a better partition.
338 unsigned NumPartitions = 1 + (j == N - 1 ? 0 : MinPartitions[j + 1]);
339 if (NumPartitions < MinPartitions[i]) {
340 // Found a better partition.
341 MinPartitions[i] = NumPartitions;
342 LastElement[i] = j;
343 }
344 }
345 }
346
347 // Iterate over the partitions, replacing with bit-test clusters in-place.
348 unsigned DstIndex = 0;
349 for (unsigned First = 0, Last; First < N; First = Last + 1) {
350 Last = LastElement[First];
351 assert(First <= Last);
352 assert(DstIndex <= First);
353
354 CaseCluster BitTestCluster;
355 if (buildBitTests(Clusters, First, Last, SI, BTCluster&: BitTestCluster)) {
356 Clusters[DstIndex++] = BitTestCluster;
357 } else {
358 size_t NumClusters = Last - First + 1;
359 std::memmove(dest: &Clusters[DstIndex], src: &Clusters[First],
360 n: sizeof(Clusters[0]) * NumClusters);
361 DstIndex += NumClusters;
362 }
363 }
364 Clusters.resize(new_size: DstIndex);
365}
366
367bool SwitchCG::SwitchLowering::buildBitTests(CaseClusterVector &Clusters,
368 unsigned First, unsigned Last,
369 const SwitchInst *SI,
370 CaseCluster &BTCluster) {
371 assert(First <= Last);
372 if (First == Last)
373 return false;
374
375 BitVector Dests(FuncInfo.MF->getNumBlockIDs());
376 unsigned NumCmps = 0;
377 for (int64_t I = First; I <= Last; ++I) {
378 assert(Clusters[I].Kind == CC_Range);
379 Dests.set(Clusters[I].MBB->getNumber());
380 NumCmps += (Clusters[I].Low == Clusters[I].High) ? 1 : 2;
381 }
382 unsigned NumDests = Dests.count();
383
384 APInt Low = Clusters[First].Low->getValue();
385 APInt High = Clusters[Last].High->getValue();
386 assert(Low.slt(High));
387
388 if (!TLI->isSuitableForBitTests(NumDests, NumCmps, Low, High, DL: *DL))
389 return false;
390
391 APInt LowBound;
392 APInt CmpRange;
393
394 const int BitWidth = TLI->getPointerTy(DL: *DL).getSizeInBits();
395 assert(TLI->rangeFitsInWord(Low, High, *DL) &&
396 "Case range must fit in bit mask!");
397
398 // Check if the clusters cover a contiguous range such that no value in the
399 // range will jump to the default statement.
400 bool ContiguousRange = true;
401 for (int64_t I = First + 1; I <= Last; ++I) {
402 if (Clusters[I].Low->getValue() != Clusters[I - 1].High->getValue() + 1) {
403 ContiguousRange = false;
404 break;
405 }
406 }
407
408 if (Low.isStrictlyPositive() && High.slt(RHS: BitWidth)) {
409 // Optimize the case where all the case values fit in a word without having
410 // to subtract minValue. In this case, we can optimize away the subtraction.
411 LowBound = APInt::getZero(numBits: Low.getBitWidth());
412 CmpRange = High;
413 ContiguousRange = false;
414 } else {
415 LowBound = Low;
416 CmpRange = High - Low;
417 }
418
419 CaseBitsVector CBV;
420 auto TotalProb = BranchProbability::getZero();
421 for (unsigned i = First; i <= Last; ++i) {
422 // Find the CaseBits for this destination.
423 unsigned j;
424 for (j = 0; j < CBV.size(); ++j)
425 if (CBV[j].BB == Clusters[i].MBB)
426 break;
427 if (j == CBV.size())
428 CBV.push_back(
429 x: CaseBits(0, Clusters[i].MBB, 0, BranchProbability::getZero()));
430 CaseBits *CB = &CBV[j];
431
432 // Update Mask, Bits and ExtraProb.
433 uint64_t Lo = (Clusters[i].Low->getValue() - LowBound).getZExtValue();
434 uint64_t Hi = (Clusters[i].High->getValue() - LowBound).getZExtValue();
435 assert(Hi >= Lo && Hi < 64 && "Invalid bit case!");
436 CB->Mask |= (-1ULL >> (63 - (Hi - Lo))) << Lo;
437 CB->Bits += Hi - Lo + 1;
438 CB->ExtraProb += Clusters[i].Prob;
439 TotalProb += Clusters[i].Prob;
440 }
441
442 BitTestInfo BTI;
443 llvm::sort(C&: CBV, Comp: [](const CaseBits &a, const CaseBits &b) {
444 // Sort by probability first, number of bits second, bit mask third.
445 if (a.ExtraProb != b.ExtraProb)
446 return a.ExtraProb > b.ExtraProb;
447 if (a.Bits != b.Bits)
448 return a.Bits > b.Bits;
449 return a.Mask < b.Mask;
450 });
451
452 for (auto &CB : CBV) {
453 MachineBasicBlock *BitTestBB =
454 FuncInfo.MF->CreateMachineBasicBlock(BB: SI->getParent());
455 BTI.push_back(Elt: BitTestCase(CB.Mask, BitTestBB, CB.BB, CB.ExtraProb));
456 }
457 BitTestCases.emplace_back(args: std::move(LowBound), args: std::move(CmpRange),
458 args: SI->getCondition(), args: Register(), args: MVT::Other, args: false,
459 args&: ContiguousRange, args: nullptr, args: nullptr, args: std::move(BTI),
460 args&: TotalProb);
461
462 BTCluster = CaseCluster::bitTests(Low: Clusters[First].Low, High: Clusters[Last].High,
463 BTCasesIndex: BitTestCases.size() - 1, Prob: TotalProb);
464 return true;
465}
466
467void SwitchCG::sortAndRangeify(CaseClusterVector &Clusters) {
468#ifndef NDEBUG
469 for (const CaseCluster &CC : Clusters)
470 assert(CC.Low == CC.High && "Input clusters must be single-case");
471#endif
472
473 llvm::sort(C&: Clusters, Comp: [](const CaseCluster &a, const CaseCluster &b) {
474 return a.Low->getValue().slt(RHS: b.Low->getValue());
475 });
476
477 // Merge adjacent clusters with the same destination.
478 const unsigned N = Clusters.size();
479 unsigned DstIndex = 0;
480 for (unsigned SrcIndex = 0; SrcIndex < N; ++SrcIndex) {
481 CaseCluster &CC = Clusters[SrcIndex];
482 const ConstantInt *CaseVal = CC.Low;
483 MachineBasicBlock *Succ = CC.MBB;
484
485 if (DstIndex != 0 && Clusters[DstIndex - 1].MBB == Succ &&
486 (CaseVal->getValue() - Clusters[DstIndex - 1].High->getValue()) == 1) {
487 // If this case has the same successor and is a neighbour, merge it into
488 // the previous cluster.
489 Clusters[DstIndex - 1].High = CaseVal;
490 Clusters[DstIndex - 1].Prob += CC.Prob;
491 } else {
492 std::memmove(dest: &Clusters[DstIndex++], src: &Clusters[SrcIndex],
493 n: sizeof(Clusters[SrcIndex]));
494 }
495 }
496 Clusters.resize(new_size: DstIndex);
497}
498
499unsigned SwitchCG::SwitchLowering::caseClusterRank(const CaseCluster &CC,
500 CaseClusterIt First,
501 CaseClusterIt Last) {
502 return std::count_if(first: First, last: Last + 1, pred: [&](const CaseCluster &X) {
503 if (X.Prob != CC.Prob)
504 return X.Prob > CC.Prob;
505
506 // Ties are broken by comparing the case value.
507 return X.Low->getValue().slt(RHS: CC.Low->getValue());
508 });
509}
510
511llvm::SwitchCG::SwitchLowering::SplitWorkItemInfo
512SwitchCG::SwitchLowering::computeSplitWorkItemInfo(
513 const SwitchWorkListItem &W) {
514 CaseClusterIt LastLeft = W.FirstCluster;
515 CaseClusterIt FirstRight = W.LastCluster;
516 auto LeftProb = LastLeft->Prob + W.DefaultProb / 2;
517 auto RightProb = FirstRight->Prob + W.DefaultProb / 2;
518
519 // Move LastLeft and FirstRight towards each other from opposite directions to
520 // find a partitioning of the clusters which balances the probability on both
521 // sides. If LeftProb and RightProb are equal, alternate which side is
522 // taken to ensure 0-probability nodes are distributed evenly.
523 unsigned I = 0;
524 while (LastLeft + 1 < FirstRight) {
525 if (LeftProb < RightProb || (LeftProb == RightProb && (I & 1)))
526 LeftProb += (++LastLeft)->Prob;
527 else
528 RightProb += (--FirstRight)->Prob;
529 I++;
530 }
531
532 while (true) {
533 // Our binary search tree differs from a typical BST in that ours can have
534 // up to three values in each leaf. The pivot selection above doesn't take
535 // that into account, which means the tree might require more nodes and be
536 // less efficient. We compensate for this here.
537
538 unsigned NumLeft = LastLeft - W.FirstCluster + 1;
539 unsigned NumRight = W.LastCluster - FirstRight + 1;
540
541 if (std::min(a: NumLeft, b: NumRight) < 3 && std::max(a: NumLeft, b: NumRight) > 3) {
542 // If one side has less than 3 clusters, and the other has more than 3,
543 // consider taking a cluster from the other side.
544
545 if (NumLeft < NumRight) {
546 // Consider moving the first cluster on the right to the left side.
547 CaseCluster &CC = *FirstRight;
548 unsigned RightSideRank = caseClusterRank(CC, First: FirstRight, Last: W.LastCluster);
549 unsigned LeftSideRank = caseClusterRank(CC, First: W.FirstCluster, Last: LastLeft);
550 if (LeftSideRank <= RightSideRank) {
551 // Moving the cluster to the left does not demote it.
552 ++LastLeft;
553 ++FirstRight;
554 continue;
555 }
556 } else {
557 assert(NumRight < NumLeft);
558 // Consider moving the last element on the left to the right side.
559 CaseCluster &CC = *LastLeft;
560 unsigned LeftSideRank = caseClusterRank(CC, First: W.FirstCluster, Last: LastLeft);
561 unsigned RightSideRank = caseClusterRank(CC, First: FirstRight, Last: W.LastCluster);
562 if (RightSideRank <= LeftSideRank) {
563 // Moving the cluster to the right does not demot it.
564 --LastLeft;
565 --FirstRight;
566 continue;
567 }
568 }
569 }
570 break;
571 }
572
573 assert(LastLeft + 1 == FirstRight);
574 assert(LastLeft >= W.FirstCluster);
575 assert(FirstRight <= W.LastCluster);
576
577 return SplitWorkItemInfo{.LastLeft: LastLeft, .FirstRight: FirstRight, .LeftProb: LeftProb, .RightProb: RightProb};
578}
579