1 //===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
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 // The LowerSwitch transformation rewrites switch instructions with a sequence
11 // of branches, which allows targets to get away with not implementing the
12 // switch instruction until it is convenient.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/SmallVector.h"
20 #include "llvm/IR/BasicBlock.h"
21 #include "llvm/IR/CFG.h"
22 #include "llvm/IR/Constants.h"
23 #include "llvm/IR/Function.h"
24 #include "llvm/IR/InstrTypes.h"
25 #include "llvm/IR/Instructions.h"
26 #include "llvm/IR/Value.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Support/Casting.h"
29 #include "llvm/Support/Compiler.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
32 #include "llvm/Transforms/Utils.h"
33 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
43 #define DEBUG_TYPE "lower-switch"
51 } // end anonymous namespace
53 // Return true iff R is covered by Ranges.
54 static bool IsInRanges(const IntRange &R,
55 const std::vector<IntRange> &Ranges) {
56 // Note: Ranges must be sorted, non-overlapping and non-adjacent.
58 // Find the first range whose High field is >= R.High,
59 // then check if the Low field is <= R.Low. If so, we
60 // have a Range that covers R.
61 auto I = std::lower_bound(
62 Ranges.begin(), Ranges.end(), R,
63 [](const IntRange &A, const IntRange &B) { return A.High < B.High; });
64 return I != Ranges.end() && I->Low <= R.Low;
69 /// Replace all SwitchInst instructions with chained branch instructions.
70 class LowerSwitch : public FunctionPass {
72 // Pass identification, replacement for typeid
75 LowerSwitch() : FunctionPass(ID) {
76 initializeLowerSwitchPass(*PassRegistry::getPassRegistry());
79 bool runOnFunction(Function &F) override;
86 CaseRange(ConstantInt *low, ConstantInt *high, BasicBlock *bb)
87 : Low(low), High(high), BB(bb) {}
90 using CaseVector = std::vector<CaseRange>;
91 using CaseItr = std::vector<CaseRange>::iterator;
94 void processSwitchInst(SwitchInst *SI, SmallPtrSetImpl<BasicBlock*> &DeleteList);
96 BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
97 ConstantInt *LowerBound, ConstantInt *UpperBound,
98 Value *Val, BasicBlock *Predecessor,
99 BasicBlock *OrigBlock, BasicBlock *Default,
100 const std::vector<IntRange> &UnreachableRanges);
101 BasicBlock *newLeafBlock(CaseRange &Leaf, Value *Val, BasicBlock *OrigBlock,
102 BasicBlock *Default);
103 unsigned Clusterify(CaseVector &Cases, SwitchInst *SI);
106 /// The comparison function for sorting the switch case values in the vector.
107 /// WARNING: Case ranges should be disjoint!
109 bool operator()(const LowerSwitch::CaseRange& C1,
110 const LowerSwitch::CaseRange& C2) {
111 const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
112 const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
113 return CI1->getValue().slt(CI2->getValue());
117 } // end anonymous namespace
119 char LowerSwitch::ID = 0;
121 // Publicly exposed interface to pass...
122 char &llvm::LowerSwitchID = LowerSwitch::ID;
124 INITIALIZE_PASS(LowerSwitch, "lowerswitch",
125 "Lower SwitchInst's to branches", false, false)
127 // createLowerSwitchPass - Interface to this file...
128 FunctionPass *llvm::createLowerSwitchPass() {
129 return new LowerSwitch();
132 bool LowerSwitch::runOnFunction(Function &F) {
133 bool Changed = false;
134 SmallPtrSet<BasicBlock*, 8> DeleteList;
136 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
137 BasicBlock *Cur = &*I++; // Advance over block so we don't traverse new blocks
139 // If the block is a dead Default block that will be deleted later, don't
140 // waste time processing it.
141 if (DeleteList.count(Cur))
144 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
146 processSwitchInst(SI, DeleteList);
150 for (BasicBlock* BB: DeleteList) {
157 /// Used for debugging purposes.
159 static raw_ostream &operator<<(raw_ostream &O,
160 const LowerSwitch::CaseVector &C) {
163 for (LowerSwitch::CaseVector::const_iterator B = C.begin(),
164 E = C.end(); B != E; ) {
165 O << *B->Low << " -" << *B->High;
166 if (++B != E) O << ", ";
172 /// Update the first occurrence of the "switch statement" BB in the PHI
173 /// node with the "new" BB. The other occurrences will:
175 /// 1) Be updated by subsequent calls to this function. Switch statements may
176 /// have more than one outcoming edge into the same BB if they all have the same
177 /// value. When the switch statement is converted these incoming edges are now
178 /// coming from multiple BBs.
179 /// 2) Removed if subsequent incoming values now share the same case, i.e.,
180 /// multiple outcome edges are condensed into one. This is necessary to keep the
181 /// number of phi values equal to the number of branches to SuccBB.
182 static void fixPhis(BasicBlock *SuccBB, BasicBlock *OrigBB, BasicBlock *NewBB,
183 unsigned NumMergedCases) {
184 for (BasicBlock::iterator I = SuccBB->begin(),
185 IE = SuccBB->getFirstNonPHI()->getIterator();
187 PHINode *PN = cast<PHINode>(I);
189 // Only update the first occurrence.
190 unsigned Idx = 0, E = PN->getNumIncomingValues();
191 unsigned LocalNumMergedCases = NumMergedCases;
192 for (; Idx != E; ++Idx) {
193 if (PN->getIncomingBlock(Idx) == OrigBB) {
194 PN->setIncomingBlock(Idx, NewBB);
199 // Remove additional occurrences coming from condensed cases and keep the
200 // number of incoming values equal to the number of branches to SuccBB.
201 SmallVector<unsigned, 8> Indices;
202 for (++Idx; LocalNumMergedCases > 0 && Idx < E; ++Idx)
203 if (PN->getIncomingBlock(Idx) == OrigBB) {
204 Indices.push_back(Idx);
205 LocalNumMergedCases--;
207 // Remove incoming values in the reverse order to prevent invalidating
208 // *successive* index.
209 for (unsigned III : llvm::reverse(Indices))
210 PN->removeIncomingValue(III);
214 /// Convert the switch statement into a binary lookup of the case values.
215 /// The function recursively builds this tree. LowerBound and UpperBound are
216 /// used to keep track of the bounds for Val that have already been checked by
217 /// a block emitted by one of the previous calls to switchConvert in the call
220 LowerSwitch::switchConvert(CaseItr Begin, CaseItr End, ConstantInt *LowerBound,
221 ConstantInt *UpperBound, Value *Val,
222 BasicBlock *Predecessor, BasicBlock *OrigBlock,
224 const std::vector<IntRange> &UnreachableRanges) {
225 unsigned Size = End - Begin;
228 // Check if the Case Range is perfectly squeezed in between
229 // already checked Upper and Lower bounds. If it is then we can avoid
230 // emitting the code that checks if the value actually falls in the range
231 // because the bounds already tell us so.
232 if (Begin->Low == LowerBound && Begin->High == UpperBound) {
233 unsigned NumMergedCases = 0;
234 if (LowerBound && UpperBound)
236 UpperBound->getSExtValue() - LowerBound->getSExtValue();
237 fixPhis(Begin->BB, OrigBlock, Predecessor, NumMergedCases);
240 return newLeafBlock(*Begin, Val, OrigBlock, Default);
243 unsigned Mid = Size / 2;
244 std::vector<CaseRange> LHS(Begin, Begin + Mid);
245 LLVM_DEBUG(dbgs() << "LHS: " << LHS << "\n");
246 std::vector<CaseRange> RHS(Begin + Mid, End);
247 LLVM_DEBUG(dbgs() << "RHS: " << RHS << "\n");
249 CaseRange &Pivot = *(Begin + Mid);
250 LLVM_DEBUG(dbgs() << "Pivot ==> " << Pivot.Low->getValue() << " -"
251 << Pivot.High->getValue() << "\n");
253 // NewLowerBound here should never be the integer minimal value.
254 // This is because it is computed from a case range that is never
255 // the smallest, so there is always a case range that has at least
257 ConstantInt *NewLowerBound = Pivot.Low;
259 // Because NewLowerBound is never the smallest representable integer
260 // it is safe here to subtract one.
261 ConstantInt *NewUpperBound = ConstantInt::get(NewLowerBound->getContext(),
262 NewLowerBound->getValue() - 1);
264 if (!UnreachableRanges.empty()) {
265 // Check if the gap between LHS's highest and NewLowerBound is unreachable.
266 int64_t GapLow = LHS.back().High->getSExtValue() + 1;
267 int64_t GapHigh = NewLowerBound->getSExtValue() - 1;
268 IntRange Gap = { GapLow, GapHigh };
269 if (GapHigh >= GapLow && IsInRanges(Gap, UnreachableRanges))
270 NewUpperBound = LHS.back().High;
273 LLVM_DEBUG(dbgs() << "LHS Bounds ==> "; if (LowerBound) {
274 dbgs() << LowerBound->getSExtValue();
275 } else { dbgs() << "NONE"; } dbgs() << " - "
276 << NewUpperBound->getSExtValue() << "\n";
277 dbgs() << "RHS Bounds ==> ";
278 dbgs() << NewLowerBound->getSExtValue() << " - "; if (UpperBound) {
279 dbgs() << UpperBound->getSExtValue() << "\n";
280 } else { dbgs() << "NONE\n"; });
282 // Create a new node that checks if the value is < pivot. Go to the
283 // left branch if it is and right branch if not.
284 Function* F = OrigBlock->getParent();
285 BasicBlock* NewNode = BasicBlock::Create(Val->getContext(), "NodeBlock");
287 ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT,
288 Val, Pivot.Low, "Pivot");
290 BasicBlock *LBranch = switchConvert(LHS.begin(), LHS.end(), LowerBound,
291 NewUpperBound, Val, NewNode, OrigBlock,
292 Default, UnreachableRanges);
293 BasicBlock *RBranch = switchConvert(RHS.begin(), RHS.end(), NewLowerBound,
294 UpperBound, Val, NewNode, OrigBlock,
295 Default, UnreachableRanges);
297 F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewNode);
298 NewNode->getInstList().push_back(Comp);
300 BranchInst::Create(LBranch, RBranch, Comp, NewNode);
304 /// Create a new leaf block for the binary lookup tree. It checks if the
305 /// switch's value == the case's value. If not, then it jumps to the default
306 /// branch. At this point in the tree, the value can't be another valid case
307 /// value, so the jump to the "default" branch is warranted.
308 BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val,
309 BasicBlock* OrigBlock,
310 BasicBlock* Default) {
311 Function* F = OrigBlock->getParent();
312 BasicBlock* NewLeaf = BasicBlock::Create(Val->getContext(), "LeafBlock");
313 F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewLeaf);
316 ICmpInst* Comp = nullptr;
317 if (Leaf.Low == Leaf.High) {
318 // Make the seteq instruction...
319 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_EQ, Val,
320 Leaf.Low, "SwitchLeaf");
322 // Make range comparison
323 if (Leaf.Low->isMinValue(true /*isSigned*/)) {
324 // Val >= Min && Val <= Hi --> Val <= Hi
325 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SLE, Val, Leaf.High,
327 } else if (Leaf.Low->isZero()) {
328 // Val >= 0 && Val <= Hi --> Val <=u Hi
329 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Val, Leaf.High,
332 // Emit V-Lo <=u Hi-Lo
333 Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
334 Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
335 Val->getName()+".off",
337 Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
338 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Add, UpperBound,
343 // Make the conditional branch...
344 BasicBlock* Succ = Leaf.BB;
345 BranchInst::Create(Succ, Default, Comp, NewLeaf);
347 // If there were any PHI nodes in this successor, rewrite one entry
348 // from OrigBlock to come from NewLeaf.
349 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
350 PHINode* PN = cast<PHINode>(I);
351 // Remove all but one incoming entries from the cluster
352 uint64_t Range = Leaf.High->getSExtValue() -
353 Leaf.Low->getSExtValue();
354 for (uint64_t j = 0; j < Range; ++j) {
355 PN->removeIncomingValue(OrigBlock);
358 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
359 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
360 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
366 /// Transform simple list of Cases into list of CaseRange's.
367 unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
368 unsigned numCmps = 0;
370 // Start with "simple" cases
371 for (auto Case : SI->cases())
372 Cases.push_back(CaseRange(Case.getCaseValue(), Case.getCaseValue(),
373 Case.getCaseSuccessor()));
375 llvm::sort(Cases, CaseCmp());
377 // Merge case into clusters
378 if (Cases.size() >= 2) {
379 CaseItr I = Cases.begin();
380 for (CaseItr J = std::next(I), E = Cases.end(); J != E; ++J) {
381 int64_t nextValue = J->Low->getSExtValue();
382 int64_t currentValue = I->High->getSExtValue();
383 BasicBlock* nextBB = J->BB;
384 BasicBlock* currentBB = I->BB;
386 // If the two neighboring cases go to the same destination, merge them
387 // into a single case.
388 assert(nextValue > currentValue && "Cases should be strictly ascending");
389 if ((nextValue == currentValue + 1) && (currentBB == nextBB)) {
391 // FIXME: Combine branch weights.
392 } else if (++I != J) {
396 Cases.erase(std::next(I), Cases.end());
399 for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) {
400 if (I->Low != I->High)
401 // A range counts double, since it requires two compares.
408 /// Replace the specified switch instruction with a sequence of chained if-then
409 /// insts in a balanced binary search.
410 void LowerSwitch::processSwitchInst(SwitchInst *SI,
411 SmallPtrSetImpl<BasicBlock*> &DeleteList) {
412 BasicBlock *CurBlock = SI->getParent();
413 BasicBlock *OrigBlock = CurBlock;
414 Function *F = CurBlock->getParent();
415 Value *Val = SI->getCondition(); // The value we are switching on...
416 BasicBlock* Default = SI->getDefaultDest();
418 // Don't handle unreachable blocks. If there are successors with phis, this
419 // would leave them behind with missing predecessors.
420 if ((CurBlock != &F->getEntryBlock() && pred_empty(CurBlock)) ||
421 CurBlock->getSinglePredecessor() == CurBlock) {
422 DeleteList.insert(CurBlock);
426 // If there is only the default destination, just branch.
427 if (!SI->getNumCases()) {
428 BranchInst::Create(Default, CurBlock);
429 SI->eraseFromParent();
433 // Prepare cases vector.
435 unsigned numCmps = Clusterify(Cases, SI);
436 LLVM_DEBUG(dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
437 << ". Total compares: " << numCmps << "\n");
438 LLVM_DEBUG(dbgs() << "Cases: " << Cases << "\n");
441 ConstantInt *LowerBound = nullptr;
442 ConstantInt *UpperBound = nullptr;
443 std::vector<IntRange> UnreachableRanges;
445 if (isa<UnreachableInst>(Default->getFirstNonPHIOrDbg())) {
446 // Make the bounds tightly fitted around the case value range, because we
447 // know that the value passed to the switch must be exactly one of the case
449 assert(!Cases.empty());
450 LowerBound = Cases.front().Low;
451 UpperBound = Cases.back().High;
453 DenseMap<BasicBlock *, unsigned> Popularity;
455 BasicBlock *PopSucc = nullptr;
457 IntRange R = {std::numeric_limits<int64_t>::min(),
458 std::numeric_limits<int64_t>::max()};
459 UnreachableRanges.push_back(R);
460 for (const auto &I : Cases) {
461 int64_t Low = I.Low->getSExtValue();
462 int64_t High = I.High->getSExtValue();
464 IntRange &LastRange = UnreachableRanges.back();
465 if (LastRange.Low == Low) {
466 // There is nothing left of the previous range.
467 UnreachableRanges.pop_back();
469 // Terminate the previous range.
470 assert(Low > LastRange.Low);
471 LastRange.High = Low - 1;
473 if (High != std::numeric_limits<int64_t>::max()) {
474 IntRange R = { High + 1, std::numeric_limits<int64_t>::max() };
475 UnreachableRanges.push_back(R);
479 int64_t N = High - Low + 1;
480 unsigned &Pop = Popularity[I.BB];
481 if ((Pop += N) > MaxPop) {
487 /* UnreachableRanges should be sorted and the ranges non-adjacent. */
488 for (auto I = UnreachableRanges.begin(), E = UnreachableRanges.end();
490 assert(I->Low <= I->High);
493 assert(Next->Low > I->High);
498 // As the default block in the switch is unreachable, update the PHI nodes
499 // (remove the entry to the default block) to reflect this.
500 Default->removePredecessor(OrigBlock);
502 // Use the most popular block as the new default, reducing the number of
504 assert(MaxPop > 0 && PopSucc);
508 Cases, [PopSucc](const CaseRange &R) { return R.BB == PopSucc; }),
511 // If there are no cases left, just branch.
513 BranchInst::Create(Default, CurBlock);
514 SI->eraseFromParent();
515 // As all the cases have been replaced with a single branch, only keep
516 // one entry in the PHI nodes.
517 for (unsigned I = 0 ; I < (MaxPop - 1) ; ++I)
518 PopSucc->removePredecessor(OrigBlock);
523 unsigned NrOfDefaults = (SI->getDefaultDest() == Default) ? 1 : 0;
524 for (const auto &Case : SI->cases())
525 if (Case.getCaseSuccessor() == Default)
528 // Create a new, empty default block so that the new hierarchy of
529 // if-then statements go to this and the PHI nodes are happy.
530 BasicBlock *NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
531 F->getBasicBlockList().insert(Default->getIterator(), NewDefault);
532 BranchInst::Create(Default, NewDefault);
534 BasicBlock *SwitchBlock =
535 switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
536 OrigBlock, OrigBlock, NewDefault, UnreachableRanges);
538 // If there are entries in any PHI nodes for the default edge, make sure
539 // to update them as well.
540 fixPhis(Default, OrigBlock, NewDefault, NrOfDefaults);
542 // Branch to our shiny new if-then stuff...
543 BranchInst::Create(SwitchBlock, OrigBlock);
545 // We are now done with the switch instruction, delete it.
546 BasicBlock *OldDefault = SI->getDefaultDest();
547 CurBlock->getInstList().erase(SI);
549 // If the Default block has no more predecessors just add it to DeleteList.
550 if (pred_begin(OldDefault) == pred_end(OldDefault))
551 DeleteList.insert(OldDefault);