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llvm-project/llvm/lib/Transforms/Utils/LowerSwitch.cpp

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//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// The LowerSwitch transformation rewrites switch instructions with a sequence
// of branches, which allows targets to get away with not implementing the
// switch instruction until it is convenient.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/STLExtras.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Pass.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "lower-switch"
namespace {
struct IntRange {
int64_t Low, High;
};
// Return true iff R is covered by Ranges.
static bool IsInRanges(const IntRange &R,
const std::vector<IntRange> &Ranges) {
// Note: Ranges must be sorted, non-overlapping and non-adjacent.
// Find the first range whose High field is >= R.High,
// then check if the Low field is <= R.Low. If so, we
// have a Range that covers R.
auto I = std::lower_bound(
Ranges.begin(), Ranges.end(), R,
[](const IntRange &A, const IntRange &B) { return A.High < B.High; });
return I != Ranges.end() && I->Low <= R.Low;
}
/// Replace all SwitchInst instructions with chained branch instructions.
class LowerSwitch : public FunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
LowerSwitch() : FunctionPass(ID) {
initializeLowerSwitchPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override;
struct CaseRange {
ConstantInt* Low;
ConstantInt* High;
BasicBlock* BB;
CaseRange(ConstantInt *low, ConstantInt *high, BasicBlock *bb)
: Low(low), High(high), BB(bb) {}
};
typedef std::vector<CaseRange> CaseVector;
typedef std::vector<CaseRange>::iterator CaseItr;
private:
void processSwitchInst(SwitchInst *SI, SmallPtrSetImpl<BasicBlock*> &DeleteList);
BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
ConstantInt *LowerBound, ConstantInt *UpperBound,
Value *Val, BasicBlock *Predecessor,
BasicBlock *OrigBlock, BasicBlock *Default,
const std::vector<IntRange> &UnreachableRanges);
BasicBlock *newLeafBlock(CaseRange &Leaf, Value *Val, BasicBlock *OrigBlock,
BasicBlock *Default);
unsigned Clusterify(CaseVector &Cases, SwitchInst *SI);
};
/// The comparison function for sorting the switch case values in the vector.
/// WARNING: Case ranges should be disjoint!
struct CaseCmp {
bool operator () (const LowerSwitch::CaseRange& C1,
const LowerSwitch::CaseRange& C2) {
const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
return CI1->getValue().slt(CI2->getValue());
}
};
}
char LowerSwitch::ID = 0;
INITIALIZE_PASS(LowerSwitch, "lowerswitch",
"Lower SwitchInst's to branches", false, false)
// Publicly exposed interface to pass...
char &llvm::LowerSwitchID = LowerSwitch::ID;
// createLowerSwitchPass - Interface to this file...
FunctionPass *llvm::createLowerSwitchPass() {
return new LowerSwitch();
}
bool LowerSwitch::runOnFunction(Function &F) {
bool Changed = false;
SmallPtrSet<BasicBlock*, 8> DeleteList;
for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
BasicBlock *Cur = &*I++; // Advance over block so we don't traverse new blocks
// If the block is a dead Default block that will be deleted later, don't
// waste time processing it.
if (DeleteList.count(Cur))
continue;
if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
Changed = true;
processSwitchInst(SI, DeleteList);
}
}
for (BasicBlock* BB: DeleteList) {
DeleteDeadBlock(BB);
}
return Changed;
}
/// Used for debugging purposes.
static raw_ostream& operator<<(raw_ostream &O,
const LowerSwitch::CaseVector &C)
LLVM_ATTRIBUTE_USED;
static raw_ostream& operator<<(raw_ostream &O,
const LowerSwitch::CaseVector &C) {
O << "[";
for (LowerSwitch::CaseVector::const_iterator B = C.begin(),
E = C.end(); B != E; ) {
O << *B->Low << " -" << *B->High;
if (++B != E) O << ", ";
}
return O << "]";
}
/// \brief Update the first occurrence of the "switch statement" BB in the PHI
/// node with the "new" BB. The other occurrences will:
///
/// 1) Be updated by subsequent calls to this function. Switch statements may
/// have more than one outcoming edge into the same BB if they all have the same
/// value. When the switch statement is converted these incoming edges are now
/// coming from multiple BBs.
/// 2) Removed if subsequent incoming values now share the same case, i.e.,
/// multiple outcome edges are condensed into one. This is necessary to keep the
/// number of phi values equal to the number of branches to SuccBB.
static void fixPhis(BasicBlock *SuccBB, BasicBlock *OrigBB, BasicBlock *NewBB,
unsigned NumMergedCases) {
for (BasicBlock::iterator I = SuccBB->begin(),
IE = SuccBB->getFirstNonPHI()->getIterator();
I != IE; ++I) {
PHINode *PN = cast<PHINode>(I);
// Only update the first occurrence.
unsigned Idx = 0, E = PN->getNumIncomingValues();
unsigned LocalNumMergedCases = NumMergedCases;
for (; Idx != E; ++Idx) {
if (PN->getIncomingBlock(Idx) == OrigBB) {
PN->setIncomingBlock(Idx, NewBB);
break;
}
}
// Remove additional occurrences coming from condensed cases and keep the
// number of incoming values equal to the number of branches to SuccBB.
SmallVector<unsigned, 8> Indices;
for (++Idx; LocalNumMergedCases > 0 && Idx < E; ++Idx)
if (PN->getIncomingBlock(Idx) == OrigBB) {
Indices.push_back(Idx);
LocalNumMergedCases--;
}
// Remove incoming values in the reverse order to prevent invalidating
// *successive* index.
for (unsigned III : reverse(Indices))
PN->removeIncomingValue(III);
}
}
/// Convert the switch statement into a binary lookup of the case values.
/// The function recursively builds this tree. LowerBound and UpperBound are
/// used to keep track of the bounds for Val that have already been checked by
/// a block emitted by one of the previous calls to switchConvert in the call
/// stack.
BasicBlock *
LowerSwitch::switchConvert(CaseItr Begin, CaseItr End, ConstantInt *LowerBound,
ConstantInt *UpperBound, Value *Val,
BasicBlock *Predecessor, BasicBlock *OrigBlock,
BasicBlock *Default,
const std::vector<IntRange> &UnreachableRanges) {
unsigned Size = End - Begin;
if (Size == 1) {
// Check if the Case Range is perfectly squeezed in between
// already checked Upper and Lower bounds. If it is then we can avoid
// emitting the code that checks if the value actually falls in the range
// because the bounds already tell us so.
if (Begin->Low == LowerBound && Begin->High == UpperBound) {
unsigned NumMergedCases = 0;
if (LowerBound && UpperBound)
NumMergedCases =
UpperBound->getSExtValue() - LowerBound->getSExtValue();
fixPhis(Begin->BB, OrigBlock, Predecessor, NumMergedCases);
return Begin->BB;
}
return newLeafBlock(*Begin, Val, OrigBlock, Default);
}
unsigned Mid = Size / 2;
std::vector<CaseRange> LHS(Begin, Begin + Mid);
DEBUG(dbgs() << "LHS: " << LHS << "\n");
std::vector<CaseRange> RHS(Begin + Mid, End);
DEBUG(dbgs() << "RHS: " << RHS << "\n");
CaseRange &Pivot = *(Begin + Mid);
DEBUG(dbgs() << "Pivot ==> "
<< Pivot.Low->getValue()
<< " -" << Pivot.High->getValue() << "\n");
// NewLowerBound here should never be the integer minimal value.
// This is because it is computed from a case range that is never
// the smallest, so there is always a case range that has at least
// a smaller value.
ConstantInt *NewLowerBound = Pivot.Low;
// Because NewLowerBound is never the smallest representable integer
// it is safe here to subtract one.
ConstantInt *NewUpperBound = ConstantInt::get(NewLowerBound->getContext(),
NewLowerBound->getValue() - 1);
if (!UnreachableRanges.empty()) {
// Check if the gap between LHS's highest and NewLowerBound is unreachable.
int64_t GapLow = LHS.back().High->getSExtValue() + 1;
int64_t GapHigh = NewLowerBound->getSExtValue() - 1;
IntRange Gap = { GapLow, GapHigh };
if (GapHigh >= GapLow && IsInRanges(Gap, UnreachableRanges))
NewUpperBound = LHS.back().High;
}
DEBUG(dbgs() << "LHS Bounds ==> ";
if (LowerBound) {
dbgs() << LowerBound->getSExtValue();
} else {
dbgs() << "NONE";
}
dbgs() << " - " << NewUpperBound->getSExtValue() << "\n";
dbgs() << "RHS Bounds ==> ";
dbgs() << NewLowerBound->getSExtValue() << " - ";
if (UpperBound) {
dbgs() << UpperBound->getSExtValue() << "\n";
} else {
dbgs() << "NONE\n";
});
// Create a new node that checks if the value is < pivot. Go to the
// left branch if it is and right branch if not.
Function* F = OrigBlock->getParent();
BasicBlock* NewNode = BasicBlock::Create(Val->getContext(), "NodeBlock");
ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT,
Val, Pivot.Low, "Pivot");
BasicBlock *LBranch = switchConvert(LHS.begin(), LHS.end(), LowerBound,
NewUpperBound, Val, NewNode, OrigBlock,
Default, UnreachableRanges);
BasicBlock *RBranch = switchConvert(RHS.begin(), RHS.end(), NewLowerBound,
UpperBound, Val, NewNode, OrigBlock,
Default, UnreachableRanges);
F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewNode);
NewNode->getInstList().push_back(Comp);
BranchInst::Create(LBranch, RBranch, Comp, NewNode);
return NewNode;
}
/// Create a new leaf block for the binary lookup tree. It checks if the
/// switch's value == the case's value. If not, then it jumps to the default
/// branch. At this point in the tree, the value can't be another valid case
/// value, so the jump to the "default" branch is warranted.
BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val,
BasicBlock* OrigBlock,
BasicBlock* Default)
{
Function* F = OrigBlock->getParent();
BasicBlock* NewLeaf = BasicBlock::Create(Val->getContext(), "LeafBlock");
F->getBasicBlockList().insert(++OrigBlock->getIterator(), NewLeaf);
// Emit comparison
ICmpInst* Comp = nullptr;
if (Leaf.Low == Leaf.High) {
// Make the seteq instruction...
Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_EQ, Val,
Leaf.Low, "SwitchLeaf");
} else {
// Make range comparison
if (Leaf.Low->isMinValue(true /*isSigned*/)) {
// Val >= Min && Val <= Hi --> Val <= Hi
Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SLE, Val, Leaf.High,
"SwitchLeaf");
} else if (Leaf.Low->isZero()) {
// Val >= 0 && Val <= Hi --> Val <=u Hi
Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Val, Leaf.High,
"SwitchLeaf");
} else {
// Emit V-Lo <=u Hi-Lo
Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
Val->getName()+".off",
NewLeaf);
Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Add, UpperBound,
"SwitchLeaf");
}
}
// Make the conditional branch...
BasicBlock* Succ = Leaf.BB;
BranchInst::Create(Succ, Default, Comp, NewLeaf);
// If there were any PHI nodes in this successor, rewrite one entry
// from OrigBlock to come from NewLeaf.
for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
PHINode* PN = cast<PHINode>(I);
// Remove all but one incoming entries from the cluster
uint64_t Range = Leaf.High->getSExtValue() -
Leaf.Low->getSExtValue();
for (uint64_t j = 0; j < Range; ++j) {
PN->removeIncomingValue(OrigBlock);
}
int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
assert(BlockIdx != -1 && "Switch didn't go to this successor??");
PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
}
return NewLeaf;
}
/// Transform simple list of Cases into list of CaseRange's.
unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
unsigned numCmps = 0;
// Start with "simple" cases
for (auto Case : SI->cases())
Cases.push_back(CaseRange(Case.getCaseValue(), Case.getCaseValue(),
Case.getCaseSuccessor()));
std::sort(Cases.begin(), Cases.end(), CaseCmp());
// Merge case into clusters
if (Cases.size() >= 2) {
CaseItr I = Cases.begin();
for (CaseItr J = std::next(I), E = Cases.end(); J != E; ++J) {
int64_t nextValue = J->Low->getSExtValue();
int64_t currentValue = I->High->getSExtValue();
BasicBlock* nextBB = J->BB;
BasicBlock* currentBB = I->BB;
// If the two neighboring cases go to the same destination, merge them
// into a single case.
assert(nextValue > currentValue && "Cases should be strictly ascending");
if ((nextValue == currentValue + 1) && (currentBB == nextBB)) {
I->High = J->High;
// FIXME: Combine branch weights.
} else if (++I != J) {
*I = *J;
}
}
Cases.erase(std::next(I), Cases.end());
}
for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) {
if (I->Low != I->High)
// A range counts double, since it requires two compares.
++numCmps;
}
return numCmps;
}
/// Replace the specified switch instruction with a sequence of chained if-then
/// insts in a balanced binary search.
void LowerSwitch::processSwitchInst(SwitchInst *SI,
SmallPtrSetImpl<BasicBlock*> &DeleteList) {
BasicBlock *CurBlock = SI->getParent();
BasicBlock *OrigBlock = CurBlock;
Function *F = CurBlock->getParent();
Value *Val = SI->getCondition(); // The value we are switching on...
BasicBlock* Default = SI->getDefaultDest();
// Don't handle unreachable blocks. If there are successors with phis, this
// would leave them behind with missing predecessors.
if ((CurBlock != &F->getEntryBlock() && pred_empty(CurBlock)) ||
CurBlock->getSinglePredecessor() == CurBlock) {
DeleteList.insert(CurBlock);
return;
}
// If there is only the default destination, just branch.
if (!SI->getNumCases()) {
BranchInst::Create(Default, CurBlock);
SI->eraseFromParent();
return;
}
// Prepare cases vector.
CaseVector Cases;
unsigned numCmps = Clusterify(Cases, SI);
DEBUG(dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
<< ". Total compares: " << numCmps << "\n");
DEBUG(dbgs() << "Cases: " << Cases << "\n");
(void)numCmps;
ConstantInt *LowerBound = nullptr;
ConstantInt *UpperBound = nullptr;
std::vector<IntRange> UnreachableRanges;
if (isa<UnreachableInst>(Default->getFirstNonPHIOrDbg())) {
// Make the bounds tightly fitted around the case value range, because we
// know that the value passed to the switch must be exactly one of the case
// values.
assert(!Cases.empty());
LowerBound = Cases.front().Low;
UpperBound = Cases.back().High;
DenseMap<BasicBlock *, unsigned> Popularity;
unsigned MaxPop = 0;
BasicBlock *PopSucc = nullptr;
IntRange R = { INT64_MIN, INT64_MAX };
UnreachableRanges.push_back(R);
for (const auto &I : Cases) {
int64_t Low = I.Low->getSExtValue();
int64_t High = I.High->getSExtValue();
IntRange &LastRange = UnreachableRanges.back();
if (LastRange.Low == Low) {
// There is nothing left of the previous range.
UnreachableRanges.pop_back();
} else {
// Terminate the previous range.
assert(Low > LastRange.Low);
LastRange.High = Low - 1;
}
if (High != INT64_MAX) {
IntRange R = { High + 1, INT64_MAX };
UnreachableRanges.push_back(R);
}
// Count popularity.
int64_t N = High - Low + 1;
unsigned &Pop = Popularity[I.BB];
if ((Pop += N) > MaxPop) {
MaxPop = Pop;
PopSucc = I.BB;
}
}
#ifndef NDEBUG
/* UnreachableRanges should be sorted and the ranges non-adjacent. */
for (auto I = UnreachableRanges.begin(), E = UnreachableRanges.end();
I != E; ++I) {
assert(I->Low <= I->High);
auto Next = I + 1;
if (Next != E) {
assert(Next->Low > I->High);
}
}
#endif
// Use the most popular block as the new default, reducing the number of
// cases.
assert(MaxPop > 0 && PopSucc);
Default = PopSucc;
Cases.erase(
remove_if(Cases,
[PopSucc](const CaseRange &R) { return R.BB == PopSucc; }),
Cases.end());
// If there are no cases left, just branch.
if (Cases.empty()) {
BranchInst::Create(Default, CurBlock);
SI->eraseFromParent();
return;
}
}
// Create a new, empty default block so that the new hierarchy of
// if-then statements go to this and the PHI nodes are happy.
BasicBlock *NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
F->getBasicBlockList().insert(Default->getIterator(), NewDefault);
BranchInst::Create(Default, NewDefault);
// If there is an entry in any PHI nodes for the default edge, make sure
// to update them as well.
for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
assert(BlockIdx != -1 && "Switch didn't go to this successor??");
PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
}
BasicBlock *SwitchBlock =
switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
OrigBlock, OrigBlock, NewDefault, UnreachableRanges);
// Branch to our shiny new if-then stuff...
BranchInst::Create(SwitchBlock, OrigBlock);
// We are now done with the switch instruction, delete it.
BasicBlock *OldDefault = SI->getDefaultDest();
CurBlock->getInstList().erase(SI);
// If the Default block has no more predecessors just add it to DeleteList.
if (pred_begin(OldDefault) == pred_end(OldDefault))
DeleteList.insert(OldDefault);
}
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