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cd52a7a381
Apparently, the style needs to be agreed upon first. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240390 91177308-0d34-0410-b5e6-96231b3b80d8
525 lines
19 KiB
C++
525 lines
19 KiB
C++
//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// The LowerSwitch transformation rewrites switch instructions with a sequence
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// of branches, which allows targets to get away with not implementing the
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// switch instruction until it is convenient.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
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#include <algorithm>
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using namespace llvm;
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#define DEBUG_TYPE "lower-switch"
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namespace {
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struct IntRange {
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int64_t Low, High;
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};
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// Return true iff R is covered by Ranges.
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static bool IsInRanges(const IntRange &R,
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const std::vector<IntRange> &Ranges) {
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// Note: Ranges must be sorted, non-overlapping and non-adjacent.
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// Find the first range whose High field is >= R.High,
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// then check if the Low field is <= R.Low. If so, we
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// have a Range that covers R.
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auto I = std::lower_bound(
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Ranges.begin(), Ranges.end(), R,
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[](const IntRange &A, const IntRange &B) { return A.High < B.High; });
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return I != Ranges.end() && I->Low <= R.Low;
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}
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/// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
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/// instructions.
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class LowerSwitch : public FunctionPass {
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public:
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static char ID; // Pass identification, replacement for typeid
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LowerSwitch() : FunctionPass(ID) {
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initializeLowerSwitchPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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// This is a cluster of orthogonal Transforms
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AU.addPreserved<UnifyFunctionExitNodes>();
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AU.addPreservedID(LowerInvokePassID);
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}
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struct CaseRange {
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ConstantInt* Low;
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ConstantInt* High;
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BasicBlock* BB;
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CaseRange(ConstantInt *low, ConstantInt *high, BasicBlock *bb)
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: Low(low), High(high), BB(bb) {}
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};
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typedef std::vector<CaseRange> CaseVector;
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typedef std::vector<CaseRange>::iterator CaseItr;
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private:
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void processSwitchInst(SwitchInst *SI);
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BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
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ConstantInt *LowerBound, ConstantInt *UpperBound,
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Value *Val, BasicBlock *Predecessor,
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BasicBlock *OrigBlock, BasicBlock *Default,
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const std::vector<IntRange> &UnreachableRanges);
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BasicBlock *newLeafBlock(CaseRange &Leaf, Value *Val, BasicBlock *OrigBlock,
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BasicBlock *Default);
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unsigned Clusterify(CaseVector &Cases, SwitchInst *SI);
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};
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/// The comparison function for sorting the switch case values in the vector.
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/// WARNING: Case ranges should be disjoint!
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struct CaseCmp {
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bool operator () (const LowerSwitch::CaseRange& C1,
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const LowerSwitch::CaseRange& C2) {
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const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
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const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
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return CI1->getValue().slt(CI2->getValue());
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}
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};
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}
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char LowerSwitch::ID = 0;
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INITIALIZE_PASS(LowerSwitch, "lowerswitch",
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"Lower SwitchInst's to branches", false, false)
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// Publicly exposed interface to pass...
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char &llvm::LowerSwitchID = LowerSwitch::ID;
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// createLowerSwitchPass - Interface to this file...
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FunctionPass *llvm::createLowerSwitchPass() {
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return new LowerSwitch();
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}
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bool LowerSwitch::runOnFunction(Function &F) {
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bool Changed = false;
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for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
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BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
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if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
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Changed = true;
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processSwitchInst(SI);
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}
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}
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return Changed;
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}
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// operator<< - Used for debugging purposes.
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//
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static raw_ostream& operator<<(raw_ostream &O,
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const LowerSwitch::CaseVector &C)
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LLVM_ATTRIBUTE_USED;
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static raw_ostream& operator<<(raw_ostream &O,
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const LowerSwitch::CaseVector &C) {
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O << "[";
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for (LowerSwitch::CaseVector::const_iterator B = C.begin(),
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E = C.end(); B != E; ) {
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O << *B->Low << " -" << *B->High;
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if (++B != E) O << ", ";
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}
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return O << "]";
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}
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// \brief Update the first occurrence of the "switch statement" BB in the PHI
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// node with the "new" BB. The other occurrences will:
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//
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// 1) Be updated by subsequent calls to this function. Switch statements may
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// have more than one outcoming edge into the same BB if they all have the same
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// value. When the switch statement is converted these incoming edges are now
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// coming from multiple BBs.
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// 2) Removed if subsequent incoming values now share the same case, i.e.,
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// multiple outcome edges are condensed into one. This is necessary to keep the
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// number of phi values equal to the number of branches to SuccBB.
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static void fixPhis(BasicBlock *SuccBB, BasicBlock *OrigBB, BasicBlock *NewBB,
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unsigned NumMergedCases) {
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for (BasicBlock::iterator I = SuccBB->begin(), IE = SuccBB->getFirstNonPHI();
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I != IE; ++I) {
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PHINode *PN = cast<PHINode>(I);
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// Only update the first occurence.
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unsigned Idx = 0, E = PN->getNumIncomingValues();
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unsigned LocalNumMergedCases = NumMergedCases;
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for (; Idx != E; ++Idx) {
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if (PN->getIncomingBlock(Idx) == OrigBB) {
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PN->setIncomingBlock(Idx, NewBB);
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break;
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}
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}
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// Remove additional occurences coming from condensed cases and keep the
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// number of incoming values equal to the number of branches to SuccBB.
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SmallVector<unsigned, 8> Indices;
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for (++Idx; LocalNumMergedCases > 0 && Idx < E; ++Idx)
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if (PN->getIncomingBlock(Idx) == OrigBB) {
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Indices.push_back(Idx);
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LocalNumMergedCases--;
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}
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// Remove incoming values in the reverse order to prevent invalidating
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// *successive* index.
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for (auto III = Indices.rbegin(), IIE = Indices.rend(); III != IIE; ++III)
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PN->removeIncomingValue(*III);
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}
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}
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// switchConvert - Convert the switch statement into a binary lookup of
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// the case values. The function recursively builds this tree.
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// LowerBound and UpperBound are used to keep track of the bounds for Val
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// that have already been checked by a block emitted by one of the previous
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// calls to switchConvert in the call stack.
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BasicBlock *
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LowerSwitch::switchConvert(CaseItr Begin, CaseItr End, ConstantInt *LowerBound,
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ConstantInt *UpperBound, Value *Val,
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BasicBlock *Predecessor, BasicBlock *OrigBlock,
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BasicBlock *Default,
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const std::vector<IntRange> &UnreachableRanges) {
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unsigned Size = End - Begin;
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if (Size == 1) {
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// Check if the Case Range is perfectly squeezed in between
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// already checked Upper and Lower bounds. If it is then we can avoid
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// emitting the code that checks if the value actually falls in the range
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// because the bounds already tell us so.
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if (Begin->Low == LowerBound && Begin->High == UpperBound) {
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unsigned NumMergedCases = 0;
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if (LowerBound && UpperBound)
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NumMergedCases =
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UpperBound->getSExtValue() - LowerBound->getSExtValue();
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fixPhis(Begin->BB, OrigBlock, Predecessor, NumMergedCases);
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return Begin->BB;
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}
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return newLeafBlock(*Begin, Val, OrigBlock, Default);
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}
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unsigned Mid = Size / 2;
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std::vector<CaseRange> LHS(Begin, Begin + Mid);
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DEBUG(dbgs() << "LHS: " << LHS << "\n");
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std::vector<CaseRange> RHS(Begin + Mid, End);
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DEBUG(dbgs() << "RHS: " << RHS << "\n");
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CaseRange &Pivot = *(Begin + Mid);
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DEBUG(dbgs() << "Pivot ==> "
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<< Pivot.Low->getValue()
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<< " -" << Pivot.High->getValue() << "\n");
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// NewLowerBound here should never be the integer minimal value.
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// This is because it is computed from a case range that is never
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// the smallest, so there is always a case range that has at least
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// a smaller value.
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ConstantInt *NewLowerBound = Pivot.Low;
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// Because NewLowerBound is never the smallest representable integer
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// it is safe here to subtract one.
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ConstantInt *NewUpperBound = ConstantInt::get(NewLowerBound->getContext(),
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NewLowerBound->getValue() - 1);
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if (!UnreachableRanges.empty()) {
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// Check if the gap between LHS's highest and NewLowerBound is unreachable.
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int64_t GapLow = LHS.back().High->getSExtValue() + 1;
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int64_t GapHigh = NewLowerBound->getSExtValue() - 1;
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IntRange Gap = { GapLow, GapHigh };
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if (GapHigh >= GapLow && IsInRanges(Gap, UnreachableRanges))
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NewUpperBound = LHS.back().High;
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}
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DEBUG(dbgs() << "LHS Bounds ==> ";
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if (LowerBound) {
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dbgs() << LowerBound->getSExtValue();
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} else {
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dbgs() << "NONE";
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}
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dbgs() << " - " << NewUpperBound->getSExtValue() << "\n";
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dbgs() << "RHS Bounds ==> ";
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dbgs() << NewLowerBound->getSExtValue() << " - ";
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if (UpperBound) {
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dbgs() << UpperBound->getSExtValue() << "\n";
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} else {
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dbgs() << "NONE\n";
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});
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// Create a new node that checks if the value is < pivot. Go to the
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// left branch if it is and right branch if not.
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Function* F = OrigBlock->getParent();
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BasicBlock* NewNode = BasicBlock::Create(Val->getContext(), "NodeBlock");
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ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT,
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Val, Pivot.Low, "Pivot");
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BasicBlock *LBranch = switchConvert(LHS.begin(), LHS.end(), LowerBound,
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NewUpperBound, Val, NewNode, OrigBlock,
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Default, UnreachableRanges);
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BasicBlock *RBranch = switchConvert(RHS.begin(), RHS.end(), NewLowerBound,
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UpperBound, Val, NewNode, OrigBlock,
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Default, UnreachableRanges);
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Function::iterator FI = OrigBlock;
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F->getBasicBlockList().insert(++FI, NewNode);
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NewNode->getInstList().push_back(Comp);
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BranchInst::Create(LBranch, RBranch, Comp, NewNode);
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return NewNode;
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}
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// newLeafBlock - Create a new leaf block for the binary lookup tree. It
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// checks if the switch's value == the case's value. If not, then it
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// jumps to the default branch. At this point in the tree, the value
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// can't be another valid case value, so the jump to the "default" branch
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// is warranted.
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//
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BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val,
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BasicBlock* OrigBlock,
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BasicBlock* Default)
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{
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Function* F = OrigBlock->getParent();
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BasicBlock* NewLeaf = BasicBlock::Create(Val->getContext(), "LeafBlock");
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Function::iterator FI = OrigBlock;
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F->getBasicBlockList().insert(++FI, NewLeaf);
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// Emit comparison
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ICmpInst* Comp = nullptr;
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if (Leaf.Low == Leaf.High) {
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// Make the seteq instruction...
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Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_EQ, Val,
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Leaf.Low, "SwitchLeaf");
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} else {
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// Make range comparison
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if (Leaf.Low->isMinValue(true /*isSigned*/)) {
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// Val >= Min && Val <= Hi --> Val <= Hi
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Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SLE, Val, Leaf.High,
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"SwitchLeaf");
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} else if (Leaf.Low->isZero()) {
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// Val >= 0 && Val <= Hi --> Val <=u Hi
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Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Val, Leaf.High,
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"SwitchLeaf");
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} else {
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// Emit V-Lo <=u Hi-Lo
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Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
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Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
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Val->getName()+".off",
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NewLeaf);
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Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
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Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Add, UpperBound,
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"SwitchLeaf");
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}
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}
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// Make the conditional branch...
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BasicBlock* Succ = Leaf.BB;
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BranchInst::Create(Succ, Default, Comp, NewLeaf);
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// If there were any PHI nodes in this successor, rewrite one entry
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// from OrigBlock to come from NewLeaf.
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for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
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PHINode* PN = cast<PHINode>(I);
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// Remove all but one incoming entries from the cluster
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uint64_t Range = Leaf.High->getSExtValue() -
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Leaf.Low->getSExtValue();
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for (uint64_t j = 0; j < Range; ++j) {
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PN->removeIncomingValue(OrigBlock);
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}
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int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
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assert(BlockIdx != -1 && "Switch didn't go to this successor??");
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PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
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}
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return NewLeaf;
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}
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// Clusterify - Transform simple list of Cases into list of CaseRange's
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unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
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unsigned numCmps = 0;
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// Start with "simple" cases
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for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); i != e; ++i)
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Cases.push_back(CaseRange(i.getCaseValue(), i.getCaseValue(),
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i.getCaseSuccessor()));
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std::sort(Cases.begin(), Cases.end(), CaseCmp());
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// Merge case into clusters
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if (Cases.size() >= 2) {
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CaseItr I = Cases.begin();
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for (CaseItr J = std::next(I), E = Cases.end(); J != E; ++J) {
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int64_t nextValue = J->Low->getSExtValue();
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int64_t currentValue = I->High->getSExtValue();
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BasicBlock* nextBB = J->BB;
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BasicBlock* currentBB = I->BB;
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// If the two neighboring cases go to the same destination, merge them
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// into a single case.
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assert(nextValue > currentValue && "Cases should be strictly ascending");
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if ((nextValue == currentValue + 1) && (currentBB == nextBB)) {
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I->High = J->High;
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// FIXME: Combine branch weights.
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} else if (++I != J) {
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*I = *J;
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}
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}
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Cases.erase(std::next(I), Cases.end());
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}
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for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) {
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if (I->Low != I->High)
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// A range counts double, since it requires two compares.
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++numCmps;
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}
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return numCmps;
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}
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// processSwitchInst - Replace the specified switch instruction with a sequence
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// of chained if-then insts in a balanced binary search.
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//
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void LowerSwitch::processSwitchInst(SwitchInst *SI) {
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BasicBlock *CurBlock = SI->getParent();
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BasicBlock *OrigBlock = CurBlock;
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Function *F = CurBlock->getParent();
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Value *Val = SI->getCondition(); // The value we are switching on...
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BasicBlock* Default = SI->getDefaultDest();
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// If there is only the default destination, just branch.
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if (!SI->getNumCases()) {
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BranchInst::Create(Default, CurBlock);
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SI->eraseFromParent();
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return;
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}
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// Prepare cases vector.
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CaseVector Cases;
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unsigned numCmps = Clusterify(Cases, SI);
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DEBUG(dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
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<< ". Total compares: " << numCmps << "\n");
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DEBUG(dbgs() << "Cases: " << Cases << "\n");
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(void)numCmps;
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ConstantInt *LowerBound = nullptr;
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ConstantInt *UpperBound = nullptr;
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std::vector<IntRange> UnreachableRanges;
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if (isa<UnreachableInst>(Default->getFirstNonPHIOrDbg())) {
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// Make the bounds tightly fitted around the case value range, becase we
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// know that the value passed to the switch must be exactly one of the case
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// values.
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assert(!Cases.empty());
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LowerBound = Cases.front().Low;
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UpperBound = Cases.back().High;
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DenseMap<BasicBlock *, unsigned> Popularity;
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unsigned MaxPop = 0;
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BasicBlock *PopSucc = nullptr;
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IntRange R = { INT64_MIN, INT64_MAX };
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UnreachableRanges.push_back(R);
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for (const auto &I : Cases) {
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int64_t Low = I.Low->getSExtValue();
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int64_t High = I.High->getSExtValue();
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IntRange &LastRange = UnreachableRanges.back();
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if (LastRange.Low == Low) {
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// There is nothing left of the previous range.
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UnreachableRanges.pop_back();
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} else {
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// Terminate the previous range.
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assert(Low > LastRange.Low);
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LastRange.High = Low - 1;
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}
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if (High != INT64_MAX) {
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IntRange R = { High + 1, INT64_MAX };
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UnreachableRanges.push_back(R);
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}
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// Count popularity.
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int64_t N = High - Low + 1;
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unsigned &Pop = Popularity[I.BB];
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if ((Pop += N) > MaxPop) {
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MaxPop = Pop;
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PopSucc = I.BB;
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}
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}
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#ifndef NDEBUG
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/* UnreachableRanges should be sorted and the ranges non-adjacent. */
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for (auto I = UnreachableRanges.begin(), E = UnreachableRanges.end();
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I != E; ++I) {
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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(std::remove_if(
|
|
Cases.begin(), Cases.end(),
|
|
[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, 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 remove it.
|
|
if (pred_begin(OldDefault) == pred_end(OldDefault))
|
|
DeleteDeadBlock(OldDefault);
|
|
}
|