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237 lines
8.5 KiB
C++
237 lines
8.5 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 was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source 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 statements with a sequence of
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// branches, which allows targets to get away with not implementing the switch
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// statement 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/Transforms/Utils/UnifyFunctionExitNodes.h"
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#include "llvm/Constants.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include <algorithm>
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#include <iostream>
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using namespace llvm;
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namespace {
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Statistic<> NumLowered("lowerswitch", "Number of SwitchInst's replaced");
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/// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
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/// instructions. Note that this cannot be a BasicBlock pass because it
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/// modifies the CFG!
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class LowerSwitch : public FunctionPass {
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public:
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virtual bool runOnFunction(Function &F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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// This is a cluster of orthogonal Transforms
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AU.addPreserved<UnifyFunctionExitNodes>();
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AU.addPreservedID(PromoteMemoryToRegisterID);
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AU.addPreservedID(LowerSelectID);
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AU.addPreservedID(LowerInvokePassID);
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AU.addPreservedID(LowerAllocationsID);
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}
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typedef std::pair<Constant*, BasicBlock*> Case;
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typedef std::vector<Case>::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, Value* Val,
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BasicBlock* OrigBlock, BasicBlock* Default);
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BasicBlock* newLeafBlock(Case& Leaf, Value* Val,
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BasicBlock* OrigBlock, BasicBlock* Default);
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};
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/// The comparison function for sorting the switch case values in the vector.
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struct CaseCmp {
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bool operator () (const LowerSwitch::Case& C1,
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const LowerSwitch::Case& C2) {
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if (const ConstantUInt* U1 = dyn_cast<const ConstantUInt>(C1.first))
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return U1->getValue() < cast<const ConstantUInt>(C2.first)->getValue();
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const ConstantSInt* S1 = dyn_cast<const ConstantSInt>(C1.first);
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return S1->getValue() < cast<const ConstantSInt>(C2.first)->getValue();
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}
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};
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RegisterOpt<LowerSwitch>
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X("lowerswitch", "Lower SwitchInst's to branches");
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}
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// Publically exposed interface to pass...
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const PassInfo *llvm::LowerSwitchID = X.getPassInfo();
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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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std::ostream& operator<<(std::ostream &O,
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const std::vector<LowerSwitch::Case> &C) {
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O << "[";
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for (std::vector<LowerSwitch::Case>::const_iterator B = C.begin(),
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E = C.end(); B != E; ) {
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O << *B->first;
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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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// 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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//
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BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
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Value* Val, BasicBlock* OrigBlock,
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BasicBlock* Default)
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{
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unsigned Size = End - Begin;
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if (Size == 1)
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return newLeafBlock(*Begin, Val, OrigBlock, Default);
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unsigned Mid = Size / 2;
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std::vector<Case> LHS(Begin, Begin + Mid);
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DEBUG(std::cerr << "LHS: " << LHS << "\n");
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std::vector<Case> RHS(Begin + Mid, End);
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DEBUG(std::cerr << "RHS: " << RHS << "\n");
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Case& Pivot = *(Begin + Mid);
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DEBUG(std::cerr << "Pivot ==> "
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<< (int64_t)cast<ConstantInt>(Pivot.first)->getRawValue()
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<< "\n");
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BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
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OrigBlock, Default);
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BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
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OrigBlock, Default);
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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 = new BasicBlock("NodeBlock");
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F->getBasicBlockList().insert(OrigBlock->getNext(), NewNode);
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SetCondInst* Comp = new SetCondInst(Instruction::SetLT, Val, Pivot.first,
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"Pivot");
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NewNode->getInstList().push_back(Comp);
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new BranchInst(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(Case& 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 = new BasicBlock("LeafBlock");
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F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
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// Make the seteq instruction...
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SetCondInst* Comp = new SetCondInst(Instruction::SetEQ, Val,
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Leaf.first, "SwitchLeaf");
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NewLeaf->getInstList().push_back(Comp);
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// Make the conditional branch...
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BasicBlock* Succ = Leaf.second;
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new BranchInst(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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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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// 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->getOperand(0); // 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, don't bother with the code below.
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if (SI->getNumOperands() == 2) {
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new BranchInst(SI->getDefaultDest(), CurBlock);
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CurBlock->getInstList().erase(SI);
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return;
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}
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// Create a new, empty default block so that the new hierarchy of
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// if-then statements go to this and the PHI nodes are happy.
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BasicBlock* NewDefault = new BasicBlock("NewDefault");
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F->getBasicBlockList().insert(Default, NewDefault);
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new BranchInst(Default, NewDefault);
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// If there is an entry in any PHI nodes for the default edge, make sure
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// to update them as well.
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for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
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PHINode *PN = cast<PHINode>(I);
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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, NewDefault);
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}
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std::vector<Case> Cases;
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// Expand comparisons for all of the non-default cases...
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for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
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Cases.push_back(Case(SI->getSuccessorValue(i), SI->getSuccessor(i)));
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std::sort(Cases.begin(), Cases.end(), CaseCmp());
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DEBUG(std::cerr << "Cases: " << Cases << "\n");
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BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
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OrigBlock, NewDefault);
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// Branch to our shiny new if-then stuff...
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new BranchInst(SwitchBlock, OrigBlock);
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// We are now done with the switch instruction, delete it.
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CurBlock->getInstList().erase(SI);
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}
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