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Bill contributed this major rewrite of the -lowerswitch pass to make it generate
logarithmic conditional branch sequences instead of linear sequences. Thanks Bill! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8928 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -7,11 +7,13 @@
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Function.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iOperators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/Pass.h"
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#include "Support/Debug.h"
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#include "Support/Statistic.h"
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namespace {
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@ -20,9 +22,30 @@ namespace {
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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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struct LowerSwitch : public FunctionPass {
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class LowerSwitch : public FunctionPass {
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public:
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bool runOnFunction(Function &F);
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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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@ -49,16 +72,107 @@ bool LowerSwitch::runOnFunction(Function &F) {
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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, std::vector<LowerSwitch::Case>& C)
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{
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O << "[";
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for (std::vector<LowerSwitch::Case>::iterator B = C.begin(), E = C.end();
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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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<< cast<ConstantUInt>(Pivot.first)->getValue() << "\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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BranchInst* Br = new BranchInst(LBranch, RBranch, Comp);
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NewNode->getInstList().push_back(Br);
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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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Instruction* Br = new BranchInst(Succ, Default, Comp);
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NewLeaf->getInstList().push_back(Br);
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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();
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PHINode* PN = dyn_cast<PHINode>(I); ++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 basic blocks. Right now we just insert an incredibly stupid
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// linear sequence of branches. It would be better to do a balanced binary
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// search eventually. FIXME
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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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// Unlink the switch instruction from it's block.
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CurBlock->getInstList().remove(SI);
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@ -70,50 +184,36 @@ void LowerSwitch::processSwitchInst(SwitchInst *SI) {
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return;
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}
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// Expand comparisons for all of the non-default cases...
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for (unsigned i = 2, e = SI->getNumOperands(); i != e; i += 2) {
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// Insert a new basic block after the current one...
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BasicBlock *NextBlock;
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if (i != e-2) {
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NextBlock = new BasicBlock("switchblock");
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F->getBasicBlockList().insert(CurBlock->getNext(), NextBlock);
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} else { // Last case, if it's not the value, go to default block.
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NextBlock = cast<BasicBlock>(SI->getDefaultDest());
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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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// Make the seteq instruction...
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Instruction *Comp = new SetCondInst(Instruction::SetEQ, Val,
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SI->getOperand(i), "switchcase");
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CurBlock->getInstList().push_back(Comp);
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NewDefault->getInstList().push_back(new BranchInst(Default));
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// Make the conditional branch...
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BasicBlock *Succ = cast<BasicBlock>(SI->getOperand(i+1));
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Instruction *Br = new BranchInst(Succ, NextBlock, Comp);
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CurBlock->getInstList().push_back(Br);
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// If there were any PHI nodes in this successor, rewrite one entry from
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// OrigBlock to come from CurBlock.
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for (BasicBlock::iterator I = Succ->begin();
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PHINode *PN = dyn_cast<PHINode>(I); ++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, CurBlock);
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}
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if (i == e-2) { // Is this looking at the default destination?
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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 = NextBlock->begin();
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PHINode *PN = dyn_cast<PHINode>(I); ++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, CurBlock);
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}
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}
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CurBlock = NextBlock; // Move on to the next condition
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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();
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PHINode *PN = dyn_cast<PHINode>(I); ++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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OrigBlock->getInstList().push_back(new BranchInst(SwitchBlock));
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// We are now done with the switch instruction, delete it.
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delete SI;
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}
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