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Bill contributed this major rewrite of the -lowerswitch pass to make it generate

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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
This commit is contained in:
Chris Lattner 2003-10-07 18:46:23 +00:00
parent 7528343cbd
commit ebbc1a5aa0
1 changed files with 144 additions and 44 deletions
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188
lib/Transforms/Utils/LowerSwitch.cpp
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@ -7,11 +7,13 @@
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/iTerminators.h"
#include "llvm/iOperators.h"
#include "llvm/iPHINode.h"
#include "llvm/Pass.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
namespace {
@ -20,9 +22,30 @@ namespace {
/// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
/// instructions. Note that this cannot be a BasicBlock pass because it
/// modifies the CFG!
struct LowerSwitch : public FunctionPass {
class LowerSwitch : public FunctionPass {
public:
bool runOnFunction(Function &F);
typedef std::pair<Constant*, BasicBlock*> Case;
typedef std::vector<Case>::iterator CaseItr;
private:
void processSwitchInst(SwitchInst *SI);
BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
BasicBlock* OrigBlock, BasicBlock* Default);
BasicBlock* newLeafBlock(Case& Leaf, Value* Val,
BasicBlock* OrigBlock, BasicBlock* Default);
};
/// The comparison function for sorting the switch case values in the vector.
struct CaseCmp {
bool operator () (const LowerSwitch::Case& C1,
const LowerSwitch::Case& C2) {
if (const ConstantUInt* U1 = dyn_cast<const ConstantUInt>(C1.first))
return U1->getValue() < cast<const ConstantUInt>(C2.first)->getValue();
const ConstantSInt* S1 = dyn_cast<const ConstantSInt>(C1.first);
return S1->getValue() < cast<const ConstantSInt>(C2.first)->getValue();
}
};
RegisterOpt<LowerSwitch>
@ -49,16 +72,107 @@ bool LowerSwitch::runOnFunction(Function &F) {
return Changed;
}
// operator<< - Used for debugging purposes.
//
std::ostream& operator << (std::ostream& O, std::vector<LowerSwitch::Case>& C)
{
O << "[";
for (std::vector<LowerSwitch::Case>::iterator B = C.begin(), E = C.end();
B != E; ) {
O << *B->first;
if (++B != E) O << ", ";
}
return O << "]";
}
// switchConvert - Convert the switch statement into a binary lookup of
// the case values. The function recursively builds this tree.
//
BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
Value* Val, BasicBlock* OrigBlock,
BasicBlock* Default)
{
unsigned Size = End - Begin;
if (Size == 1)
return newLeafBlock(*Begin, Val, OrigBlock, Default);
unsigned Mid = Size / 2;
std::vector<Case> LHS(Begin, Begin + Mid);
DEBUG(std::cerr << "LHS: " << LHS << "\n");
std::vector<Case> RHS(Begin + Mid, End);
DEBUG(std::cerr << "RHS: " << RHS << "\n");
Case& Pivot = *(Begin + Mid);
DEBUG(std::cerr << "Pivot ==> "
<< cast<ConstantUInt>(Pivot.first)->getValue() << "\n");
BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
OrigBlock, Default);
BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
OrigBlock, Default);
// 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 = new BasicBlock("NodeBlock");
F->getBasicBlockList().insert(OrigBlock->getNext(), NewNode);
SetCondInst* Comp = new SetCondInst(Instruction::SetLT, Val, Pivot.first,
"Pivot");
NewNode->getInstList().push_back(Comp);
BranchInst* Br = new BranchInst(LBranch, RBranch, Comp);
NewNode->getInstList().push_back(Br);
return NewNode;
}
// newLeafBlock - 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(Case& Leaf, Value* Val,
BasicBlock* OrigBlock,
BasicBlock* Default)
{
Function* F = OrigBlock->getParent();
BasicBlock* NewLeaf = new BasicBlock("LeafBlock");
F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
// Make the seteq instruction...
SetCondInst* Comp = new SetCondInst(Instruction::SetEQ, Val,
Leaf.first, "SwitchLeaf");
NewLeaf->getInstList().push_back(Comp);
// Make the conditional branch...
BasicBlock* Succ = Leaf.second;
Instruction* Br = new BranchInst(Succ, Default, Comp);
NewLeaf->getInstList().push_back(Br);
// If there were any PHI nodes in this successor, rewrite one entry
// from OrigBlock to come from NewLeaf.
for (BasicBlock::iterator I = Succ->begin();
PHINode* PN = dyn_cast<PHINode>(I); ++I) {
int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
assert(BlockIdx != -1 && "Switch didn't go to this successor??");
PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
}
return NewLeaf;
}
// processSwitchInst - Replace the specified switch instruction with a sequence
// of chained basic blocks. Right now we just insert an incredibly stupid
// linear sequence of branches. It would be better to do a balanced binary
// search eventually. FIXME
// of chained if-then insts in a balanced binary search.
//
void LowerSwitch::processSwitchInst(SwitchInst *SI) {
BasicBlock *CurBlock = SI->getParent();
BasicBlock *OrigBlock = CurBlock;
Function *F = CurBlock->getParent();
Value *Val = SI->getOperand(0); // The value we are switching on...
BasicBlock* Default = SI->getDefaultDest();
// Unlink the switch instruction from it's block.
CurBlock->getInstList().remove(SI);
@ -70,50 +184,36 @@ void LowerSwitch::processSwitchInst(SwitchInst *SI) {
return;
}
// Expand comparisons for all of the non-default cases...
for (unsigned i = 2, e = SI->getNumOperands(); i != e; i += 2) {
// Insert a new basic block after the current one...
BasicBlock *NextBlock;
if (i != e-2) {
NextBlock = new BasicBlock("switchblock");
F->getBasicBlockList().insert(CurBlock->getNext(), NextBlock);
} else { // Last case, if it's not the value, go to default block.
NextBlock = cast<BasicBlock>(SI->getDefaultDest());
}
// 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 = new BasicBlock("NewDefault");
F->getBasicBlockList().insert(Default, NewDefault);
// Make the seteq instruction...
Instruction *Comp = new SetCondInst(Instruction::SetEQ, Val,
SI->getOperand(i), "switchcase");
CurBlock->getInstList().push_back(Comp);
NewDefault->getInstList().push_back(new BranchInst(Default));
// Make the conditional branch...
BasicBlock *Succ = cast<BasicBlock>(SI->getOperand(i+1));
Instruction *Br = new BranchInst(Succ, NextBlock, Comp);
CurBlock->getInstList().push_back(Br);
// If there were any PHI nodes in this successor, rewrite one entry from
// OrigBlock to come from CurBlock.
for (BasicBlock::iterator I = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
assert(BlockIdx != -1 && "Switch didn't go to this successor??");
PN->setIncomingBlock((unsigned)BlockIdx, CurBlock);
}
if (i == e-2) { // Is this looking at the default destination?
// If there is an entry in any PHI nodes for the default edge, make sure
// to update them as well.
for (BasicBlock::iterator I = NextBlock->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
assert(BlockIdx != -1 && "Switch didn't go to this successor??");
PN->setIncomingBlock((unsigned)BlockIdx, CurBlock);
}
}
CurBlock = NextBlock; // Move on to the next condition
// 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();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
assert(BlockIdx != -1 && "Switch didn't go to this successor??");
PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
}
std::vector<Case> Cases;
// Expand comparisons for all of the non-default cases...
for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
Cases.push_back(Case(SI->getSuccessorValue(i), SI->getSuccessor(i)));
std::sort(Cases.begin(), Cases.end(), CaseCmp());
DEBUG(std::cerr << "Cases: " << Cases << "\n");
BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
OrigBlock, NewDefault);
// Branch to our shiny new if-then stuff...
OrigBlock->getInstList().push_back(new BranchInst(SwitchBlock));
// We are now done with the switch instruction, delete it.
delete SI;
}