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

1 files changed, 144 insertions, 44 deletions

diff --git a/lib/Transforms/Utils/LowerSwitch.cpp b/lib/Transforms/Utils/LowerSwitch.cpp
index 73ce2f77f9..f2608d9ef0 100644
--- a/lib/Transforms/Utils/LowerSwitch.cpp
+++ b/lib/Transforms/Utils/LowerSwitch.cpp
@@ -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());
-    }
-
-    // Make the seteq instruction...
-    Instruction *Comp = new SetCondInst(Instruction::SetEQ, Val,
-                                        SI->getOperand(i), "switchcase");
-    CurBlock->getInstList().push_back(Comp);
-
-    // 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);
-    }
+  // 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);
 
-    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);
-      }
-    }
+  NewDefault->getInstList().push_back(new BranchInst(Default));
 
-    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;
 }