New pass to lower switch instructions to branch instructions

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@5865 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/LowerSwitch.cpp

@@ -0,0 +1,102 @@
+//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
+//
+// The LowerSwitch transformation rewrites switch statements with a sequence of
+// branches, which allows targets to get away with not implementing the switch
+// statement until it is convenient.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Function.h"
+#include "llvm/iTerminators.h"
+#include "llvm/iOperators.h"
+#include "llvm/iPHINode.h"
+#include "llvm/Pass.h"
+#include "Support/Statistic.h"
+
+namespace {
+  Statistic<> NumLowered("lowerswitch", "Number of SwitchInst's replaced");
+
+  /// 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 {
+    bool runOnFunction(Function &F);
+    void processSwitchInst(SwitchInst *SI);
+  };
+
+  RegisterOpt<LowerSwitch>
+  X("lowerswitch", "Lower SwitchInst's to branches");
+}
+
+// createLowerSwitchPass - Interface to this file...
+Pass *createLowerSwitchPass() {
+  return new LowerSwitch();
+}
+
+bool LowerSwitch::runOnFunction(Function &F) {
+  bool Changed = false;
+
+  for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
+    BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
+
+    if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
+      Changed = true;
+      processSwitchInst(SI);
+    }
+  }
+
+  return Changed;
+}
+
+// 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
+//
+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...
+
+  // Unlink the switch instruction from it's block.
+  CurBlock->getInstList().remove(SI);
+
+  // 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 success, 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);
+    }
+
+    CurBlock = NextBlock;  // Move on to the next condition
+  }
+
+
+  // We are now done with the switch instruction, delete it.
+  delete SI;
+}