New transformation: tail recursion elimination

llvm-svn: 8618

lib/Transforms/Scalar/TailRecursionElimination.cpp

@@ -0,0 +1,96 @@
+//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
+//
+// This file implements tail recursion elimination.
+//
+// Caveats: The algorithm implemented is trivially simple.  There are several
+// improvements that could be made:
+//
+//  1. If the function has any alloca instructions, these instructions will not
+//     remain in the entry block of the function.  Doing this requires analysis
+//     to prove that the alloca is not reachable by the recursively invoked
+//     function call.
+//  2. Tail recursion is only performed if the call immediately preceeds the
+//     return instruction.  Would it be useful to generalize this somehow?
+//  3. TRE is only performed if the function returns void or if the return
+//     returns the result returned by the call.  It is possible, but unlikely,
+//     that the return returns something else (like constant 0), and can still
+//     be TRE'd.  It can be TRE'd if ALL OTHER return instructions in the
+//     function return the exact same value.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/DerivedTypes.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/Pass.h"
+#include "Support/Statistic.h"
+
+namespace {
+  Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed");
+
+  struct TailCallElim : public FunctionPass {
+    virtual bool runOnFunction(Function &F);
+  };
+  RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination");
+}
+
+
+bool TailCallElim::runOnFunction(Function &F) {
+  // If this function is a varargs function, we won't be able to PHI the args
+  // right, so don't even try to convert it...
+  if (F.getFunctionType()->isVarArg()) return false;
+
+  BasicBlock *OldEntry = 0;
+  std::vector<PHINode*> ArgumentPHIs;
+  bool MadeChange = false;
+
+  // Loop over the function, looking for any returning blocks...
+  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
+    if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
+      if (Ret != BB->begin())
+        if (CallInst *CI = dyn_cast<CallInst>(Ret->getPrev()))
+          // Make sure the tail call is to the current function, and that the
+          // return either returns void or returns the value computed by the
+          // call.
+          if (CI->getCalledFunction() == &F &&
+              (Ret->getNumOperands() == 0 || Ret->getReturnValue() == CI)) {
+            // Ohh, it looks like we found a tail call, is this the first?
+            if (!OldEntry) {
+              // Ok, so this is the first tail call we have found in this
+              // function.  Insert a new entry block into the function, allowing
+              // us to branch back to the old entry block.
+              OldEntry = &F.getEntryNode();
+              BasicBlock *NewEntry = new BasicBlock("tailrecurse", OldEntry);
+              NewEntry->getInstList().push_back(new BranchInst(OldEntry));
+              
+              // Now that we have created a new block, which jumps to the entry
+              // block, insert a PHI node for each argument of the function.
+              // For now, we initialize each PHI to only have the real arguments
+              // which are passed in.
+              Instruction *InsertPos = OldEntry->begin();
+              for (Function::aiterator I = F.abegin(), E = F.aend(); I!=E; ++I){
+                PHINode *PN = new PHINode(I->getType(), I->getName()+".tr",
+                                          InsertPos);
+                PN->addIncoming(I, NewEntry);
+                ArgumentPHIs.push_back(PN);
+              }
+            }
+            
+            // Ok, now that we know we have a pseudo-entry block WITH all of the
+            // required PHI nodes, add entries into the PHI node for the actual
+            // parameters passed into the tail-recursive call.
+            for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
+              ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
+
+            // Now that all of the PHI nodes are in place, remove the call and
+            // ret instructions, replacing them with an unconditional branch.
+            new BranchInst(OldEntry, CI);
+            BB->getInstList().pop_back();  // Remove return.
+            BB->getInstList().pop_back();  // Remove call.
+            MadeChange = true;
+            NumEliminated++;
+          }
+  
+  return MadeChange;
+}
+