2003-09-20 05:03:31 +00:00
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//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
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2003-10-20 19:43:21 +00:00
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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2003-09-20 05:03:31 +00:00
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//
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2003-12-08 05:34:54 +00:00
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// This file transforms calls of the current function (self recursion) followed
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// by a return instruction with a branch to the entry of the function, creating
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// a loop. This pass also implements the following extensions to the basic
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// algorithm:
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2003-09-20 05:03:31 +00:00
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//
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// 1. Trivial instructions between the call and return do not prevent the
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// transformation from taking place, though currently the analysis cannot
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// support moving any really useful instructions (only dead ones).
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//
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// There are several improvements that could be made:
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//
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// 1. If the function has any alloca instructions, these instructions will be
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// moved out of the entry block of the function, causing them to be
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// evaluated each time through the tail recursion. Safely keeping allocas
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// in the entry block requires analysis to proves that the tail-called
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// function does not read or write the stack object.
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// 2. Tail recursion is only performed if the call immediately preceeds the
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// return instruction. It's possible that there could be a jump between
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// the call and the return.
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// 3. TRE is only performed if the function returns void or if the return
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// returns the result returned by the call. It is possible, but unlikely,
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// that the return returns something else (like constant 0), and can still
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// be TRE'd. It can be TRE'd if ALL OTHER return instructions in the
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// function return the exact same value.
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// 4. There can be intervening operations between the call and the return that
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// prevent the TRE from occurring. For example, there could be GEP's and
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// stores to memory that will not be read or written by the call. This
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// requires some substantial analysis (such as with DSA) to prove safe to
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// move ahead of the call, but doing so could allow many more TREs to be
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// performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
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// 5. This pass could transform functions that are prevented from being tail
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// recursive by a commutative expression to use an accumulator helper
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// function, thus compiling the typical naive factorial or 'fib'
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// implementation into efficient code.
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//
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//===----------------------------------------------------------------------===//
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2003-09-20 05:14:13 +00:00
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Pass.h"
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#include "Support/Statistic.h"
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using namespace llvm;
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namespace {
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Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed");
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struct TailCallElim : public FunctionPass {
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virtual bool runOnFunction(Function &F);
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private:
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bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
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std::vector<PHINode*> &ArgumentPHIs);
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bool CanMoveAboveCall(Instruction *I, CallInst *CI);
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};
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RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination");
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}
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2003-11-11 22:41:34 +00:00
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// Public interface to the TailCallElimination pass
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FunctionPass *llvm::createTailCallEliminationPass() {
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return new TailCallElim();
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}
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bool TailCallElim::runOnFunction(Function &F) {
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// If this function is a varargs function, we won't be able to PHI the args
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// right, so don't even try to convert it...
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if (F.getFunctionType()->isVarArg()) return false;
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BasicBlock *OldEntry = 0;
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std::vector<PHINode*> ArgumentPHIs;
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bool MadeChange = false;
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// Loop over the function, looking for any returning blocks...
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for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
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if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
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MadeChange |= ProcessReturningBlock(Ret, OldEntry, ArgumentPHIs);
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return MadeChange;
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}
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// CanMoveAboveCall - Return true if it is safe to move the specified
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// instruction from after the call to before the call, assuming that all
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// instructions between the call and this instruction are movable.
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//
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bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
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// FIXME: We can move load/store/call/free instructions above the call if the
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// call does not mod/ref the memory location being processed.
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if (I->mayWriteToMemory() || isa<LoadInst>(I))
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return false;
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// Otherwise, if this is a side-effect free instruction, check to make sure
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// that it does not use the return value of the call. If it doesn't use the
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// return value of the call, it must only use things that are defined before
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// the call, or movable instructions between the call and the instruction
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// itself.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
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if (I->getOperand(i) == CI)
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return false;
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return true;
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}
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bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
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std::vector<PHINode*> &ArgumentPHIs) {
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BasicBlock *BB = Ret->getParent();
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Function *F = BB->getParent();
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if (&BB->front() == Ret) // Make sure there is something before the ret...
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return false;
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// Scan backwards from the return, checking to see if there is a tail call in
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// this block. If so, set CI to it.
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CallInst *CI;
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BasicBlock::iterator BBI = Ret;
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while (1) {
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CI = dyn_cast<CallInst>(BBI);
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if (CI && CI->getCalledFunction() == F)
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break;
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if (BBI == BB->begin())
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return false; // Didn't find a potential tail call.
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--BBI;
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}
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// Ok, we found a potential tail call. We can currently only transform the
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// tail call if all of the instructions between the call and the return are
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// movable to above the call itself, leaving the call next to the return.
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// Check that this is the case now.
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for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI)
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if (!CanMoveAboveCall(BBI, CI))
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return false; // Cannot move this instruction out of the way.
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// We can only transform call/return pairs that either ignore the return value
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// of the call and return void, or return the value returned by the tail call.
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if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI)
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return false;
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// OK! We can transform this tail call. If this is the first one found,
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// create the new entry block, allowing us to branch back to the old entry.
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if (OldEntry == 0) {
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OldEntry = &F->getEntryBlock();
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std::string OldName = OldEntry->getName(); OldEntry->setName("tailrecurse");
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BasicBlock *NewEntry = new BasicBlock(OldName, OldEntry);
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new BranchInst(OldEntry, NewEntry);
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// Now that we have created a new block, which jumps to the entry
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// block, insert a PHI node for each argument of the function.
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// For now, we initialize each PHI to only have the real arguments
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// which are passed in.
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Instruction *InsertPos = OldEntry->begin();
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for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I) {
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PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", InsertPos);
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I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
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PN->addIncoming(I, NewEntry);
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ArgumentPHIs.push_back(PN);
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}
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}
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// Ok, now that we know we have a pseudo-entry block WITH all of the
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// required PHI nodes, add entries into the PHI node for the actual
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// parameters passed into the tail-recursive call.
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for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
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ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
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// Now that all of the PHI nodes are in place, remove the call and
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// ret instructions, replacing them with an unconditional branch.
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new BranchInst(OldEntry, Ret);
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BB->getInstList().erase(Ret); // Remove return.
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BB->getInstList().erase(CI); // Remove call.
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NumEliminated++;
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return true;
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}
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