// transformation from taking place, though currently the analysis cannot
// support moving any really useful instructions (only dead ones).
// 2. This pass transforms functions that are prevented from being tail
-// recursive by an associative expression to use an accumulator variable,
-// thus compiling the typical naive factorial or 'fib' implementation into
-// efficient code.
+// recursive by an associative and commutative expression to use an
+// accumulator variable, thus compiling the typical naive factorial or
+// 'fib' implementation into efficient code.
// 3. TRE is performed if the function returns void, if the return
// returns the result returned by the call, or if the function returns a
// run-time constant on all exits from the function. It is possible, though
// evaluated each time through the tail recursion. Safely keeping allocas
// in the entry block requires analysis to proves that the tail-called
// function does not read or write the stack object.
-// 2. Tail recursion is only performed if the call immediately preceeds the
+// 2. Tail recursion is only performed if the call immediately precedes the
// return instruction. It's possible that there could be a jump between
// the call and the return.
// 3. There can be intervening operations between the call and the return that
#define DEBUG_TYPE "tailcallelim"
#include "llvm/Transforms/Scalar.h"
-#include "llvm/Transforms/Utils/Local.h"
-#include "llvm/Constants.h"
-#include "llvm/DerivedTypes.h"
-#include "llvm/Function.h"
-#include "llvm/Instructions.h"
-#include "llvm/Pass.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/InlineCost.h"
+#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
-#include "llvm/Support/CallSite.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
#include "llvm/Support/CFG.h"
-#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/CallSite.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
STATISTIC(NumEliminated, "Number of tail calls removed");
+STATISTIC(NumRetDuped, "Number of return duplicated");
STATISTIC(NumAccumAdded, "Number of accumulators introduced");
namespace {
struct TailCallElim : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
- TailCallElim() : FunctionPass(&ID) {}
+ TailCallElim() : FunctionPass(ID) {
+ initializeTailCallElimPass(*PassRegistry::getPassRegistry());
+ }
virtual bool runOnFunction(Function &F);
private:
+ CallInst *FindTRECandidate(Instruction *I,
+ bool CannotTailCallElimCallsMarkedTail);
+ bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
+ BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVector<PHINode*, 8> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail);
+ bool FoldReturnAndProcessPred(BasicBlock *BB,
+ ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVector<PHINode*, 8> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail);
bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
bool &TailCallsAreMarkedTail,
SmallVector<PHINode*, 8> &ArgumentPHIs,
}
char TailCallElim::ID = 0;
-static RegisterPass<TailCallElim> X("tailcallelim", "Tail Call Elimination");
+INITIALIZE_PASS(TailCallElim, "tailcallelim",
+ "Tail Call Elimination", false, false)
// Public interface to the TailCallElimination pass
FunctionPass *llvm::createTailCallEliminationPass() {
bool TailCallsAreMarkedTail = false;
SmallVector<PHINode*, 8> ArgumentPHIs;
bool MadeChange = false;
-
bool FunctionContainsEscapingAllocas = false;
// CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
FunctionContainsEscapingAllocas |=
CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
}
-
+
/// FIXME: The code generator produces really bad code when an 'escaping
/// alloca' is changed from being a static alloca to being a dynamic alloca.
/// Until this is resolved, disable this transformation if that would ever
return false;
// Second pass, change any tail calls to loops.
- for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
- if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
- MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
+ if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
+ bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
ArgumentPHIs,CannotTCETailMarkedCall);
+ if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
+ Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
+ TailCallsAreMarkedTail, ArgumentPHIs,
+ CannotTCETailMarkedCall);
+ MadeChange |= Change;
+ }
+ }
// If we eliminated any tail recursions, it's possible that we inserted some
// silly PHI nodes which just merge an initial value (the incoming operand)
PHINode *PN = ArgumentPHIs[i];
// If the PHI Node is a dynamic constant, replace it with the value it is.
- if (Value *PNV = PN->hasConstantValue()) {
+ if (Value *PNV = SimplifyInstruction(PN)) {
PN->replaceAllUsesWith(PNV);
PN->eraseFromParent();
}
}
}
- // Finally, if this function contains no non-escaping allocas, mark all calls
- // in the function as eligible for tail calls (there is no stack memory for
- // them to access).
- if (!FunctionContainsEscapingAllocas)
+ // Finally, if this function contains no non-escaping allocas, or calls
+ // setjmp, mark all calls in the function as eligible for tail calls
+ //(there is no stack memory for them to access).
+ if (!FunctionContainsEscapingAllocas && !F.callsFunctionThatReturnsTwice())
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (CallInst *CI = dyn_cast<CallInst>(I)) {
// call does not mod/ref the memory location being processed.
if (I->mayHaveSideEffects()) // This also handles volatile loads.
return false;
-
+
if (LoadInst *L = dyn_cast<LoadInst>(I)) {
// Loads may always be moved above calls without side effects.
if (CI->mayHaveSideEffects()) {
Function *F = CI->getParent()->getParent();
Value *ReturnedValue = 0;
- for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
- if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
- if (RI != IgnoreRI) {
- Value *RetOp = RI->getOperand(0);
-
- // We can only perform this transformation if the value returned is
- // evaluatable at the start of the initial invocation of the function,
- // instead of at the end of the evaluation.
- //
- if (!isDynamicConstant(RetOp, CI, RI))
- return 0;
-
- if (ReturnedValue && RetOp != ReturnedValue)
- return 0; // Cannot transform if differing values are returned.
- ReturnedValue = RetOp;
- }
+ for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
+ ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
+ if (RI == 0 || RI == IgnoreRI) continue;
+
+ // We can only perform this transformation if the value returned is
+ // evaluatable at the start of the initial invocation of the function,
+ // instead of at the end of the evaluation.
+ //
+ Value *RetOp = RI->getOperand(0);
+ if (!isDynamicConstant(RetOp, CI, RI))
+ return 0;
+
+ if (ReturnedValue && RetOp != ReturnedValue)
+ return 0; // Cannot transform if differing values are returned.
+ ReturnedValue = RetOp;
+ }
return ReturnedValue;
}
///
Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
CallInst *CI) {
- if (!I->isAssociative()) return 0;
+ if (!I->isAssociative() || !I->isCommutative()) return 0;
assert(I->getNumOperands() == 2 &&
- "Associative operations should have 2 args!");
+ "Associative/commutative operations should have 2 args!");
- // Exactly one operand should be the result of the call instruction...
+ // Exactly one operand should be the result of the call instruction.
if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
(I->getOperand(0) != CI && I->getOperand(1) != CI))
return 0;
return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
}
-bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
- bool &TailCallsAreMarkedTail,
- SmallVector<PHINode*, 8> &ArgumentPHIs,
- bool CannotTailCallElimCallsMarkedTail) {
- BasicBlock *BB = Ret->getParent();
+static Instruction *FirstNonDbg(BasicBlock::iterator I) {
+ while (isa<DbgInfoIntrinsic>(I))
+ ++I;
+ return &*I;
+}
+
+CallInst*
+TailCallElim::FindTRECandidate(Instruction *TI,
+ bool CannotTailCallElimCallsMarkedTail) {
+ BasicBlock *BB = TI->getParent();
Function *F = BB->getParent();
- if (&BB->front() == Ret) // Make sure there is something before the ret...
- return false;
-
+ if (&BB->front() == TI) // Make sure there is something before the terminator.
+ return 0;
+
// Scan backwards from the return, checking to see if there is a tail call in
// this block. If so, set CI to it.
- CallInst *CI;
- BasicBlock::iterator BBI = Ret;
- while (1) {
+ CallInst *CI = 0;
+ BasicBlock::iterator BBI = TI;
+ while (true) {
CI = dyn_cast<CallInst>(BBI);
if (CI && CI->getCalledFunction() == F)
break;
if (BBI == BB->begin())
- return false; // Didn't find a potential tail call.
+ return 0; // Didn't find a potential tail call.
--BBI;
}
// If this call is marked as a tail call, and if there are dynamic allocas in
// the function, we cannot perform this optimization.
if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
- return false;
+ return 0;
// As a special case, detect code like this:
// double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
// and disable this xform in this case, because the code generator will
// lower the call to fabs into inline code.
- if (BB == &F->getEntryBlock() &&
- &BB->front() == CI && &*++BB->begin() == Ret &&
- callIsSmall(F)) {
+ if (BB == &F->getEntryBlock() &&
+ FirstNonDbg(BB->front()) == CI &&
+ FirstNonDbg(llvm::next(BB->begin())) == TI &&
+ callIsSmall(CI)) {
// A single-block function with just a call and a return. Check that
// the arguments match.
CallSite::arg_iterator I = CallSite(CI).arg_begin(),
for (; I != E && FI != FE; ++I, ++FI)
if (*I != &*FI) break;
if (I == E && FI == FE)
- return false;
+ return 0;
}
- // If we are introducing accumulator recursion to eliminate associative
- // operations after the call instruction, this variable contains the initial
- // value for the accumulator. If this value is set, we actually perform
- // accumulator recursion elimination instead of simple tail recursion
- // elimination.
+ return CI;
+}
+
+bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
+ BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVector<PHINode*, 8> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ // If we are introducing accumulator recursion to eliminate operations after
+ // the call instruction that are both associative and commutative, the initial
+ // value for the accumulator is placed in this variable. If this value is set
+ // then we actually perform accumulator recursion elimination instead of
+ // simple tail recursion elimination. If the operation is an LLVM instruction
+ // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
+ // we are handling the case when the return instruction returns a constant C
+ // which is different to the constant returned by other return instructions
+ // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
+ // special case of accumulator recursion, the operation being "return C".
Value *AccumulatorRecursionEliminationInitVal = 0;
Instruction *AccumulatorRecursionInstr = 0;
// tail call if all of the instructions between the call and the return are
// movable to above the call itself, leaving the call next to the return.
// Check that this is the case now.
- for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI)
- if (!CanMoveAboveCall(BBI, CI)) {
- // If we can't move the instruction above the call, it might be because it
- // is an associative operation that could be tranformed using accumulator
- // recursion elimination. Check to see if this is the case, and if so,
- // remember the initial accumulator value for later.
- if ((AccumulatorRecursionEliminationInitVal =
- CanTransformAccumulatorRecursion(BBI, CI))) {
- // Yes, this is accumulator recursion. Remember which instruction
- // accumulates.
- AccumulatorRecursionInstr = BBI;
- } else {
- return false; // Otherwise, we cannot eliminate the tail recursion!
- }
+ BasicBlock::iterator BBI = CI;
+ for (++BBI; &*BBI != Ret; ++BBI) {
+ if (CanMoveAboveCall(BBI, CI)) continue;
+
+ // If we can't move the instruction above the call, it might be because it
+ // is an associative and commutative operation that could be transformed
+ // using accumulator recursion elimination. Check to see if this is the
+ // case, and if so, remember the initial accumulator value for later.
+ if ((AccumulatorRecursionEliminationInitVal =
+ CanTransformAccumulatorRecursion(BBI, CI))) {
+ // Yes, this is accumulator recursion. Remember which instruction
+ // accumulates.
+ AccumulatorRecursionInstr = BBI;
+ } else {
+ return false; // Otherwise, we cannot eliminate the tail recursion!
}
+ }
// We can only transform call/return pairs that either ignore the return value
// of the call and return void, ignore the value of the call and return a
if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
!isa<UndefValue>(Ret->getReturnValue()) &&
AccumulatorRecursionEliminationInitVal == 0 &&
- !getCommonReturnValue(0, CI))
- return false;
+ !getCommonReturnValue(0, CI)) {
+ // One case remains that we are able to handle: the current return
+ // instruction returns a constant, and all other return instructions
+ // return a different constant.
+ if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
+ return false; // Current return instruction does not return a constant.
+ // Check that all other return instructions return a common constant. If
+ // so, record it in AccumulatorRecursionEliminationInitVal.
+ AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
+ if (!AccumulatorRecursionEliminationInitVal)
+ return false;
+ }
+
+ BasicBlock *BB = Ret->getParent();
+ Function *F = BB->getParent();
// OK! We can transform this tail call. If this is the first one found,
// create the new entry block, allowing us to branch back to the old entry.
Instruction *InsertPos = OldEntry->begin();
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
I != E; ++I) {
- PHINode *PN = PHINode::Create(I->getType(),
+ PHINode *PN = PHINode::Create(I->getType(), 2,
I->getName() + ".tr", InsertPos);
I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
PN->addIncoming(I, NewEntry);
if (AccumulatorRecursionEliminationInitVal) {
Instruction *AccRecInstr = AccumulatorRecursionInstr;
// Start by inserting a new PHI node for the accumulator.
- PHINode *AccPN = PHINode::Create(AccRecInstr->getType(), "accumulator.tr",
- OldEntry->begin());
+ pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
+ PHINode *AccPN =
+ PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(),
+ std::distance(PB, PE) + 1,
+ "accumulator.tr", OldEntry->begin());
// Loop over all of the predecessors of the tail recursion block. For the
// real entry into the function we seed the PHI with the initial value,
// other tail recursions eliminated) the accumulator is not modified.
// Because we haven't added the branch in the current block to OldEntry yet,
// it will not show up as a predecessor.
- for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
- PI != PE; ++PI) {
- if (*PI == &F->getEntryBlock())
- AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI);
+ for (pred_iterator PI = PB; PI != PE; ++PI) {
+ BasicBlock *P = *PI;
+ if (P == &F->getEntryBlock())
+ AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
else
- AccPN->addIncoming(AccPN, *PI);
+ AccPN->addIncoming(AccPN, P);
}
- // Add an incoming argument for the current block, which is computed by our
- // associative accumulator instruction.
- AccPN->addIncoming(AccRecInstr, BB);
-
- // Next, rewrite the accumulator recursion instruction so that it does not
- // use the result of the call anymore, instead, use the PHI node we just
- // inserted.
- AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
+ if (AccRecInstr) {
+ // Add an incoming argument for the current block, which is computed by
+ // our associative and commutative accumulator instruction.
+ AccPN->addIncoming(AccRecInstr, BB);
+
+ // Next, rewrite the accumulator recursion instruction so that it does not
+ // use the result of the call anymore, instead, use the PHI node we just
+ // inserted.
+ AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
+ } else {
+ // Add an incoming argument for the current block, which is just the
+ // constant returned by the current return instruction.
+ AccPN->addIncoming(Ret->getReturnValue(), BB);
+ }
// Finally, rewrite any return instructions in the program to return the PHI
// node instead of the "initval" that they do currently. This loop will
// Now that all of the PHI nodes are in place, remove the call and
// ret instructions, replacing them with an unconditional branch.
- BranchInst::Create(OldEntry, Ret);
+ BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
+ NewBI->setDebugLoc(CI->getDebugLoc());
+
BB->getInstList().erase(Ret); // Remove return.
BB->getInstList().erase(CI); // Remove call.
++NumEliminated;
return true;
}
+
+bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB,
+ ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVector<PHINode*, 8> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ bool Change = false;
+
+ // If the return block contains nothing but the return and PHI's,
+ // there might be an opportunity to duplicate the return in its
+ // predecessors and perform TRC there. Look for predecessors that end
+ // in unconditional branch and recursive call(s).
+ SmallVector<BranchInst*, 8> UncondBranchPreds;
+ for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
+ BasicBlock *Pred = *PI;
+ TerminatorInst *PTI = Pred->getTerminator();
+ if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
+ if (BI->isUnconditional())
+ UncondBranchPreds.push_back(BI);
+ }
+
+ while (!UncondBranchPreds.empty()) {
+ BranchInst *BI = UncondBranchPreds.pop_back_val();
+ BasicBlock *Pred = BI->getParent();
+ if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){
+ DEBUG(dbgs() << "FOLDING: " << *BB
+ << "INTO UNCOND BRANCH PRED: " << *Pred);
+ EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred),
+ OldEntry, TailCallsAreMarkedTail, ArgumentPHIs,
+ CannotTailCallElimCallsMarkedTail);
+ ++NumRetDuped;
+ Change = true;
+ }
+ }
+
+ return Change;
+}
+
+bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVector<PHINode*, 8> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail);
+ if (!CI)
+ return false;
+
+ return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
+ ArgumentPHIs,
+ CannotTailCallElimCallsMarkedTail);
+}