llvm-6502/lib/Transforms/Scalar/TailRecursionElimination.cpp

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//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file transforms calls of the current function (self recursion) followed
// by a return instruction with a branch to the entry of the function, creating
// a loop. This pass also implements the following extensions to the basic
// algorithm:
//
// 1. Trivial instructions between the call and return do not prevent the
// 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.
// 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
// 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.
//
// There are several improvements that could be made:
//
// 1. If the function has any alloca instructions, these instructions will be
// moved out of the entry block of the function, causing them to be
// 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
// 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
// prevent the TRE from occurring. For example, there could be GEP's and
// stores to memory that will not be read or written by the call. This
// requires some substantial analysis (such as with DSA) to prove safe to
// move ahead of the call, but doing so could allow many more TREs to be
// performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Support/CFG.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
namespace {
Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed");
Statistic<> NumAccumAdded("tailcallelim","Number of accumulators introduced");
struct TailCallElim : public FunctionPass {
virtual bool runOnFunction(Function &F);
private:
bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
std::vector<PHINode*> &ArgumentPHIs);
bool CanMoveAboveCall(Instruction *I, CallInst *CI);
Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
};
RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination");
}
// Public interface to the TailCallElimination pass
FunctionPass *llvm::createTailCallEliminationPass() {
return new TailCallElim();
}
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()))
MadeChange |= ProcessReturningBlock(Ret, OldEntry, ArgumentPHIs);
// 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)
// with themselves. Check to see if we did and clean up our mess if so. This
// occurs when a function passes an argument straight through to its tail
// call.
if (!ArgumentPHIs.empty()) {
unsigned NumIncoming = ArgumentPHIs[0]->getNumIncomingValues();
for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
PHINode *PN = ArgumentPHIs[i];
Value *V = 0;
for (unsigned op = 0, e = NumIncoming; op != e; ++op) {
Value *Op = PN->getIncomingValue(op);
if (Op != PN) {
if (V == 0) {
V = Op; // First value seen?
} else if (V != Op) {
V = 0;
break;
}
}
}
// If the PHI Node is a dynamic constant, replace it with the value it is.
if (V) {
PN->replaceAllUsesWith(V);
PN->getParent()->getInstList().erase(PN);
}
}
}
return MadeChange;
}
/// CanMoveAboveCall - Return true if it is safe to move the specified
/// instruction from after the call to before the call, assuming that all
/// instructions between the call and this instruction are movable.
///
bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
// FIXME: We can move load/store/call/free instructions above the call if the
// call does not mod/ref the memory location being processed.
if (I->mayWriteToMemory() || isa<LoadInst>(I))
return false;
// Otherwise, if this is a side-effect free instruction, check to make sure
// that it does not use the return value of the call. If it doesn't use the
// return value of the call, it must only use things that are defined before
// the call, or movable instructions between the call and the instruction
// itself.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (I->getOperand(i) == CI)
return false;
return true;
}
// isDynamicConstant - Return true if the specified value is the same when the
// return would exit as it was when the initial iteration of the recursive
// function was executed.
//
// We currently handle static constants and arguments that are not modified as
// part of the recursion.
//
static bool isDynamicConstant(Value *V, CallInst *CI) {
if (isa<Constant>(V)) return true; // Static constants are always dyn consts
// Check to see if this is an immutable argument, if so, the value
// will be available to initialize the accumulator.
if (Argument *Arg = dyn_cast<Argument>(V)) {
// Figure out which argument number this is...
unsigned ArgNo = 0;
Function *F = CI->getParent()->getParent();
for (Function::aiterator AI = F->abegin(); &*AI != Arg; ++AI)
++ArgNo;
// If we are passing this argument into call as the corresponding
// argument operand, then the argument is dynamically constant.
// Otherwise, we cannot transform this function safely.
if (CI->getOperand(ArgNo+1) == Arg)
return true;
}
// Not a constant or immutable argument, we can't safely transform.
return false;
}
// getCommonReturnValue - Check to see if the function containing the specified
// return instruction and tail call consistently returns the same
// runtime-constant value at all exit points. If so, return the returned value.
//
static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) {
Function *F = TheRI->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 != TheRI) {
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))
return 0;
if (ReturnedValue && RetOp != ReturnedValue)
return 0; // Cannot transform if differing values are returned.
ReturnedValue = RetOp;
}
return ReturnedValue;
}
/// CanTransformAccumulatorRecursion - If the specified instruction can be
/// transformed using accumulator recursion elimination, return the constant
/// which is the start of the accumulator value. Otherwise return null.
///
Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
CallInst *CI) {
if (!I->isAssociative()) return 0;
assert(I->getNumOperands() == 2 &&
"Associative operations should have 2 args!");
// 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;
// The only user of this instruction we allow is a single return instruction.
if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
return 0;
// Ok, now we have to check all of the other return instructions in this
// function. If they return non-constants or differing values, then we cannot
// transform the function safely.
return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
}
bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
std::vector<PHINode*> &ArgumentPHIs) {
BasicBlock *BB = Ret->getParent();
Function *F = BB->getParent();
if (&BB->front() == Ret) // Make sure there is something before the ret...
return false;
// 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) {
CI = dyn_cast<CallInst>(BBI);
if (CI && CI->getCalledFunction() == F)
break;
if (BBI == BB->begin())
return false; // Didn't find a potential tail call.
--BBI;
}
// 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.
Value *AccumulatorRecursionEliminationInitVal = 0;
Instruction *AccumulatorRecursionInstr = 0;
// Ok, we found a potential tail call. We can currently only transform the
// 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!
}
}
// 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
// constant, return the value returned by the tail call, or that are being
// accumulator recursion variable eliminated.
if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI &&
AccumulatorRecursionEliminationInitVal == 0 &&
!getCommonReturnValue(Ret, CI))
return false;
// 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.
if (OldEntry == 0) {
OldEntry = &F->getEntryBlock();
std::string OldName = OldEntry->getName(); OldEntry->setName("tailrecurse");
BasicBlock *NewEntry = new BasicBlock(OldName, F, OldEntry);
new BranchInst(OldEntry, NewEntry);
// 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);
I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
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);
// If we are introducing an accumulator variable to eliminate the recursion,
// do so now. Note that we _know_ that no subsequent tail recursion
// eliminations will happen on this function because of the way the
// accumulator recursion predicate is set up.
//
if (AccumulatorRecursionEliminationInitVal) {
Instruction *AccRecInstr = AccumulatorRecursionInstr;
// Start by inserting a new PHI node for the accumulator.
PHINode *AccPN = new PHINode(AccRecInstr->getType(), "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,
// computed earlier. For any other existing branches to this block (due to
// 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);
else
AccPN->addIncoming(AccPN, *PI);
}
// 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);
// Finally, rewrite any return instructions in the program to return the PHI
// node instead of the "initval" that they do currently. This loop will
// actually rewrite the return value we are destroying, but that's ok.
for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
RI->setOperand(0, AccPN);
++NumAccumAdded;
}
// 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, Ret);
BB->getInstList().erase(Ret); // Remove return.
BB->getInstList().erase(CI); // Remove call.
++NumEliminated;
return true;
}