mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-08-27 15:29:51 +00:00
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232998 91177308-0d34-0410-b5e6-96231b3b80d8
852 lines
34 KiB
C++
852 lines
34 KiB
C++
//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
|
|
//
|
|
// The LLVM Compiler Infrastructure
|
|
//
|
|
// This file 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 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
|
|
// 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.
|
|
// 4. If it can prove that callees do not access their caller stack frame,
|
|
// they are marked as eligible for tail call elimination (by the code
|
|
// generator).
|
|
//
|
|
// 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 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
|
|
// 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.
|
|
// 4. The algorithm we use to detect if callees access their caller stack
|
|
// frames is very primitive.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#include "llvm/Transforms/Scalar.h"
|
|
#include "llvm/ADT/STLExtras.h"
|
|
#include "llvm/ADT/SmallPtrSet.h"
|
|
#include "llvm/ADT/Statistic.h"
|
|
#include "llvm/Analysis/CFG.h"
|
|
#include "llvm/Analysis/CaptureTracking.h"
|
|
#include "llvm/Analysis/InlineCost.h"
|
|
#include "llvm/Analysis/InstructionSimplify.h"
|
|
#include "llvm/Analysis/Loads.h"
|
|
#include "llvm/Analysis/TargetTransformInfo.h"
|
|
#include "llvm/IR/CFG.h"
|
|
#include "llvm/IR/CallSite.h"
|
|
#include "llvm/IR/Constants.h"
|
|
#include "llvm/IR/DataLayout.h"
|
|
#include "llvm/IR/DerivedTypes.h"
|
|
#include "llvm/IR/DiagnosticInfo.h"
|
|
#include "llvm/IR/Function.h"
|
|
#include "llvm/IR/Instructions.h"
|
|
#include "llvm/IR/IntrinsicInst.h"
|
|
#include "llvm/IR/Module.h"
|
|
#include "llvm/IR/ValueHandle.h"
|
|
#include "llvm/Pass.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;
|
|
|
|
#define DEBUG_TYPE "tailcallelim"
|
|
|
|
STATISTIC(NumEliminated, "Number of tail calls removed");
|
|
STATISTIC(NumRetDuped, "Number of return duplicated");
|
|
STATISTIC(NumAccumAdded, "Number of accumulators introduced");
|
|
|
|
namespace {
|
|
struct TailCallElim : public FunctionPass {
|
|
const TargetTransformInfo *TTI;
|
|
|
|
static char ID; // Pass identification, replacement for typeid
|
|
TailCallElim() : FunctionPass(ID) {
|
|
initializeTailCallElimPass(*PassRegistry::getPassRegistry());
|
|
}
|
|
|
|
void getAnalysisUsage(AnalysisUsage &AU) const override;
|
|
|
|
bool runOnFunction(Function &F) override;
|
|
|
|
private:
|
|
bool runTRE(Function &F);
|
|
bool markTails(Function &F, bool &AllCallsAreTailCalls);
|
|
|
|
CallInst *FindTRECandidate(Instruction *I,
|
|
bool CannotTailCallElimCallsMarkedTail);
|
|
bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
|
|
BasicBlock *&OldEntry,
|
|
bool &TailCallsAreMarkedTail,
|
|
SmallVectorImpl<PHINode *> &ArgumentPHIs,
|
|
bool CannotTailCallElimCallsMarkedTail);
|
|
bool FoldReturnAndProcessPred(BasicBlock *BB,
|
|
ReturnInst *Ret, BasicBlock *&OldEntry,
|
|
bool &TailCallsAreMarkedTail,
|
|
SmallVectorImpl<PHINode *> &ArgumentPHIs,
|
|
bool CannotTailCallElimCallsMarkedTail);
|
|
bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
|
|
bool &TailCallsAreMarkedTail,
|
|
SmallVectorImpl<PHINode *> &ArgumentPHIs,
|
|
bool CannotTailCallElimCallsMarkedTail);
|
|
bool CanMoveAboveCall(Instruction *I, CallInst *CI);
|
|
Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
|
|
};
|
|
}
|
|
|
|
char TailCallElim::ID = 0;
|
|
INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim",
|
|
"Tail Call Elimination", false, false)
|
|
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
|
|
INITIALIZE_PASS_END(TailCallElim, "tailcallelim",
|
|
"Tail Call Elimination", false, false)
|
|
|
|
// Public interface to the TailCallElimination pass
|
|
FunctionPass *llvm::createTailCallEliminationPass() {
|
|
return new TailCallElim();
|
|
}
|
|
|
|
void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const {
|
|
AU.addRequired<TargetTransformInfoWrapperPass>();
|
|
}
|
|
|
|
/// \brief Scan the specified function for alloca instructions.
|
|
/// If it contains any dynamic allocas, returns false.
|
|
static bool CanTRE(Function &F) {
|
|
// Because of PR962, we don't TRE dynamic allocas.
|
|
for (auto &BB : F) {
|
|
for (auto &I : BB) {
|
|
if (AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
|
|
if (!AI->isStaticAlloca())
|
|
return false;
|
|
}
|
|
}
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
bool TailCallElim::runOnFunction(Function &F) {
|
|
if (skipOptnoneFunction(F))
|
|
return false;
|
|
|
|
bool AllCallsAreTailCalls = false;
|
|
bool Modified = markTails(F, AllCallsAreTailCalls);
|
|
if (AllCallsAreTailCalls)
|
|
Modified |= runTRE(F);
|
|
return Modified;
|
|
}
|
|
|
|
namespace {
|
|
struct AllocaDerivedValueTracker {
|
|
// Start at a root value and walk its use-def chain to mark calls that use the
|
|
// value or a derived value in AllocaUsers, and places where it may escape in
|
|
// EscapePoints.
|
|
void walk(Value *Root) {
|
|
SmallVector<Use *, 32> Worklist;
|
|
SmallPtrSet<Use *, 32> Visited;
|
|
|
|
auto AddUsesToWorklist = [&](Value *V) {
|
|
for (auto &U : V->uses()) {
|
|
if (!Visited.insert(&U).second)
|
|
continue;
|
|
Worklist.push_back(&U);
|
|
}
|
|
};
|
|
|
|
AddUsesToWorklist(Root);
|
|
|
|
while (!Worklist.empty()) {
|
|
Use *U = Worklist.pop_back_val();
|
|
Instruction *I = cast<Instruction>(U->getUser());
|
|
|
|
switch (I->getOpcode()) {
|
|
case Instruction::Call:
|
|
case Instruction::Invoke: {
|
|
CallSite CS(I);
|
|
bool IsNocapture = !CS.isCallee(U) &&
|
|
CS.doesNotCapture(CS.getArgumentNo(U));
|
|
callUsesLocalStack(CS, IsNocapture);
|
|
if (IsNocapture) {
|
|
// If the alloca-derived argument is passed in as nocapture, then it
|
|
// can't propagate to the call's return. That would be capturing.
|
|
continue;
|
|
}
|
|
break;
|
|
}
|
|
case Instruction::Load: {
|
|
// The result of a load is not alloca-derived (unless an alloca has
|
|
// otherwise escaped, but this is a local analysis).
|
|
continue;
|
|
}
|
|
case Instruction::Store: {
|
|
if (U->getOperandNo() == 0)
|
|
EscapePoints.insert(I);
|
|
continue; // Stores have no users to analyze.
|
|
}
|
|
case Instruction::BitCast:
|
|
case Instruction::GetElementPtr:
|
|
case Instruction::PHI:
|
|
case Instruction::Select:
|
|
case Instruction::AddrSpaceCast:
|
|
break;
|
|
default:
|
|
EscapePoints.insert(I);
|
|
break;
|
|
}
|
|
|
|
AddUsesToWorklist(I);
|
|
}
|
|
}
|
|
|
|
void callUsesLocalStack(CallSite CS, bool IsNocapture) {
|
|
// Add it to the list of alloca users.
|
|
AllocaUsers.insert(CS.getInstruction());
|
|
|
|
// If it's nocapture then it can't capture this alloca.
|
|
if (IsNocapture)
|
|
return;
|
|
|
|
// If it can write to memory, it can leak the alloca value.
|
|
if (!CS.onlyReadsMemory())
|
|
EscapePoints.insert(CS.getInstruction());
|
|
}
|
|
|
|
SmallPtrSet<Instruction *, 32> AllocaUsers;
|
|
SmallPtrSet<Instruction *, 32> EscapePoints;
|
|
};
|
|
}
|
|
|
|
bool TailCallElim::markTails(Function &F, bool &AllCallsAreTailCalls) {
|
|
if (F.callsFunctionThatReturnsTwice())
|
|
return false;
|
|
AllCallsAreTailCalls = true;
|
|
|
|
// The local stack holds all alloca instructions and all byval arguments.
|
|
AllocaDerivedValueTracker Tracker;
|
|
for (Argument &Arg : F.args()) {
|
|
if (Arg.hasByValAttr())
|
|
Tracker.walk(&Arg);
|
|
}
|
|
for (auto &BB : F) {
|
|
for (auto &I : BB)
|
|
if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
|
|
Tracker.walk(AI);
|
|
}
|
|
|
|
bool Modified = false;
|
|
|
|
// Track whether a block is reachable after an alloca has escaped. Blocks that
|
|
// contain the escaping instruction will be marked as being visited without an
|
|
// escaped alloca, since that is how the block began.
|
|
enum VisitType {
|
|
UNVISITED,
|
|
UNESCAPED,
|
|
ESCAPED
|
|
};
|
|
DenseMap<BasicBlock *, VisitType> Visited;
|
|
|
|
// We propagate the fact that an alloca has escaped from block to successor.
|
|
// Visit the blocks that are propagating the escapedness first. To do this, we
|
|
// maintain two worklists.
|
|
SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
|
|
|
|
// We may enter a block and visit it thinking that no alloca has escaped yet,
|
|
// then see an escape point and go back around a loop edge and come back to
|
|
// the same block twice. Because of this, we defer setting tail on calls when
|
|
// we first encounter them in a block. Every entry in this list does not
|
|
// statically use an alloca via use-def chain analysis, but may find an alloca
|
|
// through other means if the block turns out to be reachable after an escape
|
|
// point.
|
|
SmallVector<CallInst *, 32> DeferredTails;
|
|
|
|
BasicBlock *BB = &F.getEntryBlock();
|
|
VisitType Escaped = UNESCAPED;
|
|
do {
|
|
for (auto &I : *BB) {
|
|
if (Tracker.EscapePoints.count(&I))
|
|
Escaped = ESCAPED;
|
|
|
|
CallInst *CI = dyn_cast<CallInst>(&I);
|
|
if (!CI || CI->isTailCall())
|
|
continue;
|
|
|
|
if (CI->doesNotAccessMemory()) {
|
|
// A call to a readnone function whose arguments are all things computed
|
|
// outside this function can be marked tail. Even if you stored the
|
|
// alloca address into a global, a readnone function can't load the
|
|
// global anyhow.
|
|
//
|
|
// Note that this runs whether we know an alloca has escaped or not. If
|
|
// it has, then we can't trust Tracker.AllocaUsers to be accurate.
|
|
bool SafeToTail = true;
|
|
for (auto &Arg : CI->arg_operands()) {
|
|
if (isa<Constant>(Arg.getUser()))
|
|
continue;
|
|
if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
|
|
if (!A->hasByValAttr())
|
|
continue;
|
|
SafeToTail = false;
|
|
break;
|
|
}
|
|
if (SafeToTail) {
|
|
emitOptimizationRemark(
|
|
F.getContext(), "tailcallelim", F, CI->getDebugLoc(),
|
|
"marked this readnone call a tail call candidate");
|
|
CI->setTailCall();
|
|
Modified = true;
|
|
continue;
|
|
}
|
|
}
|
|
|
|
if (Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
|
|
DeferredTails.push_back(CI);
|
|
} else {
|
|
AllCallsAreTailCalls = false;
|
|
}
|
|
}
|
|
|
|
for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
|
|
auto &State = Visited[SuccBB];
|
|
if (State < Escaped) {
|
|
State = Escaped;
|
|
if (State == ESCAPED)
|
|
WorklistEscaped.push_back(SuccBB);
|
|
else
|
|
WorklistUnescaped.push_back(SuccBB);
|
|
}
|
|
}
|
|
|
|
if (!WorklistEscaped.empty()) {
|
|
BB = WorklistEscaped.pop_back_val();
|
|
Escaped = ESCAPED;
|
|
} else {
|
|
BB = nullptr;
|
|
while (!WorklistUnescaped.empty()) {
|
|
auto *NextBB = WorklistUnescaped.pop_back_val();
|
|
if (Visited[NextBB] == UNESCAPED) {
|
|
BB = NextBB;
|
|
Escaped = UNESCAPED;
|
|
break;
|
|
}
|
|
}
|
|
}
|
|
} while (BB);
|
|
|
|
for (CallInst *CI : DeferredTails) {
|
|
if (Visited[CI->getParent()] != ESCAPED) {
|
|
// If the escape point was part way through the block, calls after the
|
|
// escape point wouldn't have been put into DeferredTails.
|
|
emitOptimizationRemark(F.getContext(), "tailcallelim", F,
|
|
CI->getDebugLoc(),
|
|
"marked this call a tail call candidate");
|
|
CI->setTailCall();
|
|
Modified = true;
|
|
} else {
|
|
AllCallsAreTailCalls = false;
|
|
}
|
|
}
|
|
|
|
return Modified;
|
|
}
|
|
|
|
bool TailCallElim::runTRE(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;
|
|
|
|
TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
|
|
BasicBlock *OldEntry = nullptr;
|
|
bool TailCallsAreMarkedTail = false;
|
|
SmallVector<PHINode*, 8> ArgumentPHIs;
|
|
bool MadeChange = false;
|
|
|
|
// If false, we cannot perform TRE on tail calls marked with the 'tail'
|
|
// attribute, because doing so would cause the stack size to increase (real
|
|
// TRE would deallocate variable sized allocas, TRE doesn't).
|
|
bool CanTRETailMarkedCall = CanTRE(F);
|
|
|
|
// Change any tail recursive calls to loops.
|
|
//
|
|
// 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
|
|
// happen. This bug is PR962.
|
|
for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; /*in loop*/) {
|
|
BasicBlock *BB = BBI++; // FoldReturnAndProcessPred may delete BB.
|
|
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
|
|
bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
|
|
ArgumentPHIs, !CanTRETailMarkedCall);
|
|
if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
|
|
Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
|
|
TailCallsAreMarkedTail, ArgumentPHIs,
|
|
!CanTRETailMarkedCall);
|
|
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)
|
|
// 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.
|
|
for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
|
|
PHINode *PN = ArgumentPHIs[i];
|
|
|
|
// If the PHI Node is a dynamic constant, replace it with the value it is.
|
|
if (Value *PNV = SimplifyInstruction(PN, F.getParent()->getDataLayout())) {
|
|
PN->replaceAllUsesWith(PNV);
|
|
PN->eraseFromParent();
|
|
}
|
|
}
|
|
|
|
return MadeChange;
|
|
}
|
|
|
|
|
|
/// 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->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()) {
|
|
// Non-volatile loads may be moved above a call with side effects if it
|
|
// does not write to memory and the load provably won't trap.
|
|
// FIXME: Writes to memory only matter if they may alias the pointer
|
|
// being loaded from.
|
|
if (CI->mayWriteToMemory() ||
|
|
!isSafeToLoadUnconditionally(L->getPointerOperand(), L,
|
|
L->getAlignment()))
|
|
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;
|
|
}
|
|
|
|
/// 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, ReturnInst *RI) {
|
|
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::arg_iterator AI = F->arg_begin(); &*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->getArgOperand(ArgNo) == Arg)
|
|
return true;
|
|
}
|
|
|
|
// Switch cases are always constant integers. If the value is being switched
|
|
// on and the return is only reachable from one of its cases, it's
|
|
// effectively constant.
|
|
if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
|
|
if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
|
|
if (SI->getCondition() == V)
|
|
return SI->getDefaultDest() != RI->getParent();
|
|
|
|
// Not a constant or immutable argument, we can't safely transform.
|
|
return false;
|
|
}
|
|
|
|
/// Check to see if the function containing the specified tail call consistently
|
|
/// returns the same runtime-constant value at all exit points except for
|
|
/// IgnoreRI. If so, return the returned value.
|
|
static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
|
|
Function *F = CI->getParent()->getParent();
|
|
Value *ReturnedValue = nullptr;
|
|
|
|
for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
|
|
ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
|
|
if (RI == nullptr || 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 nullptr;
|
|
|
|
if (ReturnedValue && RetOp != ReturnedValue)
|
|
return nullptr; // Cannot transform if differing values are returned.
|
|
ReturnedValue = RetOp;
|
|
}
|
|
return ReturnedValue;
|
|
}
|
|
|
|
/// 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() || !I->isCommutative()) return nullptr;
|
|
assert(I->getNumOperands() == 2 &&
|
|
"Associative/commutative 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 nullptr;
|
|
|
|
// The only user of this instruction we allow is a single return instruction.
|
|
if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
|
|
return nullptr;
|
|
|
|
// 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->user_back()), CI);
|
|
}
|
|
|
|
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() == TI) // Make sure there is something before the terminator.
|
|
return nullptr;
|
|
|
|
// 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 = nullptr;
|
|
BasicBlock::iterator BBI = TI;
|
|
while (true) {
|
|
CI = dyn_cast<CallInst>(BBI);
|
|
if (CI && CI->getCalledFunction() == F)
|
|
break;
|
|
|
|
if (BBI == BB->begin())
|
|
return nullptr; // 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 nullptr;
|
|
|
|
// 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() &&
|
|
FirstNonDbg(BB->front()) == CI &&
|
|
FirstNonDbg(std::next(BB->begin())) == TI &&
|
|
CI->getCalledFunction() &&
|
|
!TTI->isLoweredToCall(CI->getCalledFunction())) {
|
|
// A single-block function with just a call and a return. Check that
|
|
// the arguments match.
|
|
CallSite::arg_iterator I = CallSite(CI).arg_begin(),
|
|
E = CallSite(CI).arg_end();
|
|
Function::arg_iterator FI = F->arg_begin(),
|
|
FE = F->arg_end();
|
|
for (; I != E && FI != FE; ++I, ++FI)
|
|
if (*I != &*FI) break;
|
|
if (I == E && FI == FE)
|
|
return nullptr;
|
|
}
|
|
|
|
return CI;
|
|
}
|
|
|
|
bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
|
|
BasicBlock *&OldEntry,
|
|
bool &TailCallsAreMarkedTail,
|
|
SmallVectorImpl<PHINode *> &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 = nullptr;
|
|
Instruction *AccumulatorRecursionInstr = nullptr;
|
|
|
|
// 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.
|
|
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
|
|
// constant, return the value returned by the tail call, or that are being
|
|
// accumulator recursion variable eliminated.
|
|
if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
|
|
!isa<UndefValue>(Ret->getReturnValue()) &&
|
|
AccumulatorRecursionEliminationInitVal == nullptr &&
|
|
!getCommonReturnValue(nullptr, 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();
|
|
|
|
emitOptimizationRemark(F->getContext(), "tailcallelim", *F, CI->getDebugLoc(),
|
|
"transforming tail recursion to loop");
|
|
|
|
// 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) {
|
|
OldEntry = &F->getEntryBlock();
|
|
BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
|
|
NewEntry->takeName(OldEntry);
|
|
OldEntry->setName("tailrecurse");
|
|
BranchInst::Create(OldEntry, NewEntry);
|
|
|
|
// If this tail call is marked 'tail' and if there are any allocas in the
|
|
// entry block, move them up to the new entry block.
|
|
TailCallsAreMarkedTail = CI->isTailCall();
|
|
if (TailCallsAreMarkedTail)
|
|
// Move all fixed sized allocas from OldEntry to NewEntry.
|
|
for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
|
|
NEBI = NewEntry->begin(); OEBI != E; )
|
|
if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
|
|
if (isa<ConstantInt>(AI->getArraySize()))
|
|
AI->moveBefore(NEBI);
|
|
|
|
// 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::arg_iterator I = F->arg_begin(), E = F->arg_end();
|
|
I != E; ++I) {
|
|
PHINode *PN = PHINode::Create(I->getType(), 2,
|
|
I->getName() + ".tr", InsertPos);
|
|
I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
|
|
PN->addIncoming(I, NewEntry);
|
|
ArgumentPHIs.push_back(PN);
|
|
}
|
|
}
|
|
|
|
// If this function has self recursive calls in the tail position where some
|
|
// are marked tail and some are not, only transform one flavor or another. We
|
|
// have to choose whether we move allocas in the entry block to the new entry
|
|
// block or not, so we can't make a good choice for both. NOTE: We could do
|
|
// slightly better here in the case that the function has no entry block
|
|
// allocas.
|
|
if (TailCallsAreMarkedTail && !CI->isTailCall())
|
|
return false;
|
|
|
|
// 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->getNumArgOperands(); i != e; ++i)
|
|
ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), 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.
|
|
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,
|
|
// 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 = PB; PI != PE; ++PI) {
|
|
BasicBlock *P = *PI;
|
|
if (P == &F->getEntryBlock())
|
|
AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
|
|
else
|
|
AccPN->addIncoming(AccPN, P);
|
|
}
|
|
|
|
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
|
|
// 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.
|
|
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,
|
|
SmallVectorImpl<PHINode *> &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);
|
|
ReturnInst *RI = FoldReturnIntoUncondBranch(Ret, BB, Pred);
|
|
|
|
// Cleanup: if all predecessors of BB have been eliminated by
|
|
// FoldReturnIntoUncondBranch, delete it. It is important to empty it,
|
|
// because the ret instruction in there is still using a value which
|
|
// EliminateRecursiveTailCall will attempt to remove.
|
|
if (!BB->hasAddressTaken() && pred_begin(BB) == pred_end(BB))
|
|
BB->eraseFromParent();
|
|
|
|
EliminateRecursiveTailCall(CI, RI, OldEntry, TailCallsAreMarkedTail,
|
|
ArgumentPHIs,
|
|
CannotTailCallElimCallsMarkedTail);
|
|
++NumRetDuped;
|
|
Change = true;
|
|
}
|
|
}
|
|
|
|
return Change;
|
|
}
|
|
|
|
bool
|
|
TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
|
|
bool &TailCallsAreMarkedTail,
|
|
SmallVectorImpl<PHINode *> &ArgumentPHIs,
|
|
bool CannotTailCallElimCallsMarkedTail) {
|
|
CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail);
|
|
if (!CI)
|
|
return false;
|
|
|
|
return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
|
|
ArgumentPHIs,
|
|
CannotTailCallElimCallsMarkedTail);
|
|
}
|