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llvm-project/llvm/lib/Analysis/MustExecute.cpp

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//===- MustExecute.cpp - Printer for isGuaranteedToExecute ----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/MustExecute.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/AssemblyAnnotationWriter.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/FormattedStream.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "must-execute"
const DenseMap<BasicBlock *, ColorVector> &
LoopSafetyInfo::getBlockColors() const {
return BlockColors;
}
void LoopSafetyInfo::copyColors(BasicBlock *New, BasicBlock *Old) {
ColorVector &ColorsForNewBlock = BlockColors[New];
ColorVector &ColorsForOldBlock = BlockColors[Old];
ColorsForNewBlock = ColorsForOldBlock;
}
bool SimpleLoopSafetyInfo::blockMayThrow(const BasicBlock *BB) const {
(void)BB;
return anyBlockMayThrow();
}
bool SimpleLoopSafetyInfo::anyBlockMayThrow() const {
return MayThrow;
}
void SimpleLoopSafetyInfo::computeLoopSafetyInfo(const Loop *CurLoop) {
assert(CurLoop != nullptr && "CurLoop can't be null");
BasicBlock *Header = CurLoop->getHeader();
// Iterate over header and compute safety info.
HeaderMayThrow = !isGuaranteedToTransferExecutionToSuccessor(Header);
MayThrow = HeaderMayThrow;
// Iterate over loop instructions and compute safety info.
// Skip header as it has been computed and stored in HeaderMayThrow.
// The first block in loopinfo.Blocks is guaranteed to be the header.
assert(Header == *CurLoop->getBlocks().begin() &&
"First block must be header");
for (Loop::block_iterator BB = std::next(CurLoop->block_begin()),
BBE = CurLoop->block_end();
(BB != BBE) && !MayThrow; ++BB)
MayThrow |= !isGuaranteedToTransferExecutionToSuccessor(*BB);
computeBlockColors(CurLoop);
}
bool ICFLoopSafetyInfo::blockMayThrow(const BasicBlock *BB) const {
return ICF.hasICF(BB);
}
bool ICFLoopSafetyInfo::anyBlockMayThrow() const {
return MayThrow;
}
void ICFLoopSafetyInfo::computeLoopSafetyInfo(const Loop *CurLoop) {
assert(CurLoop != nullptr && "CurLoop can't be null");
ICF.clear();
MW.clear();
MayThrow = false;
// Figure out the fact that at least one block may throw.
for (auto &BB : CurLoop->blocks())
if (ICF.hasICF(&*BB)) {
MayThrow = true;
break;
}
computeBlockColors(CurLoop);
}
void ICFLoopSafetyInfo::insertInstructionTo(const Instruction *Inst,
const BasicBlock *BB) {
ICF.insertInstructionTo(Inst, BB);
MW.insertInstructionTo(Inst, BB);
}
void ICFLoopSafetyInfo::removeInstruction(const Instruction *Inst) {
ICF.removeInstruction(Inst);
MW.removeInstruction(Inst);
}
void LoopSafetyInfo::computeBlockColors(const Loop *CurLoop) {
// Compute funclet colors if we might sink/hoist in a function with a funclet
// personality routine.
Function *Fn = CurLoop->getHeader()->getParent();
if (Fn->hasPersonalityFn())
if (Constant *PersonalityFn = Fn->getPersonalityFn())
if (isScopedEHPersonality(classifyEHPersonality(PersonalityFn)))
BlockColors = colorEHFunclets(*Fn);
}
/// Return true if we can prove that the given ExitBlock is not reached on the
/// first iteration of the given loop. That is, the backedge of the loop must
/// be executed before the ExitBlock is executed in any dynamic execution trace.
static bool CanProveNotTakenFirstIteration(const BasicBlock *ExitBlock,
const DominatorTree *DT,
const Loop *CurLoop) {
auto *CondExitBlock = ExitBlock->getSinglePredecessor();
if (!CondExitBlock)
// expect unique exits
return false;
assert(CurLoop->contains(CondExitBlock) && "meaning of exit block");
auto *BI = dyn_cast<BranchInst>(CondExitBlock->getTerminator());
if (!BI || !BI->isConditional())
return false;
// If condition is constant and false leads to ExitBlock then we always
// execute the true branch.
if (auto *Cond = dyn_cast<ConstantInt>(BI->getCondition()))
return BI->getSuccessor(Cond->getZExtValue() ? 1 : 0) == ExitBlock;
auto *Cond = dyn_cast<CmpInst>(BI->getCondition());
if (!Cond)
return false;
// todo: this would be a lot more powerful if we used scev, but all the
// plumbing is currently missing to pass a pointer in from the pass
// Check for cmp (phi [x, preheader] ...), y where (pred x, y is known
auto *LHS = dyn_cast<PHINode>(Cond->getOperand(0));
auto *RHS = Cond->getOperand(1);
if (!LHS || LHS->getParent() != CurLoop->getHeader())
return false;
auto DL = ExitBlock->getModule()->getDataLayout();
auto *IVStart = LHS->getIncomingValueForBlock(CurLoop->getLoopPreheader());
auto *SimpleValOrNull = SimplifyCmpInst(Cond->getPredicate(),
IVStart, RHS,
{DL, /*TLI*/ nullptr,
DT, /*AC*/ nullptr, BI});
auto *SimpleCst = dyn_cast_or_null<Constant>(SimpleValOrNull);
if (!SimpleCst)
return false;
if (ExitBlock == BI->getSuccessor(0))
return SimpleCst->isZeroValue();
assert(ExitBlock == BI->getSuccessor(1) && "implied by above");
return SimpleCst->isAllOnesValue();
}
/// Collect all blocks from \p CurLoop which lie on all possible paths from
/// the header of \p CurLoop (inclusive) to BB (exclusive) into the set
/// \p Predecessors. If \p BB is the header, \p Predecessors will be empty.
static void collectTransitivePredecessors(
const Loop *CurLoop, const BasicBlock *BB,
SmallPtrSetImpl<const BasicBlock *> &Predecessors) {
assert(Predecessors.empty() && "Garbage in predecessors set?");
assert(CurLoop->contains(BB) && "Should only be called for loop blocks!");
if (BB == CurLoop->getHeader())
return;
SmallVector<const BasicBlock *, 4> WorkList;
for (auto *Pred : predecessors(BB)) {
Predecessors.insert(Pred);
WorkList.push_back(Pred);
}
while (!WorkList.empty()) {
auto *Pred = WorkList.pop_back_val();
assert(CurLoop->contains(Pred) && "Should only reach loop blocks!");
// We are not interested in backedges and we don't want to leave loop.
if (Pred == CurLoop->getHeader())
continue;
// TODO: If BB lies in an inner loop of CurLoop, this will traverse over all
// blocks of this inner loop, even those that are always executed AFTER the
// BB. It may make our analysis more conservative than it could be, see test
// @nested and @nested_no_throw in test/Analysis/MustExecute/loop-header.ll.
// We can ignore backedge of all loops containing BB to get a sligtly more
// optimistic result.
for (auto *PredPred : predecessors(Pred))
if (Predecessors.insert(PredPred).second)
WorkList.push_back(PredPred);
}
}
bool LoopSafetyInfo::allLoopPathsLeadToBlock(const Loop *CurLoop,
const BasicBlock *BB,
const DominatorTree *DT) const {
assert(CurLoop->contains(BB) && "Should only be called for loop blocks!");
// Fast path: header is always reached once the loop is entered.
if (BB == CurLoop->getHeader())
return true;
// Collect all transitive predecessors of BB in the same loop. This set will
// be a subset of the blocks within the loop.
SmallPtrSet<const BasicBlock *, 4> Predecessors;
collectTransitivePredecessors(CurLoop, BB, Predecessors);
// Make sure that all successors of, all predecessors of BB which are not
// dominated by BB, are either:
// 1) BB,
// 2) Also predecessors of BB,
// 3) Exit blocks which are not taken on 1st iteration.
// Memoize blocks we've already checked.
SmallPtrSet<const BasicBlock *, 4> CheckedSuccessors;
for (auto *Pred : Predecessors) {
// Predecessor block may throw, so it has a side exit.
if (blockMayThrow(Pred))
return false;
// BB dominates Pred, so if Pred runs, BB must run.
// This is true when Pred is a loop latch.
if (DT->dominates(BB, Pred))
continue;
for (auto *Succ : successors(Pred))
if (CheckedSuccessors.insert(Succ).second &&
Succ != BB && !Predecessors.count(Succ))
// By discharging conditions that are not executed on the 1st iteration,
// we guarantee that *at least* on the first iteration all paths from
// header that *may* execute will lead us to the block of interest. So
// that if we had virtually peeled one iteration away, in this peeled
// iteration the set of predecessors would contain only paths from
// header to BB without any exiting edges that may execute.
//
// TODO: We only do it for exiting edges currently. We could use the
// same function to skip some of the edges within the loop if we know
// that they will not be taken on the 1st iteration.
//
// TODO: If we somehow know the number of iterations in loop, the same
// check may be done for any arbitrary N-th iteration as long as N is
// not greater than minimum number of iterations in this loop.
if (CurLoop->contains(Succ) ||
!CanProveNotTakenFirstIteration(Succ, DT, CurLoop))
return false;
}
// All predecessors can only lead us to BB.
return true;
}
/// Returns true if the instruction in a loop is guaranteed to execute at least
/// once.
bool SimpleLoopSafetyInfo::isGuaranteedToExecute(const Instruction &Inst,
const DominatorTree *DT,
const Loop *CurLoop) const {
// If the instruction is in the header block for the loop (which is very
// common), it is always guaranteed to dominate the exit blocks. Since this
// is a common case, and can save some work, check it now.
if (Inst.getParent() == CurLoop->getHeader())
// If there's a throw in the header block, we can't guarantee we'll reach
// Inst unless we can prove that Inst comes before the potential implicit
// exit. At the moment, we use a (cheap) hack for the common case where
// the instruction of interest is the first one in the block.
return !HeaderMayThrow ||
Inst.getParent()->getFirstNonPHIOrDbg() == &Inst;
// If there is a path from header to exit or latch that doesn't lead to our
// instruction's block, return false.
return allLoopPathsLeadToBlock(CurLoop, Inst.getParent(), DT);
}
bool ICFLoopSafetyInfo::isGuaranteedToExecute(const Instruction &Inst,
const DominatorTree *DT,
const Loop *CurLoop) const {
return !ICF.isDominatedByICFIFromSameBlock(&Inst) &&
allLoopPathsLeadToBlock(CurLoop, Inst.getParent(), DT);
}
bool ICFLoopSafetyInfo::doesNotWriteMemoryBefore(const BasicBlock *BB,
const Loop *CurLoop) const {
assert(CurLoop->contains(BB) && "Should only be called for loop blocks!");
// Fast path: there are no instructions before header.
if (BB == CurLoop->getHeader())
return true;
// Collect all transitive predecessors of BB in the same loop. This set will
// be a subset of the blocks within the loop.
SmallPtrSet<const BasicBlock *, 4> Predecessors;
collectTransitivePredecessors(CurLoop, BB, Predecessors);
// Find if there any instruction in either predecessor that could write
// to memory.
for (auto *Pred : Predecessors)
if (MW.mayWriteToMemory(Pred))
return false;
return true;
}
bool ICFLoopSafetyInfo::doesNotWriteMemoryBefore(const Instruction &I,
const Loop *CurLoop) const {
auto *BB = I.getParent();
assert(CurLoop->contains(BB) && "Should only be called for loop blocks!");
return !MW.isDominatedByMemoryWriteFromSameBlock(&I) &&
doesNotWriteMemoryBefore(BB, CurLoop);
}
namespace {
struct MustExecutePrinter : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
MustExecutePrinter() : FunctionPass(ID) {
initializeMustExecutePrinterPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
}
bool runOnFunction(Function &F) override;
};
struct MustBeExecutedContextPrinter : public ModulePass {
static char ID;
MustBeExecutedContextPrinter() : ModulePass(ID) {
initializeMustBeExecutedContextPrinterPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
}
bool runOnModule(Module &M) override;
};
}
char MustExecutePrinter::ID = 0;
INITIALIZE_PASS_BEGIN(MustExecutePrinter, "print-mustexecute",
"Instructions which execute on loop entry", false, true)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(MustExecutePrinter, "print-mustexecute",
"Instructions which execute on loop entry", false, true)
FunctionPass *llvm::createMustExecutePrinter() {
return new MustExecutePrinter();
}
char MustBeExecutedContextPrinter::ID = 0;
INITIALIZE_PASS_BEGIN(
MustBeExecutedContextPrinter, "print-must-be-executed-contexts",
"print the must-be-executed-contexed for all instructions", false, true)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(MustBeExecutedContextPrinter,
"print-must-be-executed-contexts",
"print the must-be-executed-contexed for all instructions",
false, true)
ModulePass *llvm::createMustBeExecutedContextPrinter() {
return new MustBeExecutedContextPrinter();
}
bool MustBeExecutedContextPrinter::runOnModule(Module &M) {
// We provide non-PM analysis here because the old PM doesn't like to query
// function passes from a module pass.
SmallVector<PostDominatorTree *, 8> PDTs;
SmallVector<DominatorTree *, 8> DTs;
SmallVector<LoopInfo *, 8> LIs;
GetterTy<LoopInfo> LIGetter = [&](const Function &F) {
DominatorTree *DT = new DominatorTree(const_cast<Function &>(F));
LoopInfo *LI = new LoopInfo(*DT);
DTs.push_back(DT);
LIs.push_back(LI);
return LI;
};
GetterTy<DominatorTree> DTGetter = [&](const Function &F) {
DominatorTree *DT = new DominatorTree(const_cast<Function &>(F));
DTs.push_back(DT);
return DT;
};
GetterTy<PostDominatorTree> PDTGetter = [&](const Function &F) {
PostDominatorTree *PDT = new PostDominatorTree(const_cast<Function &>(F));
PDTs.push_back(PDT);
return PDT;
};
MustBeExecutedContextExplorer Explorer(
/* ExploreInterBlock */ true,
/* ExploreCFGForward */ true,
/* ExploreCFGBackward */ true, LIGetter, DTGetter, PDTGetter);
for (Function &F : M) {
for (Instruction &I : instructions(F)) {
dbgs() << "-- Explore context of: " << I << "\n";
for (const Instruction *CI : Explorer.range(&I))
dbgs() << " [F: " << CI->getFunction()->getName() << "] " << *CI
<< "\n";
}
}
DeleteContainerPointers(PDTs);
DeleteContainerPointers(LIs);
DeleteContainerPointers(DTs);
return false;
}
static bool isMustExecuteIn(const Instruction &I, Loop *L, DominatorTree *DT) {
// TODO: merge these two routines. For the moment, we display the best
// result obtained by *either* implementation. This is a bit unfair since no
// caller actually gets the full power at the moment.
SimpleLoopSafetyInfo LSI;
LSI.computeLoopSafetyInfo(L);
return LSI.isGuaranteedToExecute(I, DT, L) ||
isGuaranteedToExecuteForEveryIteration(&I, L);
}
namespace {
/// An assembly annotator class to print must execute information in
/// comments.
class MustExecuteAnnotatedWriter : public AssemblyAnnotationWriter {
DenseMap<const Value*, SmallVector<Loop*, 4> > MustExec;
public:
MustExecuteAnnotatedWriter(const Function &F,
DominatorTree &DT, LoopInfo &LI) {
for (auto &I: instructions(F)) {
Loop *L = LI.getLoopFor(I.getParent());
while (L) {
if (isMustExecuteIn(I, L, &DT)) {
MustExec[&I].push_back(L);
}
L = L->getParentLoop();
};
}
}
MustExecuteAnnotatedWriter(const Module &M,
DominatorTree &DT, LoopInfo &LI) {
for (auto &F : M)
for (auto &I: instructions(F)) {
Loop *L = LI.getLoopFor(I.getParent());
while (L) {
if (isMustExecuteIn(I, L, &DT)) {
MustExec[&I].push_back(L);
}
L = L->getParentLoop();
};
}
}
void printInfoComment(const Value &V, formatted_raw_ostream &OS) override {
if (!MustExec.count(&V))
return;
const auto &Loops = MustExec.lookup(&V);
const auto NumLoops = Loops.size();
if (NumLoops > 1)
OS << " ; (mustexec in " << NumLoops << " loops: ";
else
OS << " ; (mustexec in: ";
bool first = true;
for (const Loop *L : Loops) {
if (!first)
OS << ", ";
first = false;
OS << L->getHeader()->getName();
}
OS << ")";
}
};
} // namespace
bool MustExecutePrinter::runOnFunction(Function &F) {
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
MustExecuteAnnotatedWriter Writer(F, DT, LI);
F.print(dbgs(), &Writer);
return false;
}
/// Return true if \p L might be an endless loop.
static bool maybeEndlessLoop(const Loop &L) {
if (L.getHeader()->getParent()->hasFnAttribute(Attribute::WillReturn))
return false;
// TODO: Actually try to prove it is not.
// TODO: If maybeEndlessLoop is going to be expensive, cache it.
return true;
}
static bool mayContainIrreducibleControl(const Function &F, const LoopInfo *LI) {
if (!LI)
return false;
using RPOTraversal = ReversePostOrderTraversal<const Function *>;
RPOTraversal FuncRPOT(&F);
return !containsIrreducibleCFG<const BasicBlock *, const RPOTraversal,
const LoopInfo>(FuncRPOT, *LI);
}
/// Lookup \p Key in \p Map and return the result, potentially after
/// initializing the optional through \p Fn(\p args).
template <typename K, typename V, typename FnTy, typename... ArgsTy>
static V getOrCreateCachedOptional(K Key, DenseMap<K, Optional<V>> &Map,
FnTy &&Fn, ArgsTy&&... args) {
Optional<V> &OptVal = Map[Key];
if (!OptVal.hasValue())
OptVal = Fn(std::forward<ArgsTy>(args)...);
return OptVal.getValue();
}
const BasicBlock *
MustBeExecutedContextExplorer::findForwardJoinPoint(const BasicBlock *InitBB) {
const LoopInfo *LI = LIGetter(*InitBB->getParent());
const PostDominatorTree *PDT = PDTGetter(*InitBB->getParent());
LLVM_DEBUG(dbgs() << "\tFind forward join point for " << InitBB->getName()
<< (LI ? " [LI]" : "") << (PDT ? " [PDT]" : ""));
const Function &F = *InitBB->getParent();
const Loop *L = LI ? LI->getLoopFor(InitBB) : nullptr;
const BasicBlock *HeaderBB = L ? L->getHeader() : InitBB;
bool WillReturnAndNoThrow = (F.hasFnAttribute(Attribute::WillReturn) ||
(L && !maybeEndlessLoop(*L))) &&
F.doesNotThrow();
LLVM_DEBUG(dbgs() << (L ? " [in loop]" : "")
<< (WillReturnAndNoThrow ? " [WillReturn] [NoUnwind]" : "")
<< "\n");
// Determine the adjacent blocks in the given direction but exclude (self)
// loops under certain circumstances.
SmallVector<const BasicBlock *, 8> Worklist;
for (const BasicBlock *SuccBB : successors(InitBB)) {
bool IsLatch = SuccBB == HeaderBB;
// Loop latches are ignored in forward propagation if the loop cannot be
// endless and may not throw: control has to go somewhere.
if (!WillReturnAndNoThrow || !IsLatch)
Worklist.push_back(SuccBB);
}
LLVM_DEBUG(dbgs() << "\t\t#Worklist: " << Worklist.size() << "\n");
// If there are no other adjacent blocks, there is no join point.
if (Worklist.empty())
return nullptr;
// If there is one adjacent block, it is the join point.
if (Worklist.size() == 1)
return Worklist[0];
// Try to determine a join block through the help of the post-dominance
// tree. If no tree was provided, we perform simple pattern matching for one
// block conditionals and one block loops only.
const BasicBlock *JoinBB = nullptr;
if (PDT)
if (const auto *InitNode = PDT->getNode(InitBB))
if (const auto *IDomNode = InitNode->getIDom())
JoinBB = IDomNode->getBlock();
if (!JoinBB && Worklist.size() == 2) {
const BasicBlock *Succ0 = Worklist[0];
const BasicBlock *Succ1 = Worklist[1];
const BasicBlock *Succ0UniqueSucc = Succ0->getUniqueSuccessor();
const BasicBlock *Succ1UniqueSucc = Succ1->getUniqueSuccessor();
if (Succ0UniqueSucc == InitBB) {
// InitBB -> Succ0 -> InitBB
// InitBB -> Succ1 = JoinBB
JoinBB = Succ1;
} else if (Succ1UniqueSucc == InitBB) {
// InitBB -> Succ1 -> InitBB
// InitBB -> Succ0 = JoinBB
JoinBB = Succ0;
} else if (Succ0 == Succ1UniqueSucc) {
// InitBB -> Succ0 = JoinBB
// InitBB -> Succ1 -> Succ0 = JoinBB
JoinBB = Succ0;
} else if (Succ1 == Succ0UniqueSucc) {
// InitBB -> Succ0 -> Succ1 = JoinBB
// InitBB -> Succ1 = JoinBB
JoinBB = Succ1;
} else if (Succ0UniqueSucc == Succ1UniqueSucc) {
// InitBB -> Succ0 -> JoinBB
// InitBB -> Succ1 -> JoinBB
JoinBB = Succ0UniqueSucc;
}
}
if (!JoinBB && L)
JoinBB = L->getUniqueExitBlock();
if (!JoinBB)
return nullptr;
LLVM_DEBUG(dbgs() << "\t\tJoin block candidate: " << JoinBB->getName() << "\n");
// In forward direction we check if control will for sure reach JoinBB from
// InitBB, thus it can not be "stopped" along the way. Ways to "stop" control
// are: infinite loops and instructions that do not necessarily transfer
// execution to their successor. To check for them we traverse the CFG from
// the adjacent blocks to the JoinBB, looking at all intermediate blocks.
// If we know the function is "will-return" and "no-throw" there is no need
// for futher checks.
if (!F.hasFnAttribute(Attribute::WillReturn) || !F.doesNotThrow()) {
auto BlockTransfersExecutionToSuccessor = [](const BasicBlock *BB) {
return isGuaranteedToTransferExecutionToSuccessor(BB);
};
SmallPtrSet<const BasicBlock *, 16> Visited;
while (!Worklist.empty()) {
const BasicBlock *ToBB = Worklist.pop_back_val();
if (ToBB == JoinBB)
continue;
// Make sure all loops in-between are finite.
if (!Visited.insert(ToBB).second) {
if (!F.hasFnAttribute(Attribute::WillReturn)) {
if (!LI)
return nullptr;
bool MayContainIrreducibleControl = getOrCreateCachedOptional(
&F, IrreducibleControlMap, mayContainIrreducibleControl, F, LI);
if (MayContainIrreducibleControl)
return nullptr;
const Loop *L = LI->getLoopFor(ToBB);
if (L && maybeEndlessLoop(*L))
return nullptr;
}
continue;
}
// Make sure the block has no instructions that could stop control
// transfer.
bool TransfersExecution = getOrCreateCachedOptional(
ToBB, BlockTransferMap, BlockTransfersExecutionToSuccessor, ToBB);
if (!TransfersExecution)
return nullptr;
for (const BasicBlock *AdjacentBB : successors(ToBB))
Worklist.push_back(AdjacentBB);
}
}
LLVM_DEBUG(dbgs() << "\tJoin block: " << JoinBB->getName() << "\n");
return JoinBB;
}
const BasicBlock *
MustBeExecutedContextExplorer::findBackwardJoinPoint(const BasicBlock *InitBB) {
const LoopInfo *LI = LIGetter(*InitBB->getParent());
const DominatorTree *DT = DTGetter(*InitBB->getParent());
LLVM_DEBUG(dbgs() << "\tFind backward join point for " << InitBB->getName()
<< (LI ? " [LI]" : "") << (DT ? " [DT]" : ""));
// Try to determine a join block through the help of the dominance tree. If no
// tree was provided, we perform simple pattern matching for one block
// conditionals only.
if (DT)
if (const auto *InitNode = DT->getNode(InitBB))
if (const auto *IDomNode = InitNode->getIDom())
return IDomNode->getBlock();
const Loop *L = LI ? LI->getLoopFor(InitBB) : nullptr;
const BasicBlock *HeaderBB = L ? L->getHeader() : nullptr;
// Determine the predecessor blocks but ignore backedges.
SmallVector<const BasicBlock *, 8> Worklist;
for (const BasicBlock *PredBB : predecessors(InitBB)) {
bool IsBackedge =
(PredBB == InitBB) || (HeaderBB == InitBB && L->contains(PredBB));
// Loop backedges are ignored in backwards propagation: control has to come
// from somewhere.
if (!IsBackedge)
Worklist.push_back(PredBB);
}
// If there are no other predecessor blocks, there is no join point.
if (Worklist.empty())
return nullptr;
// If there is one predecessor block, it is the join point.
if (Worklist.size() == 1)
return Worklist[0];
const BasicBlock *JoinBB = nullptr;
if (Worklist.size() == 2) {
const BasicBlock *Pred0 = Worklist[0];
const BasicBlock *Pred1 = Worklist[1];
const BasicBlock *Pred0UniquePred = Pred0->getUniquePredecessor();
const BasicBlock *Pred1UniquePred = Pred1->getUniquePredecessor();
if (Pred0 == Pred1UniquePred) {
// InitBB <- Pred0 = JoinBB
// InitBB <- Pred1 <- Pred0 = JoinBB
JoinBB = Pred0;
} else if (Pred1 == Pred0UniquePred) {
// InitBB <- Pred0 <- Pred1 = JoinBB
// InitBB <- Pred1 = JoinBB
JoinBB = Pred1;
} else if (Pred0UniquePred == Pred1UniquePred) {
// InitBB <- Pred0 <- JoinBB
// InitBB <- Pred1 <- JoinBB
JoinBB = Pred0UniquePred;
}
}
if (!JoinBB && L)
JoinBB = L->getHeader();
// In backwards direction there is no need to show termination of previous
// instructions. If they do not terminate, the code afterward is dead, making
// any information/transformation correct anyway.
return JoinBB;
}
const Instruction *
MustBeExecutedContextExplorer::getMustBeExecutedNextInstruction(
MustBeExecutedIterator &It, const Instruction *PP) {
if (!PP)
return PP;
LLVM_DEBUG(dbgs() << "Find next instruction for " << *PP << "\n");
// If we explore only inside a given basic block we stop at terminators.
if (!ExploreInterBlock && PP->isTerminator()) {
LLVM_DEBUG(dbgs() << "\tReached terminator in intra-block mode, done\n");
return nullptr;
}
// If we do not traverse the call graph we check if we can make progress in
// the current function. First, check if the instruction is guaranteed to
// transfer execution to the successor.
bool TransfersExecution = isGuaranteedToTransferExecutionToSuccessor(PP);
if (!TransfersExecution)
return nullptr;
// If this is not a terminator we know that there is a single instruction
// after this one that is executed next if control is transfered. If not,
// we can try to go back to a call site we entered earlier. If none exists, we
// do not know any instruction that has to be executd next.
if (!PP->isTerminator()) {
const Instruction *NextPP = PP->getNextNode();
LLVM_DEBUG(dbgs() << "\tIntermediate instruction does transfer control\n");
return NextPP;
}
// Finally, we have to handle terminators, trivial ones first.
assert(PP->isTerminator() && "Expected a terminator!");
// A terminator without a successor is not handled yet.
if (PP->getNumSuccessors() == 0) {
LLVM_DEBUG(dbgs() << "\tUnhandled terminator\n");
return nullptr;
}
// A terminator with a single successor, we will continue at the beginning of
// that one.
if (PP->getNumSuccessors() == 1) {
LLVM_DEBUG(
dbgs() << "\tUnconditional terminator, continue with successor\n");
return &PP->getSuccessor(0)->front();
}
// Multiple successors mean we need to find the join point where control flow
// converges again. We use the findForwardJoinPoint helper function with
// information about the function and helper analyses, if available.
if (const BasicBlock *JoinBB = findForwardJoinPoint(PP->getParent()))
return &JoinBB->front();
LLVM_DEBUG(dbgs() << "\tNo join point found\n");
return nullptr;
}
const Instruction *
MustBeExecutedContextExplorer::getMustBeExecutedPrevInstruction(
MustBeExecutedIterator &It, const Instruction *PP) {
if (!PP)
return PP;
bool IsFirst = !(PP->getPrevNode());
LLVM_DEBUG(dbgs() << "Find next instruction for " << *PP
<< (IsFirst ? " [IsFirst]" : "") << "\n");
// If we explore only inside a given basic block we stop at the first
// instruction.
if (!ExploreInterBlock && IsFirst) {
LLVM_DEBUG(dbgs() << "\tReached block front in intra-block mode, done\n");
return nullptr;
}
// The block and function that contains the current position.
const BasicBlock *PPBlock = PP->getParent();
// If we are inside a block we know what instruction was executed before, the
// previous one.
if (!IsFirst) {
const Instruction *PrevPP = PP->getPrevNode();
LLVM_DEBUG(
dbgs() << "\tIntermediate instruction, continue with previous\n");
// We did not enter a callee so we simply return the previous instruction.
return PrevPP;
}
// Finally, we have to handle the case where the program point is the first in
// a block but not in the function. We use the findBackwardJoinPoint helper
// function with information about the function and helper analyses, if
// available.
if (const BasicBlock *JoinBB = findBackwardJoinPoint(PPBlock))
return &JoinBB->back();
LLVM_DEBUG(dbgs() << "\tNo join point found\n");
return nullptr;
}
MustBeExecutedIterator::MustBeExecutedIterator(
MustBeExecutedContextExplorer &Explorer, const Instruction *I)
: Explorer(Explorer), CurInst(I) {
reset(I);
}
void MustBeExecutedIterator::reset(const Instruction *I) {
Visited.clear();
resetInstruction(I);
}
void MustBeExecutedIterator::resetInstruction(const Instruction *I) {
CurInst = I;
Head = Tail = nullptr;
Visited.insert({I, ExplorationDirection::FORWARD});
Visited.insert({I, ExplorationDirection::BACKWARD});
if (Explorer.ExploreCFGForward)
Head = I;
if (Explorer.ExploreCFGBackward)
Tail = I;
}
const Instruction *MustBeExecutedIterator::advance() {
assert(CurInst && "Cannot advance an end iterator!");
Head = Explorer.getMustBeExecutedNextInstruction(*this, Head);
if (Head && Visited.insert({Head, ExplorationDirection ::FORWARD}).second)
return Head;
Head = nullptr;
Tail = Explorer.getMustBeExecutedPrevInstruction(*this, Tail);
if (Tail && Visited.insert({Tail, ExplorationDirection ::BACKWARD}).second)
return Tail;
Tail = nullptr;
return nullptr;
}
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