llvm/lib/Transforms/IPO/Attributor.cpp

//===- Attributor.cpp - Module-wide attribute deduction -------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements an inter procedural pass that deduces and/or propagating
// attributes. This is done in an abstract interpretation style fixpoint
// iteration. See the Attributor.h file comment and the class descriptions in
// that file for more information.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/IPO/Attributor.h"

#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>

using namespace llvm;

#define DEBUG_TYPE "attributor"

STATISTIC(NumFnWithExactDefinition,
          "Number of function with exact definitions");
STATISTIC(NumFnWithoutExactDefinition,
          "Number of function without exact definitions");
STATISTIC(NumAttributesTimedOut,
          "Number of abstract attributes timed out before fixpoint");
STATISTIC(NumAttributesValidFixpoint,
          "Number of abstract attributes in a valid fixpoint state");
STATISTIC(NumAttributesManifested,
          "Number of abstract attributes manifested in IR");

// TODO: Determine a good default value.
//
// In the LLVM-TS and SPEC2006, 32 seems to not induce compile time overheads
// (when run with the first 5 abstract attributes). The results also indicate
// that we never reach 32 iterations but always find a fixpoint sooner.
//
// This will become more evolved once we perform two interleaved fixpoint
// iterations: bottom-up and top-down.
static cl::opt<unsigned>
    MaxFixpointIterations("attributor-max-iterations", cl::Hidden,
                          cl::desc("Maximal number of fixpoint iterations."),
                          cl::init(32));

static cl::opt<bool> DisableAttributor(
    "attributor-disable", cl::Hidden,
    cl::desc("Disable the attributor inter-procedural deduction pass."),
    cl::init(false));

static cl::opt<bool> VerifyAttributor(
    "attributor-verify", cl::Hidden,
    cl::desc("Verify the Attributor deduction and "
             "manifestation of attributes -- may issue false-positive errors"),
    cl::init(false));

/// Logic operators for the change status enum class.
///
///{
ChangeStatus llvm::operator|(ChangeStatus l, ChangeStatus r) {
  return l == ChangeStatus::CHANGED ? l : r;
}
ChangeStatus llvm::operator&(ChangeStatus l, ChangeStatus r) {
  return l == ChangeStatus::UNCHANGED ? l : r;
}
///}

/// Helper to adjust the statistics.
static void bookkeeping(AbstractAttribute::ManifestPosition MP,
                        const Attribute &Attr) {
  if (!AreStatisticsEnabled())
    return;

  if (!Attr.isEnumAttribute())
    return;
  //switch (Attr.getKindAsEnum()) {
  //default:
  //  return;
  //}
}

/// Helper to identify the correct offset into an attribute list.
static unsigned getAttrIndex(AbstractAttribute::ManifestPosition MP,
                             unsigned ArgNo = 0) {
  switch (MP) {
  case AbstractAttribute::MP_ARGUMENT:
  case AbstractAttribute::MP_CALL_SITE_ARGUMENT:
    return ArgNo + AttributeList::FirstArgIndex;
  case AbstractAttribute::MP_FUNCTION:
    return AttributeList::FunctionIndex;
  case AbstractAttribute::MP_RETURNED:
    return AttributeList::ReturnIndex;
  }
  llvm_unreachable("Unknown manifest position!");
}

/// Return true if \p New is equal or worse than \p Old.
static bool isEqualOrWorse(const Attribute &New, const Attribute &Old) {
  if (!Old.isIntAttribute())
    return true;

  return Old.getValueAsInt() >= New.getValueAsInt();
}

/// Return true if the information provided by \p Attr was added to the
/// attribute list \p Attrs. This is only the case if it was not already present
/// in \p Attrs at the position describe by \p MP and \p ArgNo.
static bool addIfNotExistent(LLVMContext &Ctx, const Attribute &Attr,
                             AttributeList &Attrs,
                             AbstractAttribute::ManifestPosition MP,
                             unsigned ArgNo = 0) {
  unsigned AttrIdx = getAttrIndex(MP, ArgNo);

  if (Attr.isEnumAttribute()) {
    Attribute::AttrKind Kind = Attr.getKindAsEnum();
    if (Attrs.hasAttribute(AttrIdx, Kind))
      if (isEqualOrWorse(Attr, Attrs.getAttribute(AttrIdx, Kind)))
        return false;
    Attrs = Attrs.addAttribute(Ctx, AttrIdx, Attr);
    return true;
  }
  if (Attr.isStringAttribute()) {
    StringRef Kind = Attr.getKindAsString();
    if (Attrs.hasAttribute(AttrIdx, Kind))
      if (isEqualOrWorse(Attr, Attrs.getAttribute(AttrIdx, Kind)))
        return false;
    Attrs = Attrs.addAttribute(Ctx, AttrIdx, Attr);
    return true;
  }

  llvm_unreachable("Expected enum or string attribute!");
}

ChangeStatus AbstractAttribute::update(Attributor &A) {
  ChangeStatus HasChanged = ChangeStatus::UNCHANGED;
  if (getState().isAtFixpoint())
    return HasChanged;

  LLVM_DEBUG(dbgs() << "[Attributor] Update: " << *this << "\n");

  HasChanged = updateImpl(A);

  LLVM_DEBUG(dbgs() << "[Attributor] Update " << HasChanged << " " << *this
                    << "\n");

  return HasChanged;
}

ChangeStatus AbstractAttribute::manifest(Attributor &A) {
  assert(getState().isValidState() &&
         "Attempted to manifest an invalid state!");
  assert(getAssociatedValue() &&
         "Attempted to manifest an attribute without associated value!");

  ChangeStatus HasChanged = ChangeStatus::UNCHANGED;
  SmallVector<Attribute, 4> DeducedAttrs;
  getDeducedAttributes(DeducedAttrs);

  Function &ScopeFn = getAnchorScope();
  LLVMContext &Ctx = ScopeFn.getContext();
  ManifestPosition MP = getManifestPosition();

  AttributeList Attrs;
  SmallVector<unsigned, 4> ArgNos;

  // In the following some generic code that will manifest attributes in
  // DeducedAttrs if they improve the current IR. Due to the different
  // annotation positions we use the underlying AttributeList interface.
  // Note that MP_CALL_SITE_ARGUMENT can annotate multiple locations.

  switch (MP) {
  case MP_ARGUMENT:
    ArgNos.push_back(cast<Argument>(getAssociatedValue())->getArgNo());
    Attrs = ScopeFn.getAttributes();
    break;
  case MP_FUNCTION:
  case MP_RETURNED:
    ArgNos.push_back(0);
    Attrs = ScopeFn.getAttributes();
    break;
  case MP_CALL_SITE_ARGUMENT: {
    CallSite CS(&getAnchoredValue());
    for (unsigned u = 0, e = CS.getNumArgOperands(); u != e; u++)
      if (CS.getArgOperand(u) == getAssociatedValue())
        ArgNos.push_back(u);
    Attrs = CS.getAttributes();
  }
  }

  for (const Attribute &Attr : DeducedAttrs) {
    for (unsigned ArgNo : ArgNos) {
      if (!addIfNotExistent(Ctx, Attr, Attrs, MP, ArgNo))
        continue;

      HasChanged = ChangeStatus::CHANGED;
      bookkeeping(MP, Attr);
    }
  }

  if (HasChanged == ChangeStatus::UNCHANGED)
    return HasChanged;

  switch (MP) {
  case MP_ARGUMENT:
  case MP_FUNCTION:
  case MP_RETURNED:
    ScopeFn.setAttributes(Attrs);
    break;
  case MP_CALL_SITE_ARGUMENT:
    CallSite(&getAnchoredValue()).setAttributes(Attrs);
  }

  return HasChanged;
}

Function &AbstractAttribute::getAnchorScope() {
  Value &V = getAnchoredValue();
  if (isa<Function>(V))
    return cast<Function>(V);
  if (isa<Argument>(V))
    return *cast<Argument>(V).getParent();
  if (isa<Instruction>(V))
    return *cast<Instruction>(V).getFunction();
  llvm_unreachable("No scope for anchored value found!");
}

const Function &AbstractAttribute::getAnchorScope() const {
  return const_cast<AbstractAttribute *>(this)->getAnchorScope();
}

/// ----------------------------------------------------------------------------
///                               Attributor
/// ----------------------------------------------------------------------------

ChangeStatus Attributor::run() {
  // Initialize all abstract attributes.
  for (AbstractAttribute *AA : AllAbstractAttributes)
    AA->initialize(*this);

  LLVM_DEBUG(dbgs() << "[Attributor] Identified and initialized "
                    << AllAbstractAttributes.size()
                    << " abstract attributes.\n");

  // Now that all abstract attributes are collected and initialized we start the
  // abstract analysis.

  unsigned IterationCounter = 1;

  SmallVector<AbstractAttribute *, 64> ChangedAAs;
  SetVector<AbstractAttribute *> Worklist;
  Worklist.insert(AllAbstractAttributes.begin(), AllAbstractAttributes.end());

  do {
    LLVM_DEBUG(dbgs() << "\n\n[Attributor] #Iteration: " << IterationCounter
                      << ", Worklist size: " << Worklist.size() << "\n");

    // Add all abstract attributes that are potentially dependent on one that
    // changed to the work list.
    for (AbstractAttribute *ChangedAA : ChangedAAs) {
      auto &QuerriedAAs = QueryMap[ChangedAA];
      Worklist.insert(QuerriedAAs.begin(), QuerriedAAs.end());
    }

    // Reset the changed set.
    ChangedAAs.clear();

    // Update all abstract attribute in the work list and record the ones that
    // changed.
    for (AbstractAttribute *AA : Worklist)
      if (AA->update(*this) == ChangeStatus::CHANGED)
        ChangedAAs.push_back(AA);

    // Reset the work list and repopulate with the changed abstract attributes.
    // Note that dependent ones are added above.
    Worklist.clear();
    Worklist.insert(ChangedAAs.begin(), ChangedAAs.end());

  } while (!Worklist.empty() && ++IterationCounter < MaxFixpointIterations);

  LLVM_DEBUG(dbgs() << "\n[Attributor] Fixpoint iteration done after: "
                    << IterationCounter << "/" << MaxFixpointIterations
                    << " iterations\n");

  bool FinishedAtFixpoint = Worklist.empty();

  // Reset abstract arguments not settled in a sound fixpoint by now. This
  // happens when we stopped the fixpoint iteration early. Note that only the
  // ones marked as "changed" *and* the ones transitively depending on them
  // need to be reverted to a pessimistic state. Others might not be in a
  // fixpoint state but we can use the optimistic results for them anyway.
  SmallPtrSet<AbstractAttribute *, 32> Visited;
  for (unsigned u = 0; u < ChangedAAs.size(); u++) {
    AbstractAttribute *ChangedAA = ChangedAAs[u];
    if (!Visited.insert(ChangedAA).second)
      continue;

    AbstractState &State = ChangedAA->getState();
    if (!State.isAtFixpoint()) {
      State.indicatePessimisticFixpoint();

      NumAttributesTimedOut++;
    }

    auto &QuerriedAAs = QueryMap[ChangedAA];
    ChangedAAs.append(QuerriedAAs.begin(), QuerriedAAs.end());
  }

  LLVM_DEBUG({
    if (!Visited.empty())
      dbgs() << "\n[Attributor] Finalized " << Visited.size()
             << " abstract attributes.\n";
  });

  unsigned NumManifested = 0;
  unsigned NumAtFixpoint = 0;
  ChangeStatus ManifestChange = ChangeStatus::UNCHANGED;
  for (AbstractAttribute *AA : AllAbstractAttributes) {
    AbstractState &State = AA->getState();

    // If there is not already a fixpoint reached, we can now take the
    // optimistic state. This is correct because we enforced a pessimistic one
    // on abstract attributes that were transitively dependent on a changed one
    // already above.
    if (!State.isAtFixpoint())
      State.indicateOptimisticFixpoint();

    // If the state is invalid, we do not try to manifest it.
    if (!State.isValidState())
      continue;

    // Manifest the state and record if we changed the IR.
    ChangeStatus LocalChange = AA->manifest(*this);
    ManifestChange = ManifestChange | LocalChange;

    NumAtFixpoint++;
    NumManifested += (LocalChange == ChangeStatus::CHANGED);
  }

  (void)NumManifested;
  (void)NumAtFixpoint;
  LLVM_DEBUG(dbgs() << "\n[Attributor] Manifested " << NumManifested
                    << " arguments while " << NumAtFixpoint
                    << " were in a valid fixpoint state\n");

  // If verification is requested, we finished this run at a fixpoint, and the
  // IR was changed, we re-run the whole fixpoint analysis, starting at
  // re-initialization of the arguments. This re-run should not result in an IR
  // change. Though, the (virtual) state of attributes at the end of the re-run
  // might be more optimistic than the known state or the IR state if the better
  // state cannot be manifested.
  if (VerifyAttributor && FinishedAtFixpoint &&
      ManifestChange == ChangeStatus::CHANGED) {
    VerifyAttributor = false;
    ChangeStatus VerifyStatus = run();
    if (VerifyStatus != ChangeStatus::UNCHANGED)
      llvm_unreachable(
          "Attributor verification failed, re-run did result in an IR change "
          "even after a fixpoint was reached in the original run. (False "
          "positives possible!)");
    VerifyAttributor = true;
  }

  NumAttributesManifested += NumManifested;
  NumAttributesValidFixpoint += NumAtFixpoint;

  return ManifestChange;
}

void Attributor::identifyDefaultAbstractAttributes(
    Function &F, InformationCache &InfoCache,
    DenseSet</* Attribute::AttrKind */ unsigned> *Whitelist) {

  // Walk all instructions to find more attribute opportunities and also
  // interesting instructions that might be queried by abstract attributes
  // during their initialization or update.
  auto &ReadOrWriteInsts = InfoCache.FuncRWInstsMap[&F];
  auto &InstOpcodeMap = InfoCache.FuncInstOpcodeMap[&F];

  for (Instruction &I : instructions(&F)) {
    bool IsInterestingOpcode = false;

    // To allow easy access to all instructions in a function with a given
    // opcode we store them in the InfoCache. As not all opcodes are interesting
    // to concrete attributes we only cache the ones that are as identified in
    // the following switch.
    // Note: There are no concrete attributes now so this is initially empty.
    //switch (I.getOpcode()) {
    //default:
    //  break;
    //}
    if (IsInterestingOpcode)
      InstOpcodeMap[I.getOpcode()].push_back(&I);
    if (I.mayReadOrWriteMemory())
      ReadOrWriteInsts.push_back(&I);
  }
}

/// Helpers to ease debugging through output streams and print calls.
///
///{
raw_ostream &llvm::operator<<(raw_ostream &OS, ChangeStatus S) {
  return OS << (S == ChangeStatus::CHANGED ? "changed" : "unchanged");
}

raw_ostream &llvm::operator<<(raw_ostream &OS,
                              AbstractAttribute::ManifestPosition AP) {
  switch (AP) {
  case AbstractAttribute::MP_ARGUMENT:
    return OS << "arg";
  case AbstractAttribute::MP_CALL_SITE_ARGUMENT:
    return OS << "cs_arg";
  case AbstractAttribute::MP_FUNCTION:
    return OS << "fn";
  case AbstractAttribute::MP_RETURNED:
    return OS << "fn_ret";
  }
  llvm_unreachable("Unknown attribute position!");
}

raw_ostream &llvm::operator<<(raw_ostream &OS, const AbstractState &S) {
  return OS << (!S.isValidState() ? "top" : (S.isAtFixpoint() ? "fix" : ""));
}

raw_ostream &llvm::operator<<(raw_ostream &OS, const AbstractAttribute &AA) {
  AA.print(OS);
  return OS;
}

void AbstractAttribute::print(raw_ostream &OS) const {
  OS << "[" << getManifestPosition() << "][" << getAsStr() << "]["
     << AnchoredVal.getName() << "]";
}
///}

/// ----------------------------------------------------------------------------
///                       Pass (Manager) Boilerplate
/// ----------------------------------------------------------------------------

static bool runAttributorOnModule(Module &M) {
  if (DisableAttributor)
    return false;

  LLVM_DEBUG(dbgs() << "[Attributor] Run on module with " << M.size()
                    << " functions.\n");

  // Create an Attributor and initially empty information cache that is filled
  // while we identify default attribute opportunities.
  Attributor A;
  InformationCache InfoCache;

  for (Function &F : M) {
    // TODO: Not all attributes require an exact definition. Find a way to
    //       enable deduction for some but not all attributes in case the
    //       definition might be changed at runtime, see also
    //       http://lists.llvm.org/pipermail/llvm-dev/2018-February/121275.html.
    // TODO: We could always determine abstract attributes and if sufficient
    //       information was found we could duplicate the functions that do not
    //       have an exact definition.
    if (!F.hasExactDefinition()) {
      NumFnWithoutExactDefinition++;
      continue;
    }

    // For now we ignore naked and optnone functions.
    if (F.hasFnAttribute(Attribute::Naked) ||
        F.hasFnAttribute(Attribute::OptimizeNone))
      continue;

    NumFnWithExactDefinition++;

    // Populate the Attributor with abstract attribute opportunities in the
    // function and the information cache with IR information.
    A.identifyDefaultAbstractAttributes(F, InfoCache);
  }

  return A.run() == ChangeStatus::CHANGED;
}

PreservedAnalyses AttributorPass::run(Module &M, ModuleAnalysisManager &AM) {
  if (runAttributorOnModule(M)) {
    // FIXME: Think about passes we will preserve and add them here.
    return PreservedAnalyses::none();
  }
  return PreservedAnalyses::all();
}

namespace {

struct AttributorLegacyPass : public ModulePass {
  static char ID;

  AttributorLegacyPass() : ModulePass(ID) {
    initializeAttributorLegacyPassPass(*PassRegistry::getPassRegistry());
  }

  bool runOnModule(Module &M) override {
    if (skipModule(M))
      return false;
    return runAttributorOnModule(M);
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    // FIXME: Think about passes we will preserve and add them here.
    AU.setPreservesCFG();
  }
};

} // end anonymous namespace

Pass *llvm::createAttributorLegacyPass() { return new AttributorLegacyPass(); }

char AttributorLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(AttributorLegacyPass, "attributor",
                      "Deduce and propagate attributes", false, false)
INITIALIZE_PASS_END(AttributorLegacyPass, "attributor",
                    "Deduce and propagate attributes", false, false)