lib/Analysis/ScopDetection.cpp

//===----- ScopDetection.cpp  - Detect Scops --------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Detect the maximal Scops of a function.
//
// A static control part (Scop) is a subgraph of the control flow graph (CFG)
// that only has statically known control flow and can therefore be described
// within the polyhedral model.
//
// Every Scop fullfills these restrictions:
//
// * It is a single entry single exit region
//
// * Only affine linear bounds in the loops
//
// Every natural loop in a Scop must have a number of loop iterations that can
// be described as an affine linear function in surrounding loop iterators or
// parameters. (A parameter is a scalar that does not change its value during
// execution of the Scop).
//
// * Only comparisons of affine linear expressions in conditions
//
// * All loops and conditions perfectly nested
//
// The control flow needs to be structured such that it could be written using
// just 'for' and 'if' statements, without the need for any 'goto', 'break' or
// 'continue'.
//
// * Side effect free functions call
//
// Only function calls and intrinsics that do not have side effects are allowed
// (readnone).
//
// The Scop detection finds the largest Scops by checking if the largest
// region is a Scop. If this is not the case, its canonical subregions are
// checked until a region is a Scop. It is now tried to extend this Scop by
// creating a larger non canonical region.
//
//===----------------------------------------------------------------------===//

#include "polly/ScopDetection.h"

#include "polly/LinkAllPasses.h"
#include "polly/Support/ScopHelper.h"
#include "polly/Support/SCEVValidator.h"

#include "llvm/LLVMContext.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/RegionIterator.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Assembly/Writer.h"

#define DEBUG_TYPE "polly-detect"
#include "llvm/Support/Debug.h"

#include <set>

using namespace llvm;
using namespace polly;

static cl::opt<std::string>
OnlyFunction("polly-detect-only",
             cl::desc("Only detect scops in function"), cl::Hidden,
             cl::value_desc("The function name to detect scops in"),
             cl::ValueRequired, cl::init(""));

static cl::opt<bool>
IgnoreAliasing("polly-ignore-aliasing",
               cl::desc("Ignore possible aliasing of the array bases"),
               cl::Hidden, cl::init(false));

static cl::opt<bool>
AllowNonAffine("polly-allow-nonaffine",
               cl::desc("Allow non affine access functions in arrays"),
               cl::Hidden, cl::init(false));

//===----------------------------------------------------------------------===//
// Statistics.

STATISTIC(ValidRegion, "Number of regions that a valid part of Scop");

#define BADSCOP_STAT(NAME, DESC) STATISTIC(Bad##NAME##ForScop, \
                                           "Number of bad regions for Scop: "\
                                           DESC)

#define INVALID(NAME, MESSAGE) \
  do { \
    std::string Buf; \
    raw_string_ostream fmt(Buf); \
    fmt << MESSAGE; \
    fmt.flush(); \
    LastFailure = Buf; \
    DEBUG(dbgs() << MESSAGE); \
    DEBUG(dbgs() << "\n"); \
    assert(!Context.Verifying && #NAME); \
    if (!Context.Verifying) ++Bad##NAME##ForScop; \
    return false; \
  } while (0);


#define INVALID_NOVERIFY(NAME, MESSAGE) \
  do { \
    std::string Buf; \
    raw_string_ostream fmt(Buf); \
    fmt << MESSAGE; \
    fmt.flush(); \
    LastFailure = Buf; \
    DEBUG(dbgs() << MESSAGE); \
    DEBUG(dbgs() << "\n"); \
    /* DISABLED: assert(!Context.Verifying && #NAME); */ \
    if (!Context.Verifying) ++Bad##NAME##ForScop; \
    return false; \
  } while (0);


BADSCOP_STAT(CFG,             "CFG too complex");
BADSCOP_STAT(IndVar,          "Non canonical induction variable in loop");
BADSCOP_STAT(LoopBound,       "Loop bounds can not be computed");
BADSCOP_STAT(FuncCall,        "Function call with side effects appeared");
BADSCOP_STAT(AffFunc,         "Expression not affine");
BADSCOP_STAT(Scalar,          "Found scalar dependency");
BADSCOP_STAT(Alias,           "Found base address alias");
BADSCOP_STAT(SimpleRegion,    "Region not simple");
BADSCOP_STAT(Other,           "Others");

//===----------------------------------------------------------------------===//
// ScopDetection.
bool ScopDetection::isMaxRegionInScop(const Region &R) const {
  // The Region is valid only if it could be found in the set.
  return ValidRegions.count(&R);
}

std::string ScopDetection::regionIsInvalidBecause(const Region *R) const {
  if (!InvalidRegions.count(R))
    return "";

  return InvalidRegions.find(R)->second;
}

bool ScopDetection::isValidCFG(BasicBlock &BB, DetectionContext &Context) const
{
  Region &RefRegion = Context.CurRegion;
  TerminatorInst *TI = BB.getTerminator();

  // Return instructions are only valid if the region is the top level region.
  if (isa<ReturnInst>(TI) && !RefRegion.getExit() && TI->getNumOperands() == 0)
    return true;

  BranchInst *Br = dyn_cast<BranchInst>(TI);

  if (!Br)
    INVALID(CFG, "Non branch instruction terminates BB: " + BB.getName());

  if (Br->isUnconditional()) return true;

  Value *Condition = Br->getCondition();

  // UndefValue is not allowed as condition.
  if (isa<UndefValue>(Condition))
    INVALID(AffFunc, "Condition based on 'undef' value in BB: "
                     + BB.getName());

  // Only Constant and ICmpInst are allowed as condition.
  if (!(isa<Constant>(Condition) || isa<ICmpInst>(Condition)))
    INVALID(AffFunc, "Condition in BB '" + BB.getName() + "' neither "
                     "constant nor an icmp instruction");

  // Allow perfectly nested conditions.
  assert(Br->getNumSuccessors() == 2 && "Unexpected number of successors");

  if (ICmpInst *ICmp = dyn_cast<ICmpInst>(Condition)) {
    // Unsigned comparisons are not allowed. They trigger overflow problems
    // in the code generation.
    //
    // TODO: This is not sufficient and just hides bugs. However it does pretty
    // well.
    if(ICmp->isUnsigned())
      return false;

    // Are both operands of the ICmp affine?
    if (isa<UndefValue>(ICmp->getOperand(0))
        || isa<UndefValue>(ICmp->getOperand(1)))
      INVALID(AffFunc, "undef operand in branch at BB: " + BB.getName());

    const SCEV *LHS = SE->getSCEV(ICmp->getOperand(0));
    const SCEV *RHS = SE->getSCEV(ICmp->getOperand(1));

    if (!isAffineExpr(&Context.CurRegion, LHS, *SE) ||
        !isAffineExpr(&Context.CurRegion, RHS, *SE))
      INVALID(AffFunc, "Non affine branch in BB '" << BB.getName()
                        << "' with LHS: " << *LHS << " and RHS: " << *RHS);
  }

  // Allow loop exit conditions.
  Loop *L = LI->getLoopFor(&BB);
  if (L && L->getExitingBlock() == &BB)
    return true;

  // Allow perfectly nested conditions.
  Region *R = RI->getRegionFor(&BB);
  if (R->getEntry() != &BB)
    INVALID(CFG, "Not well structured condition at BB: " + BB.getName());

  return true;
}

bool ScopDetection::isValidCallInst(CallInst &CI) {
  if (CI.mayHaveSideEffects() || CI.doesNotReturn())
    return false;

  if (CI.doesNotAccessMemory())
    return true;

  Function *CalledFunction = CI.getCalledFunction();

  // Indirect calls are not supported.
  if (CalledFunction == 0)
    return false;

  // TODO: Intrinsics.
  return false;
}

bool ScopDetection::isValidMemoryAccess(Instruction &Inst,
                                        DetectionContext &Context) const {
  Value *Ptr = getPointerOperand(Inst);
  const SCEV *AccessFunction = SE->getSCEV(Ptr);
  const SCEVUnknown *BasePointer;
  Value *BaseValue;

  BasePointer = dyn_cast<SCEVUnknown>(SE->getPointerBase(AccessFunction));

  if (!BasePointer)
    INVALID(AffFunc, "No base pointer");

  BaseValue = BasePointer->getValue();

  if (isa<UndefValue>(BaseValue))
    INVALID(AffFunc, "Undefined base pointer");

  AccessFunction = SE->getMinusSCEV(AccessFunction, BasePointer);

  if (!isAffineExpr(&Context.CurRegion, AccessFunction, *SE, BaseValue) && !AllowNonAffine)
    INVALID(AffFunc, "Non affine access function" << *AccessFunction);

  // FIXME: Alias Analysis thinks IntToPtrInst aliases with alloca instructions
  // created by IndependentBlocks Pass.
  if (isa<IntToPtrInst>(BaseValue))
    INVALID(Other, "Find bad intToptr prt: " << *BaseValue);

  // Check if the base pointer of the memory access does alias with
  // any other pointer. This cannot be handled at the moment.
  AliasSet &AS =
    Context.AST.getAliasSetForPointer(BaseValue, AliasAnalysis::UnknownSize,
                                      Inst.getMetadata(LLVMContext::MD_tbaa));

  // INVALID triggers an assertion in verifying mode, if it detects that a SCoP
  // was detected by SCoP detection and that this SCoP was invalidated by a pass
  // that stated it would preserve the SCoPs.
  // We disable this check as the independent blocks pass may create memory
  // references which seem to alias, if -basicaa is not available. They actually
  // do not, but as we can not proof this without -basicaa we would fail. We
  // disable this check to not cause irrelevant verification failures.
  if (!AS.isMustAlias() && !IgnoreAliasing)
    INVALID_NOVERIFY(Alias,
                     "Possible aliasing for value: " << BaseValue->getName()
                     << "\n");

  return true;
}


bool ScopDetection::hasScalarDependency(Instruction &Inst,
                                        Region &RefRegion) const {
  for (Instruction::use_iterator UI = Inst.use_begin(), UE = Inst.use_end();
       UI != UE; ++UI)
    if (Instruction *Use = dyn_cast<Instruction>(*UI))
      if (!RefRegion.contains(Use->getParent())) {
        // DirtyHack 1: PHINode user outside the Scop is not allow, if this
        // PHINode is induction variable, the scalar to array transform may
        // break it and introduce a non-indvar PHINode, which is not allow in
        // Scop.
        // This can be fix by:
        // Introduce a IndependentBlockPrepare pass, which translate all
        // PHINodes not in Scop to array.
        // The IndependentBlockPrepare pass can also split the entry block of
        // the function to hold the alloca instruction created by scalar to
        // array.  and split the exit block of the Scop so the new create load
        // instruction for escape users will not break other Scops.
        if (isa<PHINode>(Use))
          return true;
      }

  return false;
}

bool ScopDetection::isValidInstruction(Instruction &Inst,
                                       DetectionContext &Context) const {
  // Only canonical IVs are allowed.
  if (PHINode *PN = dyn_cast<PHINode>(&Inst))
    if (!isIndVar(PN, LI))
      INVALID(IndVar, "Non canonical PHI node: " << Inst);

  // Scalar dependencies are not allowed.
  if (hasScalarDependency(Inst, Context.CurRegion))
    INVALID(Scalar, "Scalar dependency found: " << Inst);

  // We only check the call instruction but not invoke instruction.
  if (CallInst *CI = dyn_cast<CallInst>(&Inst)) {
    if (isValidCallInst(*CI))
      return true;

    INVALID(FuncCall, "Call instruction: " << Inst);
  }

  if (!Inst.mayWriteToMemory() && !Inst.mayReadFromMemory()) {
    if (isa<AllocaInst>(Inst))
      INVALID(Other, "Alloca instruction: " << Inst);

    return true;
  }

  // Check the access function.
  if (isa<LoadInst>(Inst) || isa<StoreInst>(Inst))
    return isValidMemoryAccess(Inst, Context);

  // We do not know this instruction, therefore we assume it is invalid.
  INVALID(Other, "Unknown instruction: " << Inst);
}

bool ScopDetection::isValidBasicBlock(BasicBlock &BB,
                                      DetectionContext &Context) const {
  if (!isValidCFG(BB, Context))
    return false;

  // Check all instructions, except the terminator instruction.
  for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I)
    if (!isValidInstruction(*I, Context))
      return false;

  Loop *L = LI->getLoopFor(&BB);
  if (L && L->getHeader() == &BB && !isValidLoop(L, Context))
    return false;

  return true;
}

bool ScopDetection::isValidLoop(Loop *L, DetectionContext &Context) const {
  PHINode *IndVar = L->getCanonicalInductionVariable();
  // No canonical induction variable.
  if (!IndVar)
    INVALID(IndVar, "No canonical IV at loop header: "
                    << L->getHeader()->getName());

  // Is the loop count affine?
  const SCEV *LoopCount = SE->getBackedgeTakenCount(L);
  if (!isAffineExpr(&Context.CurRegion, LoopCount, *SE))
    INVALID(LoopBound, "Non affine loop bound '" << *LoopCount << "' in loop: "
                       << L->getHeader()->getName());

  return true;
}

Region *ScopDetection::expandRegion(Region &R) {
  // Initial no valid region was found (greater than R)
  Region *LastValidRegion = NULL;
  Region *ExpandedRegion  = R.getExpandedRegion();

  DEBUG(dbgs() << "\tExpanding " << R.getNameStr() << "\n");

  while (ExpandedRegion) {
    DetectionContext Context(*ExpandedRegion, *AA, false /* verifying */);
    DEBUG(dbgs() << "\t\tTrying " << ExpandedRegion->getNameStr() << "\n");

    // Check the exit first (cheap)
    if (isValidExit(Context)) {
      // If the exit is valid check all blocks
      //  - if true, a valid region was found => store it + keep expanding
      //  - if false, .tbd. => stop  (should this really end the loop?)
      if (!allBlocksValid(Context))
        break;

      // Delete unnecessary regions (allocated by getExpandedRegion)
      if (LastValidRegion)
        delete LastValidRegion;

      // Store this region, because it is the greatest valid (encountered so far)
      LastValidRegion = ExpandedRegion;

      // Create and test the next greater region (if any)
      ExpandedRegion = ExpandedRegion->getExpandedRegion();

    } else {
      // Create and test the next greater region (if any)
      Region *TmpRegion = ExpandedRegion->getExpandedRegion();

      // Delete unnecessary regions (allocated by getExpandedRegion)
      delete ExpandedRegion;

      ExpandedRegion = TmpRegion;
    }
  }

  DEBUG(
  if (LastValidRegion)
    dbgs() << "\tto " << LastValidRegion->getNameStr() << "\n";
  else
    dbgs() << "\tExpanding " << R.getNameStr() << " failed\n";
  );

  return LastValidRegion;
}


void ScopDetection::findScops(Region &R) {
  DetectionContext Context(R, *AA, false /*verifying*/);

  LastFailure = "";

  if (isValidRegion(Context)) {
    ++ValidRegion;
    ValidRegions.insert(&R);
    return;
  }

  InvalidRegions[&R] = LastFailure;

  for (Region::iterator I = R.begin(), E = R.end(); I != E; ++I)
    findScops(**I);

  // Try to expand regions.
  //
  // As the region tree normally only contains canonical regions, non canonical
  // regions that form a Scop are not found. Therefore, those non canonical
  // regions are checked by expanding the canonical ones.

  std::vector<Region*> ToExpand;

  for (Region::iterator I = R.begin(), E = R.end(); I != E; ++I)
    ToExpand.push_back(*I);

  for (std::vector<Region*>::iterator RI = ToExpand.begin(),
       RE = ToExpand.end(); RI != RE; ++RI) {
    Region *CurrentRegion = *RI;

    // Skip invalid regions. Regions may become invalid, if they are element of
    // an already expanded region.
    if (ValidRegions.find(CurrentRegion) == ValidRegions.end())
      continue;

    Region *ExpandedR = expandRegion(*CurrentRegion);

    if (!ExpandedR)
      continue;

    R.addSubRegion(ExpandedR, true);
    ValidRegions.insert(ExpandedR);
    ValidRegions.erase(CurrentRegion);

    for (Region::iterator I = ExpandedR->begin(), E = ExpandedR->end(); I != E;
         ++I)
      ValidRegions.erase(*I);
  }
}

bool ScopDetection::allBlocksValid(DetectionContext &Context) const {
  Region &R = Context.CurRegion;

  for (Region::block_iterator I = R.block_begin(), E = R.block_end(); I != E;
       ++I)
    if (!isValidBasicBlock(**I, Context))
      return false;

  return true;
}

bool ScopDetection::isValidExit(DetectionContext &Context) const {
  Region &R = Context.CurRegion;

  // PHI nodes are not allowed in the exit basic block.
  if (BasicBlock *Exit = R.getExit()) {
    BasicBlock::iterator I = Exit->begin();
    if (I != Exit->end() && isa<PHINode> (*I))
      INVALID(Other, "PHI node in exit BB");
  }

  return true;
}

bool ScopDetection::isValidRegion(DetectionContext &Context) const {
  Region &R = Context.CurRegion;

  DEBUG(dbgs() << "Checking region: " << R.getNameStr() << "\n\t");

  // The toplevel region is no valid region.
  if (!R.getParent()) {
    DEBUG(dbgs() << "Top level region is invalid";
          dbgs() << "\n");
    return false;
  }

  // SCoP cannot contain the entry block of the function, because we need
  // to insert alloca instruction there when translate scalar to array.
  if (R.getEntry() == &(R.getEntry()->getParent()->getEntryBlock()))
    INVALID(Other, "Region containing entry block of function is invalid!");

  // Only a simple region is allowed.
  if (!R.isSimple())
    INVALID(SimpleRegion, "Region not simple: " << R.getNameStr());

  if (!isValidExit(Context))
    return false;

  if (!allBlocksValid(Context))
    return false;

  DEBUG(dbgs() << "OK\n");
  return true;
}

bool ScopDetection::isValidFunction(llvm::Function &F) {
  return !InvalidFunctions.count(&F);
}

bool ScopDetection::runOnFunction(llvm::Function &F) {
  AA = &getAnalysis<AliasAnalysis>();
  SE = &getAnalysis<ScalarEvolution>();
  LI = &getAnalysis<LoopInfo>();
  RI = &getAnalysis<RegionInfo>();
  Region *TopRegion = RI->getTopLevelRegion();

  releaseMemory();

  if (OnlyFunction != "" && F.getName() != OnlyFunction)
    return false;

  if(!isValidFunction(F))
    return false;

  findScops(*TopRegion);
  return false;
}


void polly::ScopDetection::verifyRegion(const Region &R) const {
  assert(isMaxRegionInScop(R) && "Expect R is a valid region.");
  DetectionContext Context(const_cast<Region&>(R), *AA, true /*verifying*/);
  isValidRegion(Context);
}

void polly::ScopDetection::verifyAnalysis() const {
  for (RegionSet::const_iterator I = ValidRegions.begin(),
      E = ValidRegions.end(); I != E; ++I)
    verifyRegion(**I);
}

void ScopDetection::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.addRequired<DominatorTree>();
  AU.addRequired<PostDominatorTree>();
  AU.addRequired<LoopInfo>();
  AU.addRequired<ScalarEvolution>();
  // We also need AA and RegionInfo when we are verifying analysis.
  AU.addRequiredTransitive<AliasAnalysis>();
  AU.addRequiredTransitive<RegionInfo>();
  AU.setPreservesAll();
}

void ScopDetection::print(raw_ostream &OS, const Module *) const {
  for (RegionSet::const_iterator I = ValidRegions.begin(),
      E = ValidRegions.end(); I != E; ++I)
    OS << "Valid Region for Scop: " << (*I)->getNameStr() << '\n';

  OS << "\n";
}

void ScopDetection::releaseMemory() {
  ValidRegions.clear();
  InvalidRegions.clear();
  // Do not clear the invalid function set.
}

char ScopDetection::ID = 0;

INITIALIZE_PASS_BEGIN(ScopDetection, "polly-detect",
                      "Polly - Detect static control parts (SCoPs)", false,
                      false)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTree)
INITIALIZE_PASS_DEPENDENCY(RegionInfo)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_END(ScopDetection, "polly-detect",
                    "Polly - Detect static control parts (SCoPs)", false, false)

Pass *polly::createScopDetectionPass() {
  return new ScopDetection();
}