VPlanVerifier.cpp

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//===-- VPlanVerifier.cpp -------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file defines the class VPlanVerifier, which contains utility functions
/// to check the consistency and invariants of a VPlan.
///
//===----------------------------------------------------------------------===//
 
#include "VPlanVerifier.h"
#include "VPlan.h"
#include "VPlanCFG.h"
#include "VPlanDominatorTree.h"
#include "VPlanHelpers.h"
#include "VPlanPatternMatch.h"
#include "VPlanUtils.h"
#include "llvm/ADT/SmallPtrSet.h"
 
#define DEBUG_TYPE "loop-vectorize"
 
using namespace llvm;
using namespace VPlanPatternMatch;
 
namespace {
class VPlanVerifier {
  const VPDominatorTree &VPDT;
 
  SmallPtrSet<BasicBlock *, 8> WrappedIRBBs;
 
  // Verify that phi-like recipes are at the beginning of \p VPBB, with no
  // other recipes in between. Also check that only header blocks contain
  // VPHeaderPHIRecipes.
  bool verifyPhiRecipes(const VPBasicBlock *VPBB);
 
  /// Verify that \p LastActiveLane's operand is guaranteed to be a prefix-mask.
  bool verifyLastActiveLaneRecipe(const VPInstruction &LastActiveLane) const;
 
  bool verifyVPBasicBlock(const VPBasicBlock *VPBB);
 
  bool verifyBlock(const VPBlockBase *VPB);
 
  /// Helper function that verifies the CFG invariants of the VPBlockBases
  /// within
  /// \p Region. Checks in this function are generic for VPBlockBases. They are
  /// not specific for VPBasicBlocks or VPRegionBlocks.
  bool verifyBlocksInRegion(const VPRegionBlock *Region);
 
  /// Verify the CFG invariants of VPRegionBlock \p Region and its nested
  /// VPBlockBases. Do not recurse inside nested VPRegionBlocks.
  bool verifyRegion(const VPRegionBlock *Region);
 
  /// Verify the CFG invariants of VPRegionBlock \p Region and its nested
  /// VPBlockBases. Recurse inside nested VPRegionBlocks.
  bool verifyRegionRec(const VPRegionBlock *Region);
 
public:
  VPlanVerifier(VPDominatorTree &VPDT) : VPDT(VPDT) {}
 
  bool verify(const VPlan &Plan);
};
} // namespace
 
bool VPlanVerifier::verifyPhiRecipes(const VPBasicBlock *VPBB) {
  auto RecipeI = VPBB->begin();
  auto End = VPBB->end();
  unsigned NumActiveLaneMaskPhiRecipes = 0;
  bool IsHeaderVPBB = VPBlockUtils::isHeader(VPBB, VPDT);
  while (RecipeI != End && RecipeI->isPhi()) {
    if (isa<VPActiveLaneMaskPHIRecipe>(RecipeI))
      NumActiveLaneMaskPhiRecipes++;
 
    if (IsHeaderVPBB &&
        !isa<VPHeaderPHIRecipe, VPWidenPHIRecipe, VPPhi>(*RecipeI)) {
      errs() << "Found non-header PHI recipe in header VPBB";
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
      errs() << ": ";
      RecipeI->dump();
#endif
      return false;
    }
 
    if (!IsHeaderVPBB && isa<VPHeaderPHIRecipe>(*RecipeI)) {
      errs() << "Found header PHI recipe in non-header VPBB";
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
      errs() << ": ";
      RecipeI->dump();
#endif
      return false;
    }
 
    // In region form, VPCurrentIterationPHIRecipe must be the first header phi
    // recipe. In a plain CFG VPlan, it must either be the first or second.
    if (isa<VPCurrentIterationPHIRecipe>(RecipeI) &&
        (VPBB->getPlan()->getVectorLoopRegion()
             ? RecipeI->getIterator() != VPBB->begin()
             : RecipeI->getIterator() != VPBB->begin() &&
                   RecipeI->getIterator() != std::next(VPBB->begin()))) {
      errs() << "CurrentIteration PHI is not the first/second recipe\n";
      return false;
    }
 
    // Check if the recipe operands match the number of predecessors.
    // TODO Extend to other phi-like recipes.
    if (auto *PhiIRI = dyn_cast<VPIRPhi>(&*RecipeI)) {
      if (PhiIRI->getNumOperands() != VPBB->getNumPredecessors()) {
        errs() << "Phi-like recipe with different number of operands and "
                  "predecessors.\n";
        // TODO: Print broken recipe. At the moment printing an ill-formed
        // phi-like recipe may crash.
        return false;
      }
    }
 
    RecipeI++;
  }
 
  if (!VPBB->getPlan()->isUnrolled() && NumActiveLaneMaskPhiRecipes > 1) {
    errs() << "There should be no more than one VPActiveLaneMaskPHIRecipe";
    return false;
  }
 
  while (RecipeI != End) {
    if (RecipeI->isPhi() && !isa<VPBlendRecipe>(&*RecipeI)) {
      errs() << "Found phi-like recipe after non-phi recipe";
 
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
      errs() << ": ";
      RecipeI->dump();
      errs() << "after\n";
      std::prev(RecipeI)->dump();
#endif
      return false;
    }
    RecipeI++;
  }
  return true;
}
 
static bool isKnownMonotonic(VPValue *V) {
  VPValue *X, *Y;
  if (match(V, m_Add(m_VPValue(X), m_VPValue(Y))))
    return cast<VPRecipeWithIRFlags>(V)->hasNoUnsignedWrap() &&
           isKnownMonotonic(X) && isKnownMonotonic(Y);
  if (match(V, m_StepVector()))
    return true;
  // Only handle a subset of IVs until we can guarantee there's no overflow.
  if (auto *WidenIV = dyn_cast<VPWidenIntOrFpInductionRecipe>(V))
    return WidenIV->isCanonical() || WidenIV->hasNoUnsignedWrap();
  if (auto *Steps = dyn_cast<VPScalarIVStepsRecipe>(V))
    return match(Steps->getOperand(0),
                 m_CombineOr(
                     m_CanonicalIV(),
                     m_DerivedIV(m_ZeroInt(), m_CanonicalIV(), m_One()))) &&
           match(Steps->getStepValue(), m_One());
  if (isa<VPWidenCanonicalIVRecipe>(V))
    return true;
  return vputils::isUniformAcrossVFsAndUFs(V);
}
 
bool VPlanVerifier::verifyLastActiveLaneRecipe(
    const VPInstruction &LastActiveLane) const {
  assert(LastActiveLane.getOpcode() == VPInstruction::LastActiveLane &&
         "must be called with VPInstruction::LastActiveLane");
 
  if (LastActiveLane.getNumOperands() < 1) {
    errs() << "LastActiveLane must have at least one operand\n";
    return false;
  }
 
  // All operands must be prefix-mask. This means an icmp ult/ule LHS, RHS where
  // the LHS is monotonically increasing and RHS is uniform across VFs and UF.
  for (VPValue *Op : LastActiveLane.operands()) {
    VPValue *Mask = Op;
    VPValue *HeaderMask;
 
    // Look through any `and`s with the incoming alias mask or a
    // loop_dependence_war_mask, which are always prefix masks.
    // TODO: Verify the full loop.dependence.mask chain.
    if (match(Op,
              m_c_BinaryAnd(
                  m_VPValue(HeaderMask),
                  m_CombineOr(
                      m_c_BinaryAnd(
                          m_Intrinsic<Intrinsic::loop_dependence_war_mask>(),
                          m_VPValue()),
                      m_Intrinsic<Intrinsic::loop_dependence_war_mask>(),
                      m_VPInstruction<VPInstruction::IncomingAliasMask>()))))
      Mask = HeaderMask;
 
    // The header mask is a prefix mask. Before being materialized it is the
    // loop region's abstract header mask; afterwards it is an active lane mask
    // (an intrinsic or a phi), or the icmp checked below.
    if (match(Mask, m_HeaderMask()) || isa<VPActiveLaneMaskPHIRecipe>(Mask) ||
        match(Mask, m_VPInstruction<VPInstruction::ActiveLaneMask>()))
      continue;
 
    CmpPredicate Pred;
    VPValue *LHS, *RHS;
    if (match(Mask, m_ICmp(Pred, m_VPValue(LHS), m_VPValue(RHS))) &&
        (Pred == CmpInst::ICMP_ULE || Pred == CmpInst::ICMP_ULT) &&
        isKnownMonotonic(LHS) &&
        (vputils::isUniformAcrossVFsAndUFs(RHS) ||
         match(RHS, m_EVL(m_VPValue()))))
      continue;
 
    errs() << "LastActiveLane operand ";
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
    VPSlotTracker Tracker(LastActiveLane.getParent()->getPlan());
    Op->printAsOperand(errs(), Tracker);
#endif
    errs() << " must be prefix mask (a header mask or an "
              "EVL-derived mask currently)\n";
    return false;
  }
 
  return true;
}
 
bool VPlanVerifier::verifyVPBasicBlock(const VPBasicBlock *VPBB) {
  if (!verifyPhiRecipes(VPBB))
    return false;
 
  // Verify that defs in VPBB dominate all their uses.
  DenseMap<const VPRecipeBase *, unsigned> RecipeNumbering;
  unsigned Cnt = 0;
  for (const VPRecipeBase &R : *VPBB)
    RecipeNumbering[&R] = Cnt++;
 
  for (const VPRecipeBase &R : *VPBB) {
    if (isa<VPIRInstruction>(&R) && !isa<VPIRBasicBlock>(VPBB)) {
      errs() << "VPIRInstructions ";
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
      R.dump();
      errs() << " ";
#endif
      errs() << "not in a VPIRBasicBlock!\n";
      return false;
    }
    for (const VPValue *V : R.definedValues()) {
      // Verify that each defined value has a scalar type.
      if (!V->getScalarType()) {
        errs() << "VPValue without scalar type!\n";
        return false;
      }
 
      // MaskedCond may be used from blocks it don't dominate; the block will be
      // linearized and it will dominate its users after linearization.
      if (match(&R, m_VPInstruction<VPInstruction::MaskedCond>()))
        continue;
 
      for (const VPUser *U : V->users()) {
        auto *UI = cast<VPRecipeBase>(U);
        if (isa<VPIRPhi>(UI) &&
            UI->getNumOperands() != UI->getParent()->getNumPredecessors()) {
          errs() << "Phi-like recipe with different number of operands and "
                    "predecessors.\n";
          return false;
        }
 
        if (auto *Phi = dyn_cast<VPPhiAccessors>(UI)) {
          for (const auto &[IncomingVPV, IncomingVPBB] :
               Phi->incoming_values_and_blocks()) {
            if (IncomingVPV != V)
              continue;
 
            if (VPDT.dominates(VPBB, IncomingVPBB))
              continue;
 
            errs() << "Incoming def does not dominate incoming block!\n";
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
            VPSlotTracker Tracker(VPBB->getPlan());
            IncomingVPV->getDefiningRecipe()->print(errs(), "  ", Tracker);
            errs() << "\n  does not dominate " << IncomingVPBB->getName()
                   << " for\n";
            UI->print(errs(), "  ", Tracker);
#endif
            return false;
          }
          continue;
        }
        // TODO: Also verify VPPredInstPHIRecipe.
        if (isa<VPPredInstPHIRecipe>(UI))
          continue;
 
        // If the user is in the same block, check it comes after R in the
        // block.
        if (UI->getParent() == VPBB) {
          if (RecipeNumbering[UI] >= RecipeNumbering[&R])
            continue;
        } else {
          if (VPDT.dominates(VPBB, UI->getParent()))
            continue;
        }
 
        // Recipes in blocks with a MaskedCond may be used in exit blocks; the
        // block will be linearized and its recipes will dominate their users
        // after linearization.
        bool BlockHasMaskedCond = any_of(*VPBB, [](const VPRecipeBase &R) {
          return match(&R, m_VPInstruction<VPInstruction::MaskedCond>());
        });
        if (BlockHasMaskedCond &&
            any_of(VPBB->getPlan()->getExitBlocks(), [UI](VPIRBasicBlock *EB) {
              return is_contained(EB->getPredecessors(), UI->getParent());
            })) {
          continue;
        }
 
        errs() << "Use before def!\n";
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
        VPSlotTracker Tracker(VPBB->getPlan());
        UI->print(errs(), "  ", Tracker);
        errs() << "\n  before\n";
        R.print(errs(), "  ", Tracker);
        errs() << "\n";
#endif
        return false;
      }
    }
    if (const auto *VPI = dyn_cast<VPInstruction>(&R)) {
      switch (VPI->getOpcode()) {
      case VPInstruction::LastActiveLane:
        if (!verifyLastActiveLaneRecipe(*VPI))
          return false;
        break;
      default:
        break;
      }
    }
    if (const auto *DIV = dyn_cast<VPDerivedIVRecipe>(&R)) {
      if (!DIV->getStartValue()->isDefinedOutsideLoopRegions()) {
        errs() << "VPDerivedIVRecipe must have start value defined outside "
                  "loop regions\n";
        return false;
      }
    }
    if (const auto *ScalarIVSteps = dyn_cast<VPScalarIVStepsRecipe>(&R)) {
      unsigned NumOps = ScalarIVSteps->getNumOperands();
      if (NumOps != 3 && NumOps != 4) {
        errs() << "VPScalarIVStepsRecipe must have 3 or 4 operands\n";
        return false;
      }
    }
  }
 
  auto *IRBB = dyn_cast<VPIRBasicBlock>(VPBB);
  if (!IRBB)
    return true;
 
  if (!WrappedIRBBs.insert(IRBB->getIRBasicBlock()).second) {
    errs() << "Same IR basic block used by multiple wrapper blocks!\n";
    return false;
  }
 
  return true;
}
 
bool VPlanVerifier::verifyBlock(const VPBlockBase *VPB) {
  auto *VPBB = dyn_cast<VPBasicBlock>(VPB);
  // Check block's condition bit.
  if (VPBB && !isa<VPIRBasicBlock>(VPB)) {
    // For plain CFG VPlans, verify header and latch block structure.
    if (!VPBB->getParent()) {
      if (VPBlockUtils::isHeader(VPBB, VPDT)) {
        if (VPB->getNumPredecessors() != 2) {
          errs()
              << "Header block in plain CFG VPlan must have 2 predecessors!\n";
          return false;
        }
        // Predecessor 0 is preheader, predecessor 1 is latch.
        if (!VPBlockUtils::isLatch(VPB->getPredecessors()[1], VPDT)) {
          errs() << "Header's second predecessor must be the latch!\n";
          return false;
        }
      }
 
      if (VPBlockUtils::isLatch(VPBB, VPDT)) {
        if (!match(VPBB->getTerminator(), m_Branch())) {
          errs() << "Latch block must have a branch terminator!\n";
          return false;
        }
        // Successor 0 is middle block, successor 1 is header.
        if (VPBlockUtils::isHeader(VPB->getSuccessors()[0], VPDT)) {
          errs() << "Latch's first successor must not be the header (must be "
                    "middle block)!\n";
          return false;
        }
      }
    } else if (VPB->getNumSuccessors() > 1 ||
               (VPBB->isExiting() && !VPBB->getParent()->isReplicator())) {
      if (!VPBB->getTerminator()) {
        errs() << "Block has multiple successors but doesn't "
                  "have a proper branch recipe!\n";
        return false;
      }
    } else if (VPBB->getTerminator()) {
      errs() << "Unexpected branch recipe!\n";
      return false;
    }
  }
 
  // Check block's successors.
  const auto &Successors = VPB->getSuccessors();
  for (const VPBlockBase *Succ : Successors) {
    // There must be a bi-directional link between block and successor.
    const auto &SuccPreds = Succ->getPredecessors();
    if (!is_contained(SuccPreds, VPB)) {
      errs() << "Missing predecessor link.\n";
      return false;
    }
  }
 
  // Check block's predecessors.
  const auto &Predecessors = VPB->getPredecessors();
 
  for (const VPBlockBase *Pred : Predecessors) {
    // Block and predecessor must be inside the same region.
    if (Pred->getParent() != VPB->getParent()) {
      errs() << "Predecessor is not in the same region.\n";
      return false;
    }
 
    // There must be a bi-directional link between block and predecessor.
    const auto &PredSuccs = Pred->getSuccessors();
    if (!is_contained(PredSuccs, VPB)) {
      errs() << "Missing successor link.\n";
      return false;
    }
  }
  return !VPBB || verifyVPBasicBlock(VPBB);
}
 
bool VPlanVerifier::verifyBlocksInRegion(const VPRegionBlock *Region) {
  for (const VPBlockBase *VPB : vp_depth_first_shallow(Region->getEntry())) {
    // Check block's parent.
    if (VPB->getParent() != Region) {
      errs() << "VPBlockBase has wrong parent\n";
      return false;
    }
 
    if (!verifyBlock(VPB))
      return false;
  }
  return true;
}
 
bool VPlanVerifier::verifyRegion(const VPRegionBlock *Region) {
  const VPBlockBase *Entry = Region->getEntry();
  const VPBlockBase *Exiting = Region->getExiting();
 
  // Entry and Exiting shouldn't have any predecessor/successor, respectively.
  if (Entry->hasPredecessors()) {
    errs() << "region entry block has predecessors\n";
    return false;
  }
  if (Exiting->getNumSuccessors() != 0) {
    errs() << "region exiting block has successors\n";
    return false;
  }
 
  return verifyBlocksInRegion(Region);
}
 
bool VPlanVerifier::verifyRegionRec(const VPRegionBlock *Region) {
  // Recurse inside nested regions and check all blocks inside the region.
  return verifyRegion(Region) &&
         all_of(vp_depth_first_shallow(Region->getEntry()),
                [this](const VPBlockBase *VPB) {
                  const auto *SubRegion = dyn_cast<VPRegionBlock>(VPB);
                  return !SubRegion || verifyRegionRec(SubRegion);
                });
}
 
bool VPlanVerifier::verify(const VPlan &Plan) {
  if (any_of(vp_depth_first_shallow(Plan.getEntry()),
             [this](const VPBlockBase *VPB) { return !verifyBlock(VPB); }))
    return false;
 
  // Check that the plan has a single loop region reachable from entry, and it
  // matches the one returned by getVectorLoopRegion.
  const VPRegionBlock *TopRegion = Plan.getVectorLoopRegion();
  if (any_of(VPBlockUtils::blocksOnly<const VPRegionBlock>(
                 vp_depth_first_shallow(Plan.getEntry())),
             [TopRegion](const VPRegionBlock *R) {
               return !R->isReplicator() && R != TopRegion;
             })) {
    errs() << "VPlan must have a single top-level loop region, reachable from "
              "the entry by following the last successor of each block\n";
    return false;
  }
 
  // TODO: Verify all blocks using vp_depth_first_deep iterators.
  if (!TopRegion)
    return true;
 
  if (!verifyRegionRec(TopRegion))
    return false;
 
  if (TopRegion->getParent()) {
    errs() << "VPlan Top Region should have no parent.\n";
    return false;
  }
 
  const VPBasicBlock *Entry = dyn_cast<VPBasicBlock>(TopRegion->getEntry());
  if (!Entry) {
    errs() << "VPlan entry block is not a VPBasicBlock\n";
    return false;
  }
 
  const VPBasicBlock *Exiting = dyn_cast<VPBasicBlock>(TopRegion->getExiting());
  if (!Exiting) {
    errs() << "VPlan exiting block is not a VPBasicBlock\n";
    return false;
  }
 
  if (Exiting->empty()) {
    errs() << "VPlan vector loop exiting block must end with BranchOnCount, "
              "BranchOnCond, or BranchOnTwoConds VPInstruction but is empty\n";
    return false;
  }
 
  auto *LastInst = dyn_cast<VPInstruction>(std::prev(Exiting->end()));
  if (!match(LastInst, m_Branch())) {
    errs() << "VPlan vector loop exit must end with BranchOnCount, "
              "BranchOnCond, or BranchOnTwoConds VPInstruction\n";
    return false;
  }
 
  return true;
}
 
bool llvm::verifyVPlanIsValid(const VPlan &Plan) {
  // The entry must be the root of the plan's top-level CFG: the dominator tree
  // constructed below and the verifier's block walks all start there.
  if (Plan.getEntry()->hasPredecessors()) {
    errs() << "VPlan entry block has predecessors\n";
    return false;
  }
 
  VPDominatorTree VPDT(const_cast<VPlan &>(Plan));
  VPlanVerifier Verifier(VPDT);
  return Verifier.verify(Plan);
}