SCCP.cpp

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//===-- SCCP.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
//
//===----------------------------------------------------------------------===//
//
// This file implements Interprocedural Sparse Conditional Constant Propagation.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/IPO/SCCP.h"
#include "IPOOptions.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueLattice.h"
#include "llvm/Analysis/ValueLatticeUtils.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/AttributeMask.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/ModRef.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/IPO/FunctionSpecialization.h"
#include "llvm/Transforms/Scalar/SCCP.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SCCPSolver.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "sccp"
 
STATISTIC(NumInstRemoved, "Number of instructions removed");
STATISTIC(NumArgsElimed ,"Number of arguments constant propagated");
STATISTIC(NumGlobalConst, "Number of globals found to be constant");
STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
STATISTIC(NumInstReplaced,
          "Number of instructions replaced with (simpler) instruction");
 
static void findReturnsToZap(Function &F,
                             SmallVector<ReturnInst *, 8> &ReturnsToZap,
                             SCCPSolver &Solver) {
  // We can only do this if we know that nothing else can call the function.
  if (!Solver.isArgumentTrackedFunction(&F))
    return;
 
  if (Solver.mustPreserveReturn(&F)) {
    LLVM_DEBUG(
        dbgs()
        << "Can't zap returns of the function : " << F.getName()
        << " due to present musttail or \"clang.arc.attachedcall\" call of "
           "it\n");
    return;
  }
 
  assert(
      all_of(F.users(),
             [&Solver](User *U) {
               if (isa<Instruction>(U) &&
                   !Solver.isBlockExecutable(cast<Instruction>(U)->getParent()))
                 return true;
               // Non-callsite uses are not impacted by zapping. Also, constant
               // uses (like blockaddresses) could stuck around, without being
               // used in the underlying IR, meaning we do not have lattice
               // values for them.
               if (!isa<CallBase>(U))
                 return true;
               if (U->getType()->isStructTy()) {
                 return none_of(Solver.getStructLatticeValueFor(U),
                                SCCPSolver::isOverdefined);
               }
 
               // We don't consider assume-like intrinsics to be actual address
               // captures.
               if (auto *II = dyn_cast<IntrinsicInst>(U)) {
                 if (II->isAssumeLikeIntrinsic())
                   return true;
               }
 
               return !SCCPSolver::isOverdefined(Solver.getLatticeValueFor(U));
             }) &&
      "We can only zap functions where all live users have a concrete value");
 
  for (BasicBlock &BB : F) {
    if (CallInst *CI = BB.getTerminatingMustTailCall()) {
      LLVM_DEBUG(dbgs() << "Can't zap return of the block due to present "
                        << "musttail call : " << *CI << "\n");
      (void)CI;
      return;
    }
 
    if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator()))
      if (!isa<UndefValue>(RI->getOperand(0)))
        ReturnsToZap.push_back(RI);
  }
}
 
static bool runIPSCCP(
    Module &M, const DataLayout &DL, FunctionAnalysisManager *FAM,
    std::function<const TargetLibraryInfo &(Function &)> GetTLI,
    std::function<TargetTransformInfo &(Function &)> GetTTI,
    std::function<AssumptionCache &(Function &)> GetAC,
    std::function<DominatorTree &(Function &)> GetDT,
    std::function<BlockFrequencyInfo &(Function &)> GetBFI,
    bool IsFuncSpecEnabled) {
  SCCPSolver Solver(DL, GetTLI, M.getContext());
  FunctionSpecializer Specializer(Solver, M, FAM, GetBFI, GetTLI, GetTTI,
                                  GetAC);
 
  // Loop over all functions, marking arguments to those with their addresses
  // taken or that are external as overdefined.
  for (Function &F : M) {
    if (F.isDeclaration())
      continue;
 
    DominatorTree &DT = GetDT(F);
    AssumptionCache &AC = GetAC(F);
    Solver.addPredicateInfo(F, DT, AC);
 
    // Determine if we can track the function's return values. If so, add the
    // function to the solver's set of return-tracked functions.
    if (canTrackReturnsInterprocedurally(&F))
      Solver.addTrackedFunction(&F);
 
    // Determine if we can track the function's arguments. If so, add the
    // function to the solver's set of argument-tracked functions.
    if (canTrackArgumentsInterprocedurally(&F)) {
      Solver.addArgumentTrackedFunction(&F);
      continue;
    }
 
    // Assume the function is called.
    Solver.markBlockExecutable(&F.front());
 
    for (Argument &AI : F.args())
      Solver.trackValueOfArgument(&AI);
  }
 
  // Determine if we can track any of the module's global variables. If so, add
  // the global variables we can track to the solver's set of tracked global
  // variables.
  for (GlobalVariable &G : M.globals()) {
    G.removeDeadConstantUsers();
    if (canTrackGlobalVariableInterprocedurally(&G))
      Solver.trackValueOfGlobalVariable(&G);
  }
 
  // Solve for constants.
  Solver.solveWhileResolvedUndefsIn(M);
 
  if (IsFuncSpecEnabled) {
    unsigned MaxIters = getIPOOptions(M).funcspec_max_iters;
    for (unsigned Iters = 0; Iters < MaxIters && Specializer.run(); ++Iters)
      ;
  }
 
  // Iterate over all of the instructions in the module, replacing them with
  // constants if we have found them to be of constant values.
  bool MadeChanges = false;
  for (Function &F : M) {
    if (F.isDeclaration())
      continue;
    // Skip the dead functions marked by FunctionSpecializer, avoiding removing
    // blocks in dead functions. Set MadeChanges if there is any dead function
    // that will be removed later.
    if (IsFuncSpecEnabled && Specializer.isDeadFunction(&F)) {
      MadeChanges = true;
      continue;
    }
 
    SmallVector<BasicBlock *, 512> BlocksToErase;
 
    if (Solver.isBlockExecutable(&F.front())) {
      bool ReplacedPointerArg = false;
      for (Argument &Arg : F.args()) {
        if (!Arg.use_empty() && Solver.tryToReplaceWithConstant(&Arg)) {
          ReplacedPointerArg |= Arg.getType()->isPointerTy();
          ++NumArgsElimed;
        }
      }
 
      // If we replaced an argument, we may now also access a global (currently
      // classified as "other" memory). Update memory attribute to reflect this.
      if (ReplacedPointerArg) {
        auto UpdateAttrs = [&](AttributeList AL) {
          MemoryEffects ME = AL.getMemoryEffects();
          if (ME == MemoryEffects::unknown())
            return AL;
 
          ModRefInfo ArgMemMR = ME.getModRef(IRMemLocation::ArgMem);
          ME |= MemoryEffects(IRMemLocation::ErrnoMem, ArgMemMR);
          ME |= MemoryEffects(IRMemLocation::Other, ArgMemMR);
 
          return AL.addFnAttribute(
              F.getContext(),
              Attribute::getWithMemoryEffects(F.getContext(), ME));
        };
 
        F.setAttributes(UpdateAttrs(F.getAttributes()));
        for (User *U : F.users()) {
          auto *CB = dyn_cast<CallBase>(U);
          if (!CB || CB->getCalledFunction() != &F)
            continue;
 
          CB->setAttributes(UpdateAttrs(CB->getAttributes()));
        }
      }
      MadeChanges |= ReplacedPointerArg;
    }
 
    SmallPtrSet<Value *, 32> InsertedValues;
    for (BasicBlock &BB : F) {
      if (!Solver.isBlockExecutable(&BB)) {
        LLVM_DEBUG(dbgs() << "  BasicBlock Dead:" << BB);
        ++NumDeadBlocks;
 
        MadeChanges = true;
 
        if (&BB != &F.front())
          BlocksToErase.push_back(&BB);
        continue;
      }
 
      MadeChanges |= Solver.simplifyInstsInBlock(
          BB, InsertedValues, NumInstRemoved, NumInstReplaced);
    }
 
    DominatorTree *DT = FAM->getCachedResult<DominatorTreeAnalysis>(F);
    PostDominatorTree *PDT = FAM->getCachedResult<PostDominatorTreeAnalysis>(F);
    DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Lazy);
    // Change dead blocks to unreachable. We do it after replacing constants
    // in all executable blocks, because changeToUnreachable may remove PHI
    // nodes in executable blocks we found values for. The function's entry
    // block is not part of BlocksToErase, so we have to handle it separately.
    for (BasicBlock *BB : BlocksToErase) {
      NumInstRemoved += changeToUnreachable(&*BB->getFirstNonPHIIt(),
                                            /*PreserveLCSSA=*/false, &DTU);
    }
    if (!Solver.isBlockExecutable(&F.front()))
      NumInstRemoved += changeToUnreachable(&*F.front().getFirstNonPHIIt(),
                                            /*PreserveLCSSA=*/false, &DTU);
 
    BasicBlock *NewUnreachableBB = nullptr;
    for (BasicBlock &BB : F)
      MadeChanges |= Solver.removeNonFeasibleEdges(&BB, DTU, NewUnreachableBB);
 
    for (BasicBlock *DeadBB : BlocksToErase)
      if (!DeadBB->hasAddressTaken())
        DTU.deleteBB(DeadBB);
 
    Solver.removeSSACopies(F);
  }
 
  // If we inferred constant or undef return values for a function, we replaced
  // all call uses with the inferred value.  This means we don't need to bother
  // actually returning anything from the function.  Replace all return
  // instructions with return undef.
  //
  // Do this in two stages: first identify the functions we should process, then
  // actually zap their returns.  This is important because we can only do this
  // if the address of the function isn't taken.  In cases where a return is the
  // last use of a function, the order of processing functions would affect
  // whether other functions are optimizable.
  SmallVector<ReturnInst*, 8> ReturnsToZap;
 
  Solver.inferReturnAttributes();
  Solver.inferArgAttributes();
  for (const auto &[F, ReturnValue] : Solver.getTrackedRetVals()) {
    assert(!F->getReturnType()->isVoidTy() &&
           "should not track void functions");
    if (SCCPSolver::isReplaceableConstant(ReturnValue) ||
        ReturnValue.isUnknownOrUndef())
      findReturnsToZap(*F, ReturnsToZap, Solver);
  }
 
  for (auto *F : Solver.getMRVFunctionsTracked()) {
    assert(F->getReturnType()->isStructTy() &&
           "The return type should be a struct");
    StructType *STy = cast<StructType>(F->getReturnType());
    if (Solver.isStructLatticeConstant(F, STy))
      findReturnsToZap(*F, ReturnsToZap, Solver);
  }
 
  // Zap all returns which we've identified as zap to change.
  SmallSetVector<Function *, 8> FuncZappedReturn;
  for (ReturnInst *RI : ReturnsToZap) {
    Function *F = RI->getParent()->getParent();
    RI->setOperand(0, PoisonValue::get(F->getReturnType()));
    // Record all functions that are zapped.
    FuncZappedReturn.insert(F);
  }
 
  // Remove the returned attribute for zapped functions and the
  // corresponding call sites.
  // Also remove any attributes that convert an undef return value into
  // immediate undefined behavior
  AttributeMask UBImplyingAttributes =
      AttributeFuncs::getUBImplyingAttributes();
  for (Function *F : FuncZappedReturn) {
    for (Argument &A : F->args())
      F->removeParamAttr(A.getArgNo(), Attribute::Returned);
    F->removeRetAttrs(UBImplyingAttributes);
    for (Use &U : F->uses()) {
      CallBase *CB = dyn_cast<CallBase>(U.getUser());
      if (!CB) {
        assert(isa<Constant>(U.getUser()) &&
               all_of(U.getUser()->users(), [](const User *UserUser) {
                 return cast<IntrinsicInst>(UserUser)->isAssumeLikeIntrinsic();
               }));
        continue;
      }
 
      for (Use &Arg : CB->args())
        CB->removeParamAttr(CB->getArgOperandNo(&Arg), Attribute::Returned);
      CB->removeRetAttrs(UBImplyingAttributes);
    }
  }
 
  // If we inferred constant or undef values for globals variables, we can
  // delete the global and any stores that remain to it.
  for (const auto &[GV, LV] : Solver.getTrackedGlobals()) {
    // We may have inferred a constant that is not replaceable (e.g., due to
    // different pointer provenance), so skip deleting the global in this case.
    if (!SCCPSolver::isReplaceableConstant(LV) && !LV.isUnknownOrUndef())
      continue;
    LLVM_DEBUG(dbgs() << "Found that GV '" << GV->getName()
                      << "' is constant!\n");
    for (User *U : make_early_inc_range(GV->users())) {
      // We can remove LoadInst here. The LoadInsts in dead functions marked by
      // FuncSpec are not simplified to constants, thus poison them.
      assert((isa<StoreInst>(U) || isa<LoadInst>(U)) &&
             "Only Store|Load Instruction can be user of GlobalVariable at "
             "reaching here.");
      Instruction *I = cast<Instruction>(U);
      if (isa<LoadInst>(I))
        I->replaceAllUsesWith(PoisonValue::get(I->getType()));
      I->eraseFromParent();
    }
 
    // Try to create a debug constant expression for the global variable
    // initializer value.
    SmallVector<DIGlobalVariableExpression *, 1> GVEs;
    GV->getDebugInfo(GVEs);
    if (GVEs.size() == 1) {
      DIBuilder DIB(M);
      if (DIExpression *InitExpr = getExpressionForConstant(
              DIB, *GV->getInitializer(), *GV->getValueType()))
        GVEs[0]->replaceOperandWith(1, InitExpr);
    }
 
    MadeChanges = true;
    M.eraseGlobalVariable(GV);
    ++NumGlobalConst;
  }
 
  return MadeChanges;
}
 
PreservedAnalyses IPSCCPPass::run(Module &M, ModuleAnalysisManager &AM) {
  const DataLayout &DL = M.getDataLayout();
  auto &FAM = AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
  auto GetTLI = [&FAM](Function &F) -> const TargetLibraryInfo & {
    return FAM.getResult<TargetLibraryAnalysis>(F);
  };
  auto GetTTI = [&FAM](Function &F) -> TargetTransformInfo & {
    return FAM.getResult<TargetIRAnalysis>(F);
  };
  auto GetAC = [&FAM](Function &F) -> AssumptionCache & {
    return FAM.getResult<AssumptionAnalysis>(F);
  };
  auto GetDT = [&FAM](Function &F) -> DominatorTree & {
    return FAM.getResult<DominatorTreeAnalysis>(F);
  };
  auto GetBFI = [&FAM](Function &F) -> BlockFrequencyInfo & {
    return FAM.getResult<BlockFrequencyAnalysis>(F);
  };
 
 
  if (!runIPSCCP(M, DL, &FAM, GetTLI, GetTTI, GetAC, GetDT, GetBFI,
                 isFuncSpecEnabled()))
    return PreservedAnalyses::all();
 
  PreservedAnalyses PA;
  PA.preserve<DominatorTreeAnalysis>();
  PA.preserve<PostDominatorTreeAnalysis>();
  PA.preserve<FunctionAnalysisManagerModuleProxy>();
  return PA;
}