SCCP.cpp

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//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
//
// 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 sparse conditional constant propagation and merging:
//
// Specifically, this:
//   * Assumes values are constant unless proven otherwise
//   * Assumes BasicBlocks are dead unless proven otherwise
//   * Proves values to be constant, and replaces them with constants
//   * Proves conditional branches to be unconditional
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/Scalar/SCCP.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueLatticeUtils.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.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(NumDeadBlocks , "Number of basic blocks unreachable");
STATISTIC(NumInstReplaced,
          "Number of instructions replaced with (simpler) instruction");
 
// runSCCP() - Run the Sparse Conditional Constant Propagation algorithm,
// and return true if the function was modified.
static bool runSCCP(Function &F, const DataLayout &DL,
                    const TargetLibraryInfo *TLI, DomTreeUpdater &DTU) {
  LLVM_DEBUG(dbgs() << "SCCP on function '" << F.getName() << "'\n");
  SCCPSolver Solver(
      DL, [TLI](Function &F) -> const TargetLibraryInfo & { return *TLI; },
      F.getContext());
 
  // While we don't do any actual inter-procedural analysis, still track
  // return values so we can infer attributes.
  if (canTrackReturnsInterprocedurally(&F))
    Solver.addTrackedFunction(&F);
 
  // Mark the first block of the function as being executable.
  Solver.markBlockExecutable(&F.front());
 
  // Initialize arguments based on attributes.
  for (Argument &AI : F.args())
    Solver.trackValueOfArgument(&AI);
 
  // Solve for constants.
  bool ResolvedUndefs = true;
  while (ResolvedUndefs) {
    Solver.solve();
    LLVM_DEBUG(dbgs() << "RESOLVING UNDEFs\n");
    ResolvedUndefs = Solver.resolvedUndefsIn(F);
  }
 
  bool MadeChanges = false;
 
  // If we decided that there are basic blocks that are dead in this function,
  // delete their contents now.  Note that we cannot actually delete the blocks,
  // as we cannot modify the CFG of the function.
 
  SmallPtrSet<Value *, 32> InsertedValues;
  SmallVector<BasicBlock *, 8> BlocksToErase;
  for (BasicBlock &BB : F) {
    if (!Solver.isBlockExecutable(&BB)) {
      LLVM_DEBUG(dbgs() << "  BasicBlock Dead:" << BB);
      ++NumDeadBlocks;
      BlocksToErase.push_back(&BB);
      MadeChanges = true;
      continue;
    }
 
    MadeChanges |= Solver.simplifyInstsInBlock(BB, InsertedValues,
                                               NumInstRemoved, NumInstReplaced);
  }
 
  // Remove unreachable blocks and non-feasible edges.
  for (BasicBlock *DeadBB : BlocksToErase)
    NumInstRemoved += changeToUnreachable(DeadBB->getFirstNonPHI(),
                                          /*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.inferReturnAttributes();
 
  return MadeChanges;
}
 
PreservedAnalyses SCCPPass::run(Function &F, FunctionAnalysisManager &AM) {
  const DataLayout &DL = F.getDataLayout();
  auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
  auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
  if (!runSCCP(F, DL, &TLI, DTU))
    return PreservedAnalyses::all();
 
  auto PA = PreservedAnalyses();
  PA.preserve<DominatorTreeAnalysis>();
  return PA;
}