llvm-project/llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp

//===- ReduceBasicBlocks.cpp - Specialized Delta Pass ---------------------===//
//
// 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 a function which calls the Generic Delta pass in order
// to reduce uninteresting BasicBlocks from defined functions.
//
//===----------------------------------------------------------------------===//

#include "ReduceBasicBlocks.h"
#include "Utils.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/raw_ostream.h"
#include <vector>

using namespace llvm;

/// Replaces BB Terminator with one that only contains Chunk BBs
static void replaceBranchTerminator(BasicBlock &BB,
                                    const DenseSet<BasicBlock *> &BBsToKeep) {
  auto *Term = BB.getTerminator();
  std::vector<BasicBlock *> ChunkSuccessors;
  for (auto *Succ : successors(&BB))
    if (BBsToKeep.count(Succ))
      ChunkSuccessors.push_back(Succ);

  // BB only references Chunk BBs
  if (ChunkSuccessors.size() == Term->getNumSuccessors())
    return;

  bool IsBranch = isa<BranchInst>(Term) || isa<InvokeInst>(Term);
  Value *Address = nullptr;
  if (auto *IndBI = dyn_cast<IndirectBrInst>(Term))
    Address = IndBI->getAddress();

  Term->replaceAllUsesWith(getDefaultValue(Term->getType()));
  Term->eraseFromParent();

  if (ChunkSuccessors.empty()) {
    // Scan forward in BB list to try find a block that is kept.
    Function &F = *BB.getParent();
    Function::iterator FI = BB.getIterator();
    FI++;
    while (FI != F.end()) {
      auto &FIB = *FI;
      if (BBsToKeep.count(&FIB) && !isa<PHINode>(FIB.begin())) {
        BranchInst::Create(&FIB, &BB);
        return;
      }
      FI++;
    }
    // If that fails then resort to replacing with a ret.
    auto *FnRetTy = BB.getParent()->getReturnType();
    ReturnInst::Create(BB.getContext(),
                       FnRetTy->isVoidTy() ? nullptr : getDefaultValue(FnRetTy),
                       &BB);
    return;
  }

  if (IsBranch)
    BranchInst::Create(ChunkSuccessors[0], &BB);

  if (Address) {
    auto *NewIndBI =
        IndirectBrInst::Create(Address, ChunkSuccessors.size(), &BB);
    for (auto *Dest : ChunkSuccessors)
      NewIndBI->addDestination(Dest);
  }
}

/// Removes uninteresting BBs from switch, if the default case ends up being
/// uninteresting, the switch is replaced with a void return (since it has to be
/// replace with something)
static void
removeUninterestingBBsFromSwitch(SwitchInst &SwInst,
                                 const DenseSet<BasicBlock *> &BBsToKeep) {
  if (!BBsToKeep.count(SwInst.getDefaultDest())) {
    auto *FnRetTy = SwInst.getParent()->getParent()->getReturnType();
    ReturnInst::Create(SwInst.getContext(),
                       FnRetTy->isVoidTy() ? nullptr : getDefaultValue(FnRetTy),
                       SwInst.getParent());
    SwInst.eraseFromParent();
  } else
    for (int I = 0, E = SwInst.getNumCases(); I != E; ++I) {
      auto Case = SwInst.case_begin() + I;
      if (!BBsToKeep.count(Case->getCaseSuccessor())) {
        SwInst.removeCase(Case);
        --I;
        --E;
      }
    }
}

/// Removes out-of-chunk arguments from functions, and modifies their calls
/// accordingly. It also removes allocations of out-of-chunk arguments.
static void extractBasicBlocksFromModule(Oracle &O, Module &Program) {
  DenseSet<BasicBlock *> BBsToKeep;

  SmallVector<BasicBlock *> BBsToDelete;
  for (auto &F : Program) {
    for (auto &BB : F) {
      if (O.shouldKeep())
        BBsToKeep.insert(&BB);
      else {
        BBsToDelete.push_back(&BB);
        // Remove out-of-chunk BB from successor phi nodes
        for (auto *Succ : successors(&BB))
          Succ->removePredecessor(&BB);
      }
    }
  }

  // Replace terminators that reference out-of-chunk BBs
  for (auto &F : Program)
    for (auto &BB : F) {
      if (auto *SwInst = dyn_cast<SwitchInst>(BB.getTerminator()))
        removeUninterestingBBsFromSwitch(*SwInst, BBsToKeep);
      else
        replaceBranchTerminator(BB, BBsToKeep);
    }

  // Replace out-of-chunk switch uses
  for (auto &BB : BBsToDelete) {
    // Instructions might be referenced in other BBs
    for (auto &I : *BB)
      I.replaceAllUsesWith(getDefaultValue(I.getType()));
    if (BB->getParent()->size() == 1) {
      // this is the last basic block of the function, thus we must also make
      // sure to remove comdat and set linkage to external
      auto F = BB->getParent();
      F->deleteBody();
      F->setComdat(nullptr);
    } else {
      BB->eraseFromParent();
    }
  }
}

void llvm::reduceBasicBlocksDeltaPass(TestRunner &Test) {
  outs() << "*** Reducing Basic Blocks...\n";
  runDeltaPass(Test, extractBasicBlocksFromModule);
}