lld/lib/Passes/LayoutPass.cpp

//===- Passes/LayoutPass.cpp - Layout atoms -------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "LayoutPass"

#include "lld/Passes/LayoutPass.h"

#include "llvm/Support/Debug.h"

using namespace lld;

/// The function compares atoms by sorting atoms in the following order
/// a) Sorts atoms with the same permissions
/// b) Sorts atoms with the same content Type
/// c) Sorts atoms by Section position preference
/// d) Sorts atoms by how they follow / precede each atom
/// e) Sorts atoms on how they appear using File Ordinality
/// f) Sorts atoms on how they appear within the File
bool LayoutPass::CompareAtoms::operator()(const DefinedAtom *left,
                                          const DefinedAtom *right) {
  DEBUG(llvm::dbgs() << "Sorting " << left->name() << " " << right->name() << "\n");
  if (left == right)
    return false;

  DEBUG(llvm::dbgs() << "Sorting by perms\n");

  // Sort same permissions together.
  DefinedAtom::ContentPermissions leftPerms = left->permissions();
  DefinedAtom::ContentPermissions rightPerms = right->permissions();
  if (leftPerms != rightPerms)
    return leftPerms < rightPerms;

  DEBUG(llvm::dbgs() << "Sorting by contentType\n");

  // Sort same content types together.
  DefinedAtom::ContentType leftType = left->contentType();
  DefinedAtom::ContentType rightType = right->contentType();
  if (leftType != rightType)
    return leftType < rightType;

  // TO DO: Sort atoms in customs sections together.

  DEBUG(llvm::dbgs() << "Sorting by sectionPos\n");

  // Sort by section position preference.
  DefinedAtom::SectionPosition leftPos = left->sectionPosition();
  DefinedAtom::SectionPosition rightPos = right->sectionPosition();
  bool leftSpecialPos = (leftPos != DefinedAtom::sectionPositionAny);
  bool rightSpecialPos = (rightPos != DefinedAtom::sectionPositionAny);
  if (leftSpecialPos || rightSpecialPos) {
    if (leftPos != rightPos)
      return leftPos < rightPos;
  }

  DEBUG(llvm::dbgs() << "Sorting by override\n");

  AtomToOrdinalT::const_iterator lPos = _layout._ordinalOverrideMap.find(left);
  AtomToOrdinalT::const_iterator rPos = _layout._ordinalOverrideMap.find(right);
  AtomToOrdinalT::const_iterator end = _layout._ordinalOverrideMap.end();
  if (lPos != end) {
    if (rPos != end) {
      // both left and right are overridden, so compare overridden ordinals
      if (lPos->second != rPos->second)
        return lPos->second < rPos->second;
    } else {
      // left is overridden and right is not, so left < right
      return true;
    }
  } else {
    if (rPos != end) {
      // right is overridden and left is not, so right < left
      return false;
    } else {
      // neither are overridden, 
      // fall into default sorting below
    }
  }

  DEBUG(llvm::dbgs() << "Sorting by .o order\n");

  // Sort by .o order.
  const File *leftFile = &left->file();
  const File *rightFile = &right->file();
  if (leftFile != rightFile)
    return leftFile->ordinal() < rightFile->ordinal();

  DEBUG(llvm::dbgs() << "Sorting by ordinal\n");

  // Sort by atom order with .o file.
  uint64_t leftOrdinal = left->ordinal();
  uint64_t rightOrdinal = right->ordinal();
  if (leftOrdinal != rightOrdinal)
    return leftOrdinal < rightOrdinal;

  DEBUG(llvm::dbgs() << "Unordered\n");

  return false;
}

/// This pass builds the followon tables described by two DenseMaps
/// followOnRoots and followonNexts.
/// The followOnRoots map contains a mapping of a DefinedAtom to its root
/// The followOnNexts map contains a mapping of what DefinedAtom follows the
/// current Atom
/// The algorithm follows a very simple approach
/// a) If the atom is first seen, then make that as the root atom
/// b) The targetAtom which this Atom contains, has the root thats set to the 
///    root of the current atom
/// c) If the targetAtom is part of a different tree and the root of the
///    targetAtom is itself, Chain all the atoms that are contained in the tree
///    to the current Tree
/// d) If the targetAtom is part of a different chain and the root of the
///    targetAtom until the targetAtom has all atoms of size 0, then chain the
///    targetAtoms and its tree to the current chain
void LayoutPass::buildFollowOnTable(MutableFile::DefinedAtomRange &range) {
  for (auto ai : range) {
    for (const Reference *r : *ai) {
      if (r->kind() == lld::Reference::kindLayoutAfter) {
        const DefinedAtom *targetAtom = llvm::dyn_cast<DefinedAtom>(r->target());
        _followOnNexts[ai] = targetAtom;
        // If we find a followon for the first time, lets make that
        // atom as the root atom
        if (_followOnRoots.count(ai) == 0) {
          _followOnRoots[ai] = ai;
        }
        // If the targetAtom is not a root of any chain, lets make 
        // the root of the targetAtom to the root of the current chain
        auto iter = _followOnRoots.find(targetAtom);
        if (iter == _followOnRoots.end()) {
          _followOnRoots[targetAtom] = _followOnRoots[ai];
        } else {
          // The followon is part of another chain
          if (iter->second == targetAtom) {
            const DefinedAtom *a = targetAtom;
            while (true) {
              _followOnRoots[a] = _followOnRoots[ai];
              // Set all the follow on's for the targetAtom to be
              // the current root
              AtomToAtomT::iterator targetFollowOnAtomsIter =
                  _followOnNexts.find(a);

              if (targetFollowOnAtomsIter != _followOnNexts.end())
                a = targetFollowOnAtomsIter->second;
              else
                break;
            }      // while true
          } else { // the atom could be part of chain already
                   // Get to the root of the chain
            const DefinedAtom *a = _followOnRoots[targetAtom];
            const DefinedAtom *targetPrevAtom = nullptr;

            // If the size of the atom is 0, and the target 
            // is already part of a chain, lets bring the current
            // atom into the chain
            size_t currentAtomSize = (*ai).size();

            // Lets add to the chain only if the atoms that 
            // appear before the targetAtom in the chain 
            // are of size 0
            bool foundNonZeroSizeAtom = false;
            while (true) {
              targetPrevAtom = a;

              // Set all the follow on's for the targetAtom to be
              // the current root
              AtomToAtomT::iterator targetFollowOnAtomsIter =
                  _followOnNexts.find(a);

              if (targetFollowOnAtomsIter != _followOnNexts.end())
                a = targetFollowOnAtomsIter->second;
              else
                break;

              if ((a->size() != 0) && (currentAtomSize != 0)) {
                foundNonZeroSizeAtom = true;
                break;
              }

              if (a == targetAtom)
                break;

            } // while true
            if (foundNonZeroSizeAtom) {
              // TODO: print warning that an impossible layout 
              // is being desired by the user
              // Continue to the next atom 
              break;
            }

            // If the atom is a zero sized atom, then make the target
            // follow the zero sized atom, as the zero sized atom may be
            // a weak symbol
            if ((currentAtomSize == 0) && (targetPrevAtom)) {
              _followOnNexts[targetPrevAtom] = ai;
              _followOnRoots[ai] = _followOnRoots[targetPrevAtom];
              _followOnNexts[ai] = targetAtom;
            } else {
              _followOnNexts[ai] = _followOnRoots[targetAtom];
              // Set the root of all atoms in the 
              a = _followOnRoots[targetAtom];
              while (true) {
                _followOnRoots[a] = _followOnRoots[ai];
                // Set all the follow on's for the targetAtom to be
                // the current root
                AtomToAtomT::iterator targetFollowOnAtomsIter =
                    _followOnNexts.find(a);
                if (targetFollowOnAtomsIter != _followOnNexts.end())
                  a = targetFollowOnAtomsIter->second;
                else
                  break;
              } // while true
            } // end else (currentAtomSize != 0)
          }   // end else
        }     // else
      }       // kindLayoutAfter
    }         // Reference
  }           // range
}

/// This pass builds the followon tables using InGroup relationships
/// The algorithm follows a very simple approach
/// a) If the rootAtom is not part of any root, create a new root with the
///    as the head
/// b) If the current Atom root is not found, then make the current atoms root
///    point to the rootAtom
/// c) If the root of the current Atom is itself a root of some other tree
///    make all the atoms in the chain point to the ingroup reference
/// d) Check to see if the current atom is part of the chain from the rootAtom
///    if not add the atom to the chain, so that the current atom is part of the
///    the chain where the rootAtom is in
void LayoutPass::buildInGroupTable(MutableFile::DefinedAtomRange &range) {
  // This table would convert precededby references to follow on 
  // references so that we have only one table 
  for (auto ai : range) {
    for (const Reference *r : *ai) {
      if (r->kind() == lld::Reference::kindInGroup) {
        const DefinedAtom *rootAtom = llvm::dyn_cast<DefinedAtom>(r->target());
        // If the root atom is not part of any root
        // create a new root 
        if (_followOnRoots.count(rootAtom) == 0) {
          _followOnRoots[rootAtom] = rootAtom;
        }
        // If the current Atom has not been seen yet and there is no root 
        // that has been set, set the root of the atom to the targetAtom 
        // as the targetAtom points to the ingroup root
        auto iter = _followOnRoots.find(ai);
        if (iter == _followOnRoots.end()) {
          _followOnRoots[ai] = rootAtom;
        }
        else if (iter->second == ai) {
          if (iter->second != rootAtom) {
            const DefinedAtom *a = iter->second;
            // Change all the followon next references to the ingroup reference root
            while (true) {
              _followOnRoots[a] = rootAtom;
              // Set all the follow on's for the targetAtom to be
              // the current root
              AtomToAtomT::iterator targetFollowOnAtomsIter =
                  _followOnNexts.find(a);
              if (targetFollowOnAtomsIter != _followOnNexts.end())
                a = targetFollowOnAtomsIter->second;
              else
                break;
            } // while true
          }
        }
        else {
          // TODO : Flag an error that the root of the tree
          // is different, Here is an example
          // Say there are atoms
          // chain 1 : a->b->c
          // chain 2 : d->e->f
          // and e,f have their ingroup reference as a
          // this could happen only if the root of e,f that is d
          // has root as 'a'
          continue;
        }

        // Check if the current atom is part of the chain
        bool isAtomInChain = false;
        const DefinedAtom *lastAtom = rootAtom;
        while (true) {
          AtomToAtomT::iterator followOnAtomsIter = 
                  _followOnNexts.find(lastAtom);
          if (followOnAtomsIter != _followOnNexts.end()) {
            lastAtom = followOnAtomsIter->second;
            if (lastAtom == ai) {
              isAtomInChain = true;
              break;
            }
          }
          else 
            break;
        } // findAtomInChain

        if (!isAtomInChain) 
          _followOnNexts[lastAtom] = ai;
      }
    }
  }
}

/// This pass builds the followon tables using Preceded By relationships
/// The algorithm follows a very simple approach
/// a) If the targetAtom is not part of any root and the current atom is not
///    part of any root, create a chain with the current atom as root and 
///    the targetAtom as following the current atom
/// b) Chain the targetAtom to the current Atom if the targetAtom is not part 
///    of any chain and the currentAtom has no followOn's
/// c) If the targetAtom is part of a different tree and the root of the
///    targetAtom is itself, and if the current atom is not part of any root
///    chain all the atoms together 
/// d) If the current atom has no followon and the root of the targetAtom is 
///    not equal to the root of the current atom(the targetAtom is not in the 
///    same chain), chain all the atoms that are lead by the targetAtom into 
///    the current chain
void LayoutPass::buildPrecededByTable(MutableFile::DefinedAtomRange &range) {
  // This table would convert precededby references to follow on 
  // references so that we have only one table 
  for (auto ai : range) {
    for (const Reference *r : *ai) {
      if (r->kind() == lld::Reference::kindLayoutBefore) {
        const DefinedAtom *targetAtom = llvm::dyn_cast<DefinedAtom>(r->target());
        // Is the targetAtom not chained
        if (_followOnRoots.count(targetAtom) == 0) {
          // Is the current atom not part of any root ?
          if (_followOnRoots.count(ai) == 0) {
            _followOnRoots[ai] = ai;
            _followOnNexts[ai] = targetAtom;
            _followOnRoots[targetAtom] = _followOnRoots[ai];
          } else if (_followOnNexts.count(ai) == 0) {
            // Chain the targetAtom to the current Atom
            // if the currentAtom has no followon references
            _followOnNexts[ai] = targetAtom;
            _followOnRoots[targetAtom] = _followOnRoots[ai];
          }
        } else if (_followOnRoots.find(targetAtom)->second == targetAtom) {
          // Is the targetAtom in chain with the targetAtom as the root ?
          bool changeRoots = false;
          if (_followOnRoots.count(ai) == 0) {
            _followOnRoots[ai] = ai;
            _followOnNexts[ai] = targetAtom;
            _followOnRoots[targetAtom] = _followOnRoots[ai];
            changeRoots = true;
          } else if (_followOnNexts.count(ai) == 0) {
            // Chain the targetAtom to the current Atom
            // if the currentAtom has no followon references
            if (_followOnRoots[ai] != _followOnRoots[targetAtom]) {
              _followOnNexts[ai] = targetAtom;
              _followOnRoots[targetAtom] = _followOnRoots[ai];
              changeRoots = true;
            }
          }
          // Change the roots of the targetAtom and its chain to 
          // the current atoms root
          if (changeRoots) {
            const DefinedAtom *a = _followOnRoots[targetAtom];
            while (true) {
              _followOnRoots[a] = _followOnRoots[ai];
              // Set all the follow on's for the targetAtom to be
              // the current root
              AtomToAtomT::iterator targetFollowOnAtomsIter =
                  _followOnNexts.find(a);
              if (targetFollowOnAtomsIter != _followOnNexts.end())
                a = targetFollowOnAtomsIter->second;
              else
                break;
            }
          } // changeRoots
        }   // Is targetAtom root 
      }     // kindLayoutBefore
    }       //  Reference
  }         // atom iteration
}           // end function 


/// Build an ordinal override map by traversing the followon chain, and
/// assigning ordinals to each atom, if the atoms have their ordinals 
/// already assigned skip the atom and move to the next. This is the 
/// main map thats used to sort the atoms while comparing two atoms together
void LayoutPass::buildOrdinalOverrideMap(MutableFile::DefinedAtomRange &range) {
  uint64_t index = 0;
  for (auto ai : range) {
    const DefinedAtom *atom = ai;
    AtomToAtomT::iterator start = _followOnRoots.find(atom);
    if (start != _followOnRoots.end()) {
      for (const DefinedAtom *nextAtom = start->second; nextAtom != NULL;
           nextAtom = _followOnNexts[nextAtom]) {
        AtomToOrdinalT::iterator pos = _ordinalOverrideMap.find(nextAtom);
        if (pos == _ordinalOverrideMap.end()) {
          _ordinalOverrideMap[nextAtom] = index++;
        }
      }
    } else {
      _ordinalOverrideMap[atom] = index;
    }
  }
}

/// Perform the actual pass 
void LayoutPass::perform(MutableFile &mergedFile) {
  MutableFile::DefinedAtomRange atomRange = mergedFile.definedAtoms();

  // Build follow on tables
  buildFollowOnTable(atomRange);

  // Build Ingroup reference table
  buildInGroupTable(atomRange);

  // Build preceded by tables
  buildPrecededByTable(atomRange);

  // Build override maps
  buildOrdinalOverrideMap(atomRange);

  // sort the atoms
  std::sort(atomRange.begin(), atomRange.end(), _compareAtoms);
}