/src/solidity/libyul/backends/evm/StackHelpers.h

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/*
  This file is part of solidity.

  solidity is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  solidity is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with solidity.  If not, see <http://www.gnu.org/licenses/>.
*/
// SPDX-License-Identifier: GPL-3.0

#pragma once

#include <libyul/backends/evm/ControlFlowGraph.h>
#include <libyul/Exceptions.h>
#include <libyul/Utilities.h>

#include <libsolutil/Visitor.h>

#include <range/v3/algorithm/all_of.hpp>
#include <range/v3/algorithm/any_of.hpp>
#include <range/v3/view/enumerate.hpp>
#include <range/v3/view/iota.hpp>
#include <range/v3/view/reverse.hpp>
#include <range/v3/view/take.hpp>

namespace solidity::yul
{

inline std::string stackSlotToString(StackSlot const& _slot, Dialect const& _dialect)
{
  return std::visit(util::GenericVisitor{
    [&](FunctionCallReturnLabelSlot const& _ret) -> std::string { return "RET[" + std::string(resolveFunctionName(_ret.call.get().functionName, _dialect)) + "]"; },
    [](FunctionReturnLabelSlot const&) -> std::string { return "RET"; },
    [](VariableSlot const& _var) { return _var.variable.get().name.str(); },
    [](LiteralSlot const& _lit) { return toCompactHexWithPrefix(_lit.value); },
    [&](TemporarySlot const& _tmp) -> std::string { return "TMP[" + std::string(resolveFunctionName(_tmp.call.get().functionName, _dialect)) + ", " + std::to_string(_tmp.index) + "]"; },
    [](JunkSlot const&) -> std::string { return "JUNK"; }
  }, _slot);
}

inline std::string stackToString(Stack const& _stack, Dialect const& _dialect)
{
  std::string result("[ ");
  for (auto const& slot: _stack)
    result += stackSlotToString(slot, _dialect) + ' ';
  result += ']';
  return result;
}


// Used as an interface for the stack shuffler below.
// The shuffle operation class is expected to internally keep track of a current stack layout (the "source layout")
// that the shuffler is supposed to shuffle to a fixed target stack layout.
// The shuffler works iteratively. At each iteration it instantiates an instance of the shuffle operations and
// queries it for various information about the current source stack layout and the target layout, as described
// in the interface below.
// Based on that information the shuffler decides which is the next optimal operation to perform on the stack
// and calls the corresponding entry point in the shuffling operations (swap, pushOrDupTarget or pop).
template<typename ShuffleOperations>
concept ShuffleOperationConcept = requires(ShuffleOperations ops, size_t sourceOffset, size_t targetOffset, size_t depth) {
  // Returns true, iff the current slot at sourceOffset in source layout is a suitable slot at targetOffset.
  { ops.isCompatible(sourceOffset, targetOffset) } -> std::convertible_to<bool>;
  // Returns true, iff the slots at the two given source offsets are identical.
  { ops.sourceIsSame(sourceOffset, sourceOffset) } -> std::convertible_to<bool>;
  // Returns a positive integer n, if the slot at the given source offset needs n more copies.
  // Returns a negative integer -n, if the slot at the given source offsets occurs n times too many.
  // Returns zero if the amount of occurrences, in the current source layout, of the slot at the given source offset
  // matches the desired amount of occurrences in the target.
  { ops.sourceMultiplicity(sourceOffset) } -> std::convertible_to<int>;
  // Returns a positive integer n, if the slot at the given target offset needs n more copies.
  // Returns a negative integer -n, if the slot at the given target offsets occurs n times too many.
  // Returns zero if the amount of occurrences, in the current source layout, of the slot at the given target offset
  // matches the desired amount of occurrences in the target.
  { ops.targetMultiplicity(targetOffset) } -> std::convertible_to<int>;
  // Returns true, iff any slot is compatible with the given target offset.
  { ops.targetIsArbitrary(targetOffset) } -> std::convertible_to<bool>;
  // Returns the number of slots in the source layout.
  { ops.sourceSize() } -> std::convertible_to<size_t>;
  // Returns the number of slots in the target layout.
  { ops.targetSize() } -> std::convertible_to<size_t>;
  // Swaps the top most slot in the source with the slot `depth` slots below the top.
  // In terms of EVM opcodes this is supposed to be a `SWAP<depth>`.
  // In terms of vectors this is supposed to be `std::swap(source.at(source.size() - depth - 1, source.top))`.
  { ops.swap(depth) };
  // Pops the top most slot in the source, i.e. the slot at offset ops.sourceSize() - 1.
  // In terms of EVM opcodes this is `POP`.
  // In terms of vectors this is `source.pop();`.
  { ops.pop() };
  // Dups or pushes the slot that is supposed to end up at the given target offset.
  { ops.pushOrDupTarget(targetOffset) };
  // Maximum reachable depth with swaps and dups.
  { ops.reachableStackDepth } -> std::convertible_to<size_t>;
};

/// Helper class that can perform shuffling of a source stack layout to a target stack layout via
/// abstracted shuffle operations.
template<ShuffleOperationConcept ShuffleOperations>
class Shuffler
{
public:
  /// Executes the stack shuffling operations. Instantiates an instance of ShuffleOperations
  /// in each iteration. Each iteration performs exactly one operation that modifies the stack.
  /// After `shuffle`, source and target have the same size and all slots in the source layout are
  /// compatible with the slots at the same target offset.
  template<typename... Args>
  static void shuffle(Args&&... args)
  {
    bool needsMoreShuffling = true;
    // The shuffling algorithm should always terminate in polynomial time, but we provide a limit
    // in case it does not terminate due to a bug.
    size_t iterationCount = 0;
    while (iterationCount < 1000 && (needsMoreShuffling = shuffleStep(std::forward<Args>(args)...)))
      ++iterationCount;
    yulAssert(!needsMoreShuffling, "Could not create stack layout after 1000 iterations.");
  }
private:
  // If dupping an ideal slot causes a slot that will still be required to become unreachable, then dup
  // the latter slot first.
  // @returns true, if it performed a dup.
  static bool dupDeepSlotIfRequired(ShuffleOperations& _ops)
  {
    // Check if the stack is large enough for anything to potentially become unreachable.
    if (_ops.sourceSize() < (_ops.reachableStackDepth - 1))
      return false;
    // Check whether any deep slot might still be needed later (i.e. we still need to reach it with a DUP or SWAP).
    for (size_t sourceOffset: ranges::views::iota(0u, _ops.sourceSize() - (_ops.reachableStackDepth - 1)))
    {
      // This slot needs to be moved.
      if (!_ops.isCompatible(sourceOffset, sourceOffset))
      {
        // If the current top fixes the slot, swap it down now.
        if (_ops.isCompatible(_ops.sourceSize() - 1, sourceOffset))
        {
          _ops.swap(_ops.sourceSize() - sourceOffset - 1);
          return true;
        }
        // Bring up a slot to fix this now, if possible.
        if (bringUpTargetSlot(_ops, sourceOffset))
          return true;
        // Otherwise swap up the slot that will fix the offending slot.
        for (auto offset: ranges::views::iota(sourceOffset + 1, _ops.sourceSize()))
          if (_ops.isCompatible(offset, sourceOffset))
          {
            _ops.swap(_ops.sourceSize() - offset - 1);
            return true;
          }
        // Otherwise give up - we will need stack compression or stack limit evasion.
      }
      // We need another copy of this slot.
      else if (_ops.sourceMultiplicity(sourceOffset) > 0)
      {
        // If this slot occurs again later, we skip this occurrence.
        if (ranges::any_of(
          ranges::views::iota(sourceOffset + 1, _ops.sourceSize()),
          [&](size_t _offset) { return _ops.sourceIsSame(sourceOffset, _offset); }
        ))
          continue;
        // Bring up the target slot that would otherwise become unreachable.
        for (size_t targetOffset: ranges::views::iota(0u, _ops.targetSize()))
          if (!_ops.targetIsArbitrary(targetOffset) && _ops.isCompatible(sourceOffset, targetOffset))
          {
            _ops.pushOrDupTarget(targetOffset);
            return true;
          }
      }
    }
    return false;
  }
  /// Finds a slot to dup or push with the aim of eventually fixing @a _targetOffset in the target.
  /// In the simplest case, the slot at @a _targetOffset has a multiplicity > 0, i.e. it can directly be dupped or pushed
  /// and the next iteration will fix @a _targetOffset.
  /// But, in general, there may already be enough copies of the slot that is supposed to end up at @a _targetOffset
  /// on stack, s.t. it cannot be dupped again. In that case there has to be a copy of the desired slot on stack already
  /// elsewhere that is not yet in place (`nextOffset` below). The fact that ``nextOffset`` is not in place means that
  /// we can (recursively) try bringing up the slot that is supposed to end up at ``nextOffset`` in the *target*.
  /// When the target slot at ``nextOffset`` is fixed, the current source slot at ``nextOffset`` will be
  /// at the stack top, which is the slot required at @a _targetOffset.
  static bool bringUpTargetSlot(ShuffleOperations& _ops, size_t _targetOffset)
  {
    std::list<size_t> toVisit{_targetOffset};
    std::set<size_t> visited;

    while (!toVisit.empty())
    {
      auto offset = *toVisit.begin();
      toVisit.erase(toVisit.begin());
      visited.emplace(offset);
      if (_ops.targetMultiplicity(offset) > 0)
      {
        _ops.pushOrDupTarget(offset);
        return true;
      }
      // There must be another slot we can dup/push that will lead to the target slot at ``offset`` to be fixed.
      for (auto nextOffset: ranges::views::iota(0u, std::min(_ops.sourceSize(), _ops.targetSize())))
        if (
          !_ops.isCompatible(nextOffset, nextOffset) &&
          _ops.isCompatible(nextOffset, offset)
        )
          if (!visited.count(nextOffset))
            toVisit.emplace_back(nextOffset);
    }
    return false;
  }
  /// Performs a single stack operation, transforming the source layout closer to the target layout.
  template<typename... Args>
  static bool shuffleStep(Args&&... args)
  {
    ShuffleOperations ops{std::forward<Args>(args)...};

    // All source slots are final.
    if (ranges::all_of(
      ranges::views::iota(0u, ops.sourceSize()),
      [&](size_t _index) { return ops.isCompatible(_index, _index); }
    ))
    {
      // Bring up all remaining target slots, if any, or terminate otherwise.
      if (ops.sourceSize() < ops.targetSize())
      {
        if (!dupDeepSlotIfRequired(ops))
          yulAssert(bringUpTargetSlot(ops, ops.sourceSize()), "");
        return true;
      }
      return false;
    }

    size_t sourceTop = ops.sourceSize() - 1;
    // If we no longer need the current stack top, we pop it, unless we need an arbitrary slot at this position
    // in the target.
    if (
      ops.sourceMultiplicity(sourceTop) < 0 &&
      !ops.targetIsArbitrary(sourceTop)
    )
    {
      ops.pop();
      return true;
    }

    yulAssert(ops.targetSize() > 0, "");

    // If the top is not supposed to be exactly what is on top right now, try to find a lower position to swap it to.
    if (!ops.isCompatible(sourceTop, sourceTop) || ops.targetIsArbitrary(sourceTop))
      for (size_t offset: ranges::views::iota(0u, std::min(ops.sourceSize(), ops.targetSize())))
        // It makes sense to swap to a lower position, if
        if (
          !ops.isCompatible(offset, offset) && // The lower slot is not already in position.
          !ops.sourceIsSame(offset, sourceTop) && // We would not just swap identical slots.
          ops.isCompatible(sourceTop, offset) // The lower position wants to have this slot.
        )
        {
          // We cannot swap that deep.
          if (ops.sourceSize() - offset - 1 > ops.reachableStackDepth)
          {
            // If there is a reachable slot to be removed, park the current top there.
            for (size_t swapDepth: ranges::views::iota(1u, ops.reachableStackDepth + 1u) | ranges::views::reverse)
              if (ops.sourceMultiplicity(ops.sourceSize() - 1 - swapDepth) < 0)
              {
                ops.swap(swapDepth);
                if (ops.targetIsArbitrary(sourceTop))
                  // Usually we keep a slot that is to-be-removed, if the current top is arbitrary.
                  // However, since we are in a stack-too-deep situation, pop it immediately
                  // to compress the stack (we can always push back junk in the end).
                  ops.pop();
                return true;
              }
            // Otherwise we rely on stack compression or stack-to-memory.
          }
          ops.swap(ops.sourceSize() - offset - 1);
          return true;
        }

    // ops.sourceSize() > ops.targetSize() cannot be true anymore, since if the source top is no longer required,
    // we already popped it, and if it is required, we already swapped it down to a suitable target position.
    yulAssert(ops.sourceSize() <= ops.targetSize(), "");

    // If a lower slot should be removed, try to bring up the slot that should end up there and bring it up.
    // Note that after the cases above, there will always be a target slot to duplicate in this case.
    for (size_t offset: ranges::views::iota(0u, ops.sourceSize()))
      if (
        !ops.isCompatible(offset, offset) && // The lower slot is not already in position.
        ops.sourceMultiplicity(offset) < 0 && // We have too many copies of this slot.
        offset <= ops.targetSize() && // There is a target slot at this position.
        !ops.targetIsArbitrary(offset) // And that target slot is not arbitrary.
      )
      {
        if (!dupDeepSlotIfRequired(ops))
          yulAssert(bringUpTargetSlot(ops, offset), "");
        return true;
      }

    // At this point we want to keep all slots.
    for (size_t i = 0; i < ops.sourceSize(); ++i)
      yulAssert(ops.sourceMultiplicity(i) >= 0, "");
    yulAssert(ops.sourceSize() <= ops.targetSize(), "");

    // If the top is not in position, try to find a slot that wants to be at the top and swap it up.
    if (!ops.isCompatible(sourceTop, sourceTop))
      for (size_t sourceOffset: ranges::views::iota(0u, ops.sourceSize()))
        if (
          !ops.isCompatible(sourceOffset, sourceOffset) &&
          ops.isCompatible(sourceOffset, sourceTop)
        )
        {
          ops.swap(ops.sourceSize() - sourceOffset - 1);
          return true;
        }

    // If we still need more slots, produce a suitable one.
    if (ops.sourceSize() < ops.targetSize())
    {
      if (!dupDeepSlotIfRequired(ops))
        yulAssert(bringUpTargetSlot(ops, ops.sourceSize()), "");
      return true;
    }

    // The stack has the correct size, each slot has the correct number of copies and the top is in position.
    yulAssert(ops.sourceSize() == ops.targetSize(), "");
    size_t size = ops.sourceSize();
    for (size_t i = 0; i < ops.sourceSize(); ++i)
      yulAssert(ops.sourceMultiplicity(i) == 0 && (ops.targetIsArbitrary(i) || ops.targetMultiplicity(i) == 0), "");
    yulAssert(ops.isCompatible(sourceTop, sourceTop), "");

    auto swappableOffsets = ranges::views::iota(size > ops.reachableStackDepth + 1u ? size - (ops.reachableStackDepth + 1u) : 0u, size);

    // If we find a lower slot that is out of position, but also compatible with the top, swap that up.
    for (size_t offset: swappableOffsets)
      if (!ops.isCompatible(offset, offset) && ops.isCompatible(sourceTop, offset))
      {
        ops.swap(size - offset - 1);
        return true;
      }
    // Swap up any reachable slot that is still out of position.
    for (size_t offset: swappableOffsets)
      if (!ops.isCompatible(offset, offset) && !ops.sourceIsSame(offset, sourceTop))
      {
        ops.swap(size - offset - 1);
        return true;
      }
    // We are in a stack-too-deep situation and try to reduce the stack size.
    // If the current top is merely kept since the target slot is arbitrary, pop it.
    if (ops.targetIsArbitrary(sourceTop) && ops.sourceMultiplicity(sourceTop) <= 0)
    {
      ops.pop();
      return true;
    }
    // If any reachable slot is merely kept, since the target slot is arbitrary, swap it up and pop it.
    for (size_t offset: swappableOffsets)
      if (ops.targetIsArbitrary(offset) && ops.sourceMultiplicity(offset) <= 0)
      {
        ops.swap(size - offset - 1);
        ops.pop();
        return true;
      }
    // We cannot avoid a stack-too-deep error. Repeat the above without restricting to reachable slots.
    for (size_t offset: ranges::views::iota(0u, size))
      if (!ops.isCompatible(offset, offset) && ops.isCompatible(sourceTop, offset))
      {
        ops.swap(size - offset - 1);
        return true;
      }
    for (size_t offset: ranges::views::iota(0u, size))
      if (!ops.isCompatible(offset, offset) && !ops.sourceIsSame(offset, sourceTop))
      {
        ops.swap(size - offset - 1);
        return true;
      }
    yulAssert(false, "");

    // FIXME: Workaround for spurious GCC 12.1 warning (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105794)
    throw std::exception();
  }
};

/// A simple optimized map for mapping StackSlots to ints.
class Multiplicity
{
public:
  int& operator[](StackSlot const& _slot)
  {
    if (auto* p = std::get_if<FunctionCallReturnLabelSlot>(&_slot))
      return m_functionCallReturnLabelSlotMultiplicity[*p];
    if (std::holds_alternative<FunctionReturnLabelSlot>(_slot))
      return m_functionReturnLabelSlotMultiplicity;
    if (auto* p = std::get_if<VariableSlot>(&_slot))
      return m_variableSlotMultiplicity[*p];
    if (auto* p = std::get_if<LiteralSlot>(&_slot))
      return m_literalSlotMultiplicity[*p];
    if (auto* p = std::get_if<TemporarySlot>(&_slot))
      return m_temporarySlotMultiplicity[*p];
    yulAssert(std::holds_alternative<JunkSlot>(_slot));
    return m_junkSlotMultiplicity;
  }

  int at(StackSlot const& _slot) const
  {
    if (auto* p = std::get_if<FunctionCallReturnLabelSlot>(&_slot))
      return m_functionCallReturnLabelSlotMultiplicity.at(*p);
    if (std::holds_alternative<FunctionReturnLabelSlot>(_slot))
      return m_functionReturnLabelSlotMultiplicity;
    if (auto* p = std::get_if<VariableSlot>(&_slot))
      return m_variableSlotMultiplicity.at(*p);
    if (auto* p = std::get_if<LiteralSlot>(&_slot))
      return m_literalSlotMultiplicity.at(*p);
    if (auto* p = std::get_if<TemporarySlot>(&_slot))
      return m_temporarySlotMultiplicity.at(*p);
    yulAssert(std::holds_alternative<JunkSlot>(_slot));
    return m_junkSlotMultiplicity;
  }

private:
  std::map<FunctionCallReturnLabelSlot, int> m_functionCallReturnLabelSlotMultiplicity;
  int m_functionReturnLabelSlotMultiplicity = 0;
  std::map<VariableSlot, int> m_variableSlotMultiplicity;
  std::map<LiteralSlot, int> m_literalSlotMultiplicity;
  std::map<TemporarySlot, int> m_temporarySlotMultiplicity;
  int m_junkSlotMultiplicity = 0;
};

/// Transforms @a _currentStack to @a _targetStack, invoking the provided shuffling operations.
/// Modifies @a _currentStack itself after each invocation of the shuffling operations.
/// @a _swap is a function with signature void(unsigned) that is called when the top most slot is swapped with
/// the slot `depth` slots below the top. In terms of EVM opcodes this is supposed to be a `SWAP<depth>`.
/// @a _pushOrDup is a function with signature void(StackSlot const&) that is called to push or dup the slot given as
/// its argument to the stack top.
/// @a _pop is a function with signature void() that is called when the top most slot is popped.
template<typename Swap, typename PushOrDup, typename Pop>
void createStackLayout(
  Stack& _currentStack,
  Stack const& _targetStack,
  Swap _swap,
  PushOrDup _pushOrDup,
  Pop _pop,
  size_t _reachableStackDepth
)
{
  struct ShuffleOperations
  {
    Stack& currentStack;
    Stack const& targetStack;
    Swap swapCallback;
    PushOrDup pushOrDupCallback;
    Pop popCallback;
    Multiplicity multiplicity;
    size_t reachableStackDepth;
    ShuffleOperations(
      Stack& _currentStack,
      Stack const& _targetStack,
      Swap _swap,
      PushOrDup _pushOrDup,
      Pop _pop,
      size_t _reachableStackDepth
    ):
      currentStack(_currentStack),
      targetStack(_targetStack),
      swapCallback(_swap),
      pushOrDupCallback(_pushOrDup),
      popCallback(_pop),
      reachableStackDepth(_reachableStackDepth)
    {
      for (auto const& slot: currentStack)
        --multiplicity[slot];
      for (auto&& [offset, slot]: targetStack | ranges::views::enumerate)
        if (std::holds_alternative<JunkSlot>(slot) && offset < currentStack.size())
          ++multiplicity[currentStack.at(offset)];
        else
          ++multiplicity[slot];
    }
    bool isCompatible(size_t _source, size_t _target)
    {
      return
        _source < currentStack.size() &&
        _target < targetStack.size() &&
        (
          std::holds_alternative<JunkSlot>(targetStack.at(_target)) ||
          currentStack.at(_source) == targetStack.at(_target)
        );
    }
    bool sourceIsSame(size_t _lhs, size_t _rhs) { return currentStack.at(_lhs) == currentStack.at(_rhs); }
    int sourceMultiplicity(size_t _offset) { return multiplicity.at(currentStack.at(_offset)); }
    int targetMultiplicity(size_t _offset) { return multiplicity.at(targetStack.at(_offset)); }
    bool targetIsArbitrary(size_t offset)
    {
      return offset < targetStack.size() && std::holds_alternative<JunkSlot>(targetStack.at(offset));
    }
    void swap(size_t _i)
    {
      swapCallback(static_cast<unsigned>(_i));
      std::swap(currentStack.at(currentStack.size() - _i - 1), currentStack.back());
    }
    size_t sourceSize() { return currentStack.size(); }
    size_t targetSize() { return targetStack.size(); }
    void pop()
    {
      popCallback();
      currentStack.pop_back();
    }
    void pushOrDupTarget(size_t _offset)
    {
      auto const& targetSlot = targetStack.at(_offset);
      pushOrDupCallback(targetSlot);
      currentStack.push_back(targetSlot);
    }
  };

  Shuffler<ShuffleOperations>::shuffle(_currentStack, _targetStack, _swap, _pushOrDup, _pop, _reachableStackDepth);

  yulAssert(_currentStack.size() == _targetStack.size(), "");
  for (auto&& [current, target]: ranges::zip_view(_currentStack, _targetStack))
    if (std::holds_alternative<JunkSlot>(target))
      current = JunkSlot{};
    else
      yulAssert(current == target, "");
}

}

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Created: 2025-09-08 08:10