solidity/libyul/backends/evm/StackLayoutGenerator.cpp

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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
/**
* Stack layout generator for Yul to EVM code generation.
*/
#include <libyul/backends/evm/StackLayoutGenerator.h>
#include <libyul/backends/evm/StackHelpers.h>
#include <libevmasm/GasMeter.h>
#include <libsolutil/Algorithms.h>
#include <libsolutil/cxx20.h>
#include <libsolutil/Visitor.h>
#include <range/v3/algorithm/any_of.hpp>
#include <range/v3/algorithm/find.hpp>
#include <range/v3/range/conversion.hpp>
#include <range/v3/view/all.hpp>
#include <range/v3/view/concat.hpp>
#include <range/v3/view/drop.hpp>
#include <range/v3/view/drop_last.hpp>
#include <range/v3/view/filter.hpp>
#include <range/v3/view/iota.hpp>
#include <range/v3/view/map.hpp>
#include <range/v3/view/reverse.hpp>
#include <range/v3/view/take.hpp>
#include <range/v3/view/take_last.hpp>
#include <range/v3/view/transform.hpp>
using namespace solidity;
using namespace solidity::yul;
using namespace std;
StackLayout StackLayoutGenerator::run(CFG const& _cfg)
{
StackLayout stackLayout;
StackLayoutGenerator{stackLayout}.processEntryPoint(*_cfg.entry);
for (auto& functionInfo: _cfg.functionInfo | ranges::views::values)
StackLayoutGenerator{stackLayout}.processEntryPoint(*functionInfo.entry);
return stackLayout;
}
map<YulString, vector<StackLayoutGenerator::StackTooDeep>> StackLayoutGenerator::reportStackTooDeep(CFG const& _cfg)
{
map<YulString, vector<StackLayoutGenerator::StackTooDeep>> stackTooDeepErrors;
stackTooDeepErrors[YulString{}] = reportStackTooDeep(_cfg, YulString{});
for (auto const& function: _cfg.functions)
if (auto errors = reportStackTooDeep(_cfg, function->name); !errors.empty())
stackTooDeepErrors[function->name] = move(errors);
return stackTooDeepErrors;
}
vector<StackLayoutGenerator::StackTooDeep> StackLayoutGenerator::reportStackTooDeep(CFG const& _cfg, YulString _functionName)
{
StackLayout stackLayout;
CFG::FunctionInfo const* functionInfo = nullptr;
if (!_functionName.empty())
{
functionInfo = &ranges::find(
_cfg.functionInfo,
_functionName,
util::mapTuple([](auto&&, auto&& info) { return info.function.name; })
)->second;
yulAssert(functionInfo, "Function not found.");
}
StackLayoutGenerator generator{stackLayout};
CFG::BasicBlock const* entry = functionInfo ? functionInfo->entry : _cfg.entry;
generator.processEntryPoint(*entry);
return generator.reportStackTooDeep(*entry);
}
StackLayoutGenerator::StackLayoutGenerator(StackLayout& _layout): m_layout(_layout)
{
}
namespace
{
/// @returns all stack too deep errors that would occur when shuffling @a _source to @a _target.
vector<StackLayoutGenerator::StackTooDeep> findStackTooDeep(Stack const& _source, Stack const& _target)
{
Stack currentStack = _source;
vector<StackLayoutGenerator::StackTooDeep> stackTooDeepErrors;
auto getVariableChoices = [](auto&& _range) {
vector<YulString> result;
for (auto const& slot: _range)
if (auto const* variableSlot = get_if<VariableSlot>(&slot))
if (!util::contains(result, variableSlot->variable.get().name))
result.push_back(variableSlot->variable.get().name);
return result;
};
::createStackLayout(
currentStack,
_target,
[&](unsigned _i)
{
if (_i > 16)
stackTooDeepErrors.emplace_back(StackLayoutGenerator::StackTooDeep{
_i - 16,
getVariableChoices(currentStack | ranges::views::take_last(_i + 1))
});
},
[&](StackSlot const& _slot)
{
if (canBeFreelyGenerated(_slot))
return;
if (
auto depth = util::findOffset(currentStack | ranges::views::reverse, _slot);
depth && *depth >= 16
)
stackTooDeepErrors.emplace_back(StackLayoutGenerator::StackTooDeep{
*depth - 15,
getVariableChoices(currentStack | ranges::views::take_last(*depth + 1))
});
},
[&]() {}
);
return stackTooDeepErrors;
}
/// @returns the ideal stack to have before executing an operation that outputs @a _operationOutput, s.t.
/// shuffling to @a _post is cheap (excluding the input of the operation itself).
/// If @a _generateSlotOnTheFly returns true for a slot, this slot should not occur in the ideal stack, but
/// rather be generated on the fly during shuffling.
template<typename Callable>
Stack createIdealLayout(Stack const& _operationOutput, Stack const& _post, Callable _generateSlotOnTheFly)
{
struct PreviousSlot { size_t slot; };
// Determine the number of slots that have to be on stack before executing the operation (excluding
// the inputs of the operation itself).
// That is slots that should not be generated on the fly and are not outputs of the operation.
size_t preOperationLayoutSize = _post.size();
for (auto const& slot: _post)
if (util::contains(_operationOutput, slot) || _generateSlotOnTheFly(slot))
--preOperationLayoutSize;
// The symbolic layout directly after the operation has the form
// PreviousSlot{0}, ..., PreviousSlot{n}, [output<0>], ..., [output<m>]
auto layout = ranges::views::iota(0u, preOperationLayoutSize) |
ranges::views::transform([](size_t _index) { return PreviousSlot{_index}; }) |
ranges::to<vector<variant<PreviousSlot, StackSlot>>>;
layout += _operationOutput;
// Shortcut for trivial case.
if (layout.empty())
return Stack{};
// Next we will shuffle the layout to the post stack using ShuffleOperations
// that are aware of PreviousSlot's.
struct ShuffleOperations
{
vector<variant<PreviousSlot, StackSlot>>& layout;
Stack const& post;
std::set<StackSlot> outputs;
std::map<StackSlot, int> multiplicity;
Callable generateSlotOnTheFly;
ShuffleOperations(
vector<variant<PreviousSlot, StackSlot>>& _layout,
Stack const& _post,
Callable _generateSlotOnTheFly
): layout(_layout), post(_post), generateSlotOnTheFly(_generateSlotOnTheFly)
{
for (auto const& layoutSlot: layout)
if (StackSlot const* slot = get_if<StackSlot>(&layoutSlot))
outputs.insert(*slot);
for (auto const& layoutSlot: layout)
if (StackSlot const* slot = get_if<StackSlot>(&layoutSlot))
--multiplicity[*slot];
for (auto&& slot: post)
if (outputs.count(slot) || generateSlotOnTheFly(slot))
++multiplicity[slot];
}
bool isCompatible(size_t _source, size_t _target)
{
return
_source < layout.size() &&
_target < post.size() &&
(
std::holds_alternative<JunkSlot>(post.at(_target)) ||
std::visit(util::GenericVisitor{
[&](PreviousSlot const&) {
return !outputs.count(post.at(_target)) && !generateSlotOnTheFly(post.at(_target));
},
[&](StackSlot const& _s) { return _s == post.at(_target); }
}, layout.at(_source))
);
}
bool sourceIsSame(size_t _lhs, size_t _rhs)
{
return std::visit(util::GenericVisitor{
[&](PreviousSlot const&, PreviousSlot const&) { return true; },
[&](StackSlot const& _lhs, StackSlot const& _rhs) { return _lhs == _rhs; },
[&](auto const&, auto const&) { return false; }
}, layout.at(_lhs), layout.at(_rhs));
}
int sourceMultiplicity(size_t _offset)
{
return std::visit(util::GenericVisitor{
[&](PreviousSlot const&) { return 0; },
[&](StackSlot const& _s) { return multiplicity.at(_s); }
}, layout.at(_offset));
}
int targetMultiplicity(size_t _offset)
{
if (!outputs.count(post.at(_offset)) && !generateSlotOnTheFly(post.at(_offset)))
return 0;
return multiplicity.at(post.at(_offset));
}
bool targetIsArbitrary(size_t _offset)
{
return _offset < post.size() && std::holds_alternative<JunkSlot>(post.at(_offset));
}
void swap(size_t _i)
{
yulAssert(!holds_alternative<PreviousSlot>(layout.at(layout.size() - _i - 1)) || !holds_alternative<PreviousSlot>(layout.back()), "");
std::swap(layout.at(layout.size() - _i - 1), layout.back());
}
size_t sourceSize() { return layout.size(); }
size_t targetSize() { return post.size(); }
void pop() { layout.pop_back(); }
void pushOrDupTarget(size_t _offset) { layout.push_back(post.at(_offset)); }
};
Shuffler<ShuffleOperations>::shuffle(layout, _post, _generateSlotOnTheFly);
// Now we can construct the ideal layout before the operation.
// "layout" has shuffled the PreviousSlot{x} to new places using minimal operations to move the operation
// output in place. The resulting permutation of the PreviousSlot yields the ideal positions of slots
// before the operation, i.e. if PreviousSlot{2} is at a position at which _post contains VariableSlot{"tmp"},
// then we want the variable tmp in the slot at offset 2 in the layout before the operation.
vector<optional<StackSlot>> idealLayout(_post.size(), nullopt);
2021-12-15 23:11:59 +00:00
for (auto&& [slot, idealPosition]: ranges::zip_view(_post, layout))
if (PreviousSlot* previousSlot = std::get_if<PreviousSlot>(&idealPosition))
idealLayout.at(previousSlot->slot) = slot;
// The tail of layout must have contained the operation outputs and will not have been assigned slots in the last loop.
while (!idealLayout.empty() && !idealLayout.back())
idealLayout.pop_back();
yulAssert(idealLayout.size() == preOperationLayoutSize, "");
return idealLayout | ranges::views::transform([](optional<StackSlot> s) {
yulAssert(s, "");
return *s;
}) | ranges::to<Stack>;
}
}
Stack StackLayoutGenerator::propagateStackThroughOperation(Stack _exitStack, CFG::Operation const& _operation, bool _aggressiveStackCompression)
{
// Enable aggressive stack compression for recursive calls.
if (auto const* functionCall = get_if<CFG::FunctionCall>(&_operation.operation))
if (functionCall->recursive)
_aggressiveStackCompression = true;
// This is a huge tradeoff between code size, gas cost and stack size.
auto generateSlotOnTheFly = [&](StackSlot const& _slot) {
return _aggressiveStackCompression && canBeFreelyGenerated(_slot);
};
// Determine the ideal permutation of the slots in _exitLayout that are not operation outputs (and not to be
// generated on the fly), s.t. shuffling the `stack + _operation.output` to _exitLayout is cheap.
Stack stack = createIdealLayout(_operation.output, _exitStack, generateSlotOnTheFly);
// Make sure the resulting previous slots do not overlap with any assignmed variables.
if (auto const* assignment = get_if<CFG::Assignment>(&_operation.operation))
for (auto& stackSlot: stack)
if (auto const* varSlot = get_if<VariableSlot>(&stackSlot))
yulAssert(!util::contains(assignment->variables, *varSlot), "");
// Since stack+_operation.output can be easily shuffled to _exitLayout, the desired layout before the operation
// is stack+_operation.input;
stack += _operation.input;
// Store the exact desired operation entry layout. The stored layout will be recreated by the code transform
// before executing the operation. However, this recreation can produce slots that can be freely generated or
// are duplicated, i.e. we can compress the stack afterwards without causing problems for code generation later.
m_layout.operationEntryLayout[&_operation] = stack;
// Remove anything from the stack top that can be freely generated or dupped from deeper on the stack.
while (!stack.empty())
{
if (canBeFreelyGenerated(stack.back()))
stack.pop_back();
else if (auto offset = util::findOffset(stack | ranges::views::reverse | ranges::views::drop(1), stack.back()))
{
if (*offset + 2 < 16)
stack.pop_back();
else
break;
}
else
break;
}
return stack;
}
Stack StackLayoutGenerator::propagateStackThroughBlock(Stack _exitStack, CFG::BasicBlock const& _block, bool _aggressiveStackCompression)
{
Stack stack = _exitStack;
for (auto&& [idx, operation]: _block.operations | ranges::views::enumerate | ranges::views::reverse)
{
Stack newStack = propagateStackThroughOperation(stack, operation, _aggressiveStackCompression);
if (!_aggressiveStackCompression && !findStackTooDeep(newStack, stack).empty())
// If we had stack errors, run again with aggressive stack compression.
return propagateStackThroughBlock(move(_exitStack), _block, true);
stack = move(newStack);
}
return stack;
}
void StackLayoutGenerator::processEntryPoint(CFG::BasicBlock const& _entry)
{
list<CFG::BasicBlock const*> toVisit{&_entry};
set<CFG::BasicBlock const*> visited;
// TODO: check whether visiting only a subset of these in the outer iteration below is enough.
list<pair<CFG::BasicBlock const*, CFG::BasicBlock const*>> backwardsJumps = collectBackwardsJumps(_entry);
while (!toVisit.empty())
{
// First calculate stack layouts without walking backwards jumps, i.e. assuming the current preliminary
// entry layout of the backwards jump target as the initial exit layout of the backwards-jumping block.
while (!toVisit.empty())
{
CFG::BasicBlock const *block = *toVisit.begin();
toVisit.pop_front();
if (visited.count(block))
continue;
if (std::optional<Stack> exitLayout = getExitLayoutOrStageDependencies(*block, visited, toVisit))
{
visited.emplace(block);
auto& info = m_layout.blockInfos[block];
info.exitLayout = *exitLayout;
info.entryLayout = propagateStackThroughBlock(info.exitLayout, *block);
for (auto entry: block->entries)
toVisit.emplace_back(entry);
}
else
continue;
}
// Determine which backwards jumps still require fixing and stage revisits of appropriate nodes.
for (auto [jumpingBlock, target]: backwardsJumps)
// This block jumps backwards, but does not provide all slots required by the jump target on exit.
// Therefore we need to visit the subgraph between ``target`` and ``jumpingBlock`` again.
if (ranges::any_of(
m_layout.blockInfos[target].entryLayout,
[exitLayout = m_layout.blockInfos[jumpingBlock].exitLayout](StackSlot const& _slot) {
return !util::contains(exitLayout, _slot);
}
))
{
// In particular we can visit backwards starting from ``jumpingBlock`` and mark all entries to-be-visited-
// again until we hit ``target``.
toVisit.emplace_front(jumpingBlock);
// Since we are likely to permute the entry layout of ``target``, we also visit its entries again.
// This is not required for correctness, since the set of stack slots will match, but it may move some
// required stack shuffling from the loop condition to outside the loop.
for (CFG::BasicBlock const* entry: target->entries)
visited.erase(entry);
util::BreadthFirstSearch<CFG::BasicBlock const*>{{jumpingBlock}}.run(
[&visited, target = target](CFG::BasicBlock const* _block, auto _addChild) {
visited.erase(_block);
if (_block == target)
return;
for (auto const* entry: _block->entries)
_addChild(entry);
}
);
// While the shuffled layout for ``target`` will be compatible, it can be worthwhile propagating
// it further up once more.
// This would mean not stopping at _block == target above, resp. even doing visited.clear() here, revisiting the entire graph.
// This is a tradeoff between the runtime of this process and the optimality of the result.
// Also note that while visiting the entire graph again *can* be helpful, it can also be detrimental.
}
}
stitchConditionalJumps(_entry);
fillInJunk(_entry);
}
optional<Stack> StackLayoutGenerator::getExitLayoutOrStageDependencies(
CFG::BasicBlock const& _block,
set<CFG::BasicBlock const*> const& _visited,
list<CFG::BasicBlock const*>& _toVisit
) const
{
return std::visit(util::GenericVisitor{
[&](CFG::BasicBlock::MainExit const&) -> std::optional<Stack>
{
// On the exit of the outermost block the stack can be empty.
return Stack{};
},
[&](CFG::BasicBlock::Jump const& _jump) -> std::optional<Stack>
{
if (_jump.backwards)
{
// Choose the best currently known entry layout of the jump target as initial exit.
// Note that this may not yet be the final layout.
if (auto* info = util::valueOrNullptr(m_layout.blockInfos, _jump.target))
return info->entryLayout;
return Stack{};
}
// If the current iteration has already visited the jump target, start from its entry layout.
if (_visited.count(_jump.target))
return m_layout.blockInfos.at(_jump.target).entryLayout;
// Otherwise stage the jump target for visit and defer the current block.
_toVisit.emplace_front(_jump.target);
return nullopt;
},
[&](CFG::BasicBlock::ConditionalJump const& _conditionalJump) -> std::optional<Stack>
{
bool zeroVisited = _visited.count(_conditionalJump.zero);
bool nonZeroVisited = _visited.count(_conditionalJump.nonZero);
if (zeroVisited && nonZeroVisited)
{
// If the current iteration has already visited both jump targets, start from its entry layout.
Stack stack = combineStack(
m_layout.blockInfos.at(_conditionalJump.zero).entryLayout,
m_layout.blockInfos.at(_conditionalJump.nonZero).entryLayout
);
// Additionally, the jump condition has to be at the stack top at exit.
stack.emplace_back(_conditionalJump.condition);
return stack;
}
// If one of the jump targets has not been visited, stage it for visit and defer the current block.
if (!zeroVisited)
_toVisit.emplace_front(_conditionalJump.zero);
if (!nonZeroVisited)
_toVisit.emplace_front(_conditionalJump.nonZero);
return nullopt;
},
[&](CFG::BasicBlock::FunctionReturn const& _functionReturn) -> std::optional<Stack>
{
// A function return needs the return variables and the function return label slot on stack.
yulAssert(_functionReturn.info, "");
Stack stack = _functionReturn.info->returnVariables | ranges::views::transform([](auto const& _varSlot){
return StackSlot{_varSlot};
}) | ranges::to<Stack>;
stack.emplace_back(FunctionReturnLabelSlot{_functionReturn.info->function});
return stack;
},
[&](CFG::BasicBlock::Terminated const&) -> std::optional<Stack>
{
// A terminating block can have an empty stack on exit.
return Stack{};
},
}, _block.exit);
}
list<pair<CFG::BasicBlock const*, CFG::BasicBlock const*>> StackLayoutGenerator::collectBackwardsJumps(CFG::BasicBlock const& _entry) const
{
list<pair<CFG::BasicBlock const*, CFG::BasicBlock const*>> backwardsJumps;
util::BreadthFirstSearch<CFG::BasicBlock const*>{{&_entry}}.run([&](CFG::BasicBlock const* _block, auto _addChild) {
std::visit(util::GenericVisitor{
[&](CFG::BasicBlock::MainExit const&) {},
[&](CFG::BasicBlock::Jump const& _jump)
{
if (_jump.backwards)
backwardsJumps.emplace_back(_block, _jump.target);
_addChild(_jump.target);
},
[&](CFG::BasicBlock::ConditionalJump const& _conditionalJump)
{
_addChild(_conditionalJump.zero);
_addChild(_conditionalJump.nonZero);
},
[&](CFG::BasicBlock::FunctionReturn const&) {},
[&](CFG::BasicBlock::Terminated const&) {},
}, _block->exit);
});
return backwardsJumps;
}
void StackLayoutGenerator::stitchConditionalJumps(CFG::BasicBlock const& _block)
{
util::BreadthFirstSearch<CFG::BasicBlock const*> breadthFirstSearch{{&_block}};
breadthFirstSearch.run([&](CFG::BasicBlock const* _block, auto _addChild) {
auto& info = m_layout.blockInfos.at(_block);
std::visit(util::GenericVisitor{
[&](CFG::BasicBlock::MainExit const&) {},
[&](CFG::BasicBlock::Jump const& _jump)
{
if (!_jump.backwards)
_addChild(_jump.target);
},
[&](CFG::BasicBlock::ConditionalJump const& _conditionalJump)
{
auto& zeroTargetInfo = m_layout.blockInfos.at(_conditionalJump.zero);
auto& nonZeroTargetInfo = m_layout.blockInfos.at(_conditionalJump.nonZero);
Stack exitLayout = info.exitLayout;
// The last block must have produced the condition at the stack top.
yulAssert(!exitLayout.empty(), "");
yulAssert(exitLayout.back() == _conditionalJump.condition, "");
// The condition is consumed by the jump.
exitLayout.pop_back();
auto fixJumpTargetEntry = [&](Stack const& _originalEntryLayout) -> Stack {
Stack newEntryLayout = exitLayout;
// Whatever the block being jumped to does not actually require, can be marked as junk.
for (auto& slot: newEntryLayout)
if (!util::contains(_originalEntryLayout, slot))
slot = JunkSlot{};
// Make sure everything the block being jumped to requires is actually present or can be generated.
for (auto const& slot: _originalEntryLayout)
yulAssert(canBeFreelyGenerated(slot) || util::contains(newEntryLayout, slot), "");
return newEntryLayout;
};
zeroTargetInfo.entryLayout = fixJumpTargetEntry(zeroTargetInfo.entryLayout);
nonZeroTargetInfo.entryLayout = fixJumpTargetEntry(nonZeroTargetInfo.entryLayout);
_addChild(_conditionalJump.zero);
_addChild(_conditionalJump.nonZero);
},
[&](CFG::BasicBlock::FunctionReturn const&) {},
[&](CFG::BasicBlock::Terminated const&) { },
}, _block->exit);
});
}
Stack StackLayoutGenerator::combineStack(Stack const& _stack1, Stack const& _stack2)
{
// TODO: it would be nicer to replace this by a constructive algorithm.
// Currently it uses a reduced version of the Heap Algorithm to partly brute-force, which seems
// to work decently well.
Stack commonPrefix;
for (auto&& [slot1, slot2]: ranges::zip_view(_stack1, _stack2))
{
if (!(slot1 == slot2))
break;
commonPrefix.emplace_back(slot1);
}
Stack stack1Tail = _stack1 | ranges::views::drop(commonPrefix.size()) | ranges::to<Stack>;
Stack stack2Tail = _stack2 | ranges::views::drop(commonPrefix.size()) | ranges::to<Stack>;
if (stack1Tail.empty())
return commonPrefix + compressStack(stack2Tail);
if (stack2Tail.empty())
return commonPrefix + compressStack(stack1Tail);
Stack candidate;
for (auto slot: stack1Tail)
if (!util::contains(candidate, slot))
candidate.emplace_back(slot);
for (auto slot: stack2Tail)
if (!util::contains(candidate, slot))
candidate.emplace_back(slot);
cxx20::erase_if(candidate, [](StackSlot const& slot) {
return holds_alternative<LiteralSlot>(slot) || holds_alternative<FunctionCallReturnLabelSlot>(slot);
});
auto evaluate = [&](Stack const& _candidate) -> size_t {
size_t numOps = 0;
Stack testStack = _candidate;
auto swap = [&](unsigned _swapDepth) { ++numOps; if (_swapDepth > 16) numOps += 1000; };
auto dupOrPush = [&](StackSlot const& _slot)
{
if (canBeFreelyGenerated(_slot))
return;
auto depth = util::findOffset(ranges::concat_view(commonPrefix, testStack) | ranges::views::reverse, _slot);
if (depth && *depth >= 16)
numOps += 1000;
};
createStackLayout(testStack, stack1Tail, swap, dupOrPush, [&](){});
testStack = _candidate;
createStackLayout(testStack, stack2Tail, swap, dupOrPush, [&](){});
return numOps;
};
// See https://en.wikipedia.org/wiki/Heap's_algorithm
size_t n = candidate.size();
Stack bestCandidate = candidate;
size_t bestCost = evaluate(candidate);
std::vector<size_t> c(n, 0);
size_t i = 1;
while (i < n)
{
if (c[i] < i)
{
if (i & 1)
std::swap(candidate.front(), candidate[i]);
else
std::swap(candidate[c[i]], candidate[i]);
size_t cost = evaluate(candidate);
if (cost < bestCost)
{
bestCost = cost;
bestCandidate = candidate;
}
++c[i];
// Note that for a proper implementation of the Heap algorithm this would need to revert back to ``i = 1.``
// However, the incorrect implementation produces decent result and the proper version would have n!
// complexity and is thereby not feasible.
++i;
}
else
{
c[i] = 0;
++i;
}
}
return commonPrefix + bestCandidate;
}
vector<StackLayoutGenerator::StackTooDeep> StackLayoutGenerator::reportStackTooDeep(CFG::BasicBlock const& _entry) const
{
vector<StackTooDeep> stackTooDeepErrors;
util::BreadthFirstSearch<CFG::BasicBlock const*> breadthFirstSearch{{&_entry}};
breadthFirstSearch.run([&](CFG::BasicBlock const* _block, auto _addChild) {
Stack currentStack = m_layout.blockInfos.at(_block).entryLayout;
for (auto const& operation: _block->operations)
{
Stack& operationEntry = m_layout.operationEntryLayout.at(&operation);
stackTooDeepErrors += findStackTooDeep(currentStack, operationEntry);
currentStack = operationEntry;
for (size_t i = 0; i < operation.input.size(); i++)
currentStack.pop_back();
currentStack += operation.output;
}
// Do not attempt to create the exit layout m_layout.blockInfos.at(_block).exitLayout here,
// since the code generator will directly move to the target entry layout.
std::visit(util::GenericVisitor{
[&](CFG::BasicBlock::MainExit const&) {},
[&](CFG::BasicBlock::Jump const& _jump)
{
Stack const& targetLayout = m_layout.blockInfos.at(_jump.target).entryLayout;
stackTooDeepErrors += findStackTooDeep(currentStack, targetLayout);
if (!_jump.backwards)
_addChild(_jump.target);
},
[&](CFG::BasicBlock::ConditionalJump const& _conditionalJump)
{
for (Stack const& targetLayout: {
m_layout.blockInfos.at(_conditionalJump.zero).entryLayout,
m_layout.blockInfos.at(_conditionalJump.nonZero).entryLayout
})
stackTooDeepErrors += findStackTooDeep(currentStack, targetLayout);
_addChild(_conditionalJump.zero);
_addChild(_conditionalJump.nonZero);
},
[&](CFG::BasicBlock::FunctionReturn const&) {},
[&](CFG::BasicBlock::Terminated const&) {},
}, _block->exit);
});
return stackTooDeepErrors;
}
Stack StackLayoutGenerator::compressStack(Stack _stack)
{
optional<size_t> firstDupOffset;
do
{
if (firstDupOffset)
{
std::swap(_stack.at(*firstDupOffset), _stack.back());
_stack.pop_back();
firstDupOffset.reset();
}
for (auto&& [depth, slot]: _stack | ranges::views::reverse | ranges::views::enumerate)
if (canBeFreelyGenerated(slot))
{
firstDupOffset = _stack.size() - depth - 1;
break;
}
else if (auto dupDepth = util::findOffset(_stack | ranges::views::reverse | ranges::views::drop(depth + 1), slot))
if (depth + *dupDepth <= 16)
{
firstDupOffset = _stack.size() - depth - 1;
break;
}
}
while (firstDupOffset);
return _stack;
}
void StackLayoutGenerator::fillInJunk(CFG::BasicBlock const& _block)
{
/// Recursively adds junk to the subgraph starting on @a _entry.
/// Since it is only called on cut-vertices, the full subgraph retains proper stack balance.
auto addJunkRecursive = [&](CFG::BasicBlock const* _entry, size_t _numJunk) {
util::BreadthFirstSearch<CFG::BasicBlock const*> breadthFirstSearch{{_entry}};
breadthFirstSearch.run([&](CFG::BasicBlock const* _block, auto _addChild) {
auto& blockInfo = m_layout.blockInfos.at(_block);
blockInfo.entryLayout = Stack{_numJunk, JunkSlot{}} + move(blockInfo.entryLayout);
for (auto const& operation: _block->operations)
{
auto& operationEntryLayout = m_layout.operationEntryLayout.at(&operation);
operationEntryLayout = Stack{_numJunk, JunkSlot{}} + move(operationEntryLayout);
}
blockInfo.exitLayout = Stack{_numJunk, JunkSlot{}} + move(blockInfo.exitLayout);
std::visit(util::GenericVisitor{
[&](CFG::BasicBlock::MainExit const&) {},
[&](CFG::BasicBlock::Jump const& _jump)
{
_addChild(_jump.target);
},
[&](CFG::BasicBlock::ConditionalJump const& _conditionalJump)
{
_addChild(_conditionalJump.zero);
_addChild(_conditionalJump.nonZero);
},
[&](CFG::BasicBlock::FunctionReturn const&) { yulAssert(false); },
[&](CFG::BasicBlock::Terminated const&) {},
}, _block->exit);
});
};
/// @returns the number of operations required to transform @a _source to @a _target.
auto evaluateTransform = [](Stack _source, Stack const& _target) -> size_t {
size_t opGas = 0;
auto swap = [&](unsigned _swapDepth)
{
if (_swapDepth > 16)
opGas += 1000;
else
opGas += evmasm::GasMeter::runGas(evmasm::swapInstruction(_swapDepth));
};
auto dupOrPush = [&](StackSlot const& _slot)
{
if (canBeFreelyGenerated(_slot))
opGas += evmasm::GasMeter::runGas(evmasm::pushInstruction(32));
else
{
auto depth = util::findOffset(_source | ranges::views::reverse, _slot);
yulAssert(depth);
if (*depth < 16)
opGas += evmasm::GasMeter::runGas(evmasm::dupInstruction(static_cast<unsigned>(*depth + 1)));
else
opGas += 1000;
}
};
auto pop = [&]() { opGas += evmasm::GasMeter::runGas(evmasm::Instruction::POP); };
createStackLayout(_source, _target, swap, dupOrPush, pop);
return opGas;
};
/// Traverses the CFG and at each block that allows junk, i.e. that is a cut-vertex that never leads to a function
/// return, checks if adding junk reduces the shuffling cost upon entering and if so recursively adds junk
/// to the spanned subgraph.
util::BreadthFirstSearch<CFG::BasicBlock const*>{{&_block}}.run([&](CFG::BasicBlock const* _block, auto _addChild) {
std::visit(util::GenericVisitor{
[&](CFG::BasicBlock::MainExit const&) {},
[&](CFG::BasicBlock::Jump const& _jump)
{
_addChild(_jump.target);
},
[&](CFG::BasicBlock::ConditionalJump const& _conditionalJump)
{
for (CFG::BasicBlock* exit: {_conditionalJump.zero, _conditionalJump.nonZero})
if (exit->allowsJunk())
{
auto& blockInfo = m_layout.blockInfos.at(exit);
Stack entryLayout = blockInfo.entryLayout;
Stack nextLayout = exit->operations.empty() ? blockInfo.exitLayout : m_layout.operationEntryLayout.at(&exit->operations.front());
size_t bestCost = evaluateTransform(entryLayout, nextLayout);
size_t bestNumJunk = 0;
size_t maxJunk = entryLayout.size();
for (size_t numJunk = 1; numJunk <= maxJunk; ++numJunk)
{
size_t cost = evaluateTransform(entryLayout, Stack{numJunk, JunkSlot{}} + nextLayout);
if (cost < bestCost)
{
bestCost = cost;
bestNumJunk = numJunk;
}
}
if (bestNumJunk > 0)
{
addJunkRecursive(exit, bestNumJunk);
blockInfo.entryLayout = entryLayout;
}
}
_addChild(_conditionalJump.zero);
_addChild(_conditionalJump.nonZero);
},
[&](CFG::BasicBlock::FunctionReturn const&) {},
[&](CFG::BasicBlock::Terminated const&) {},
}, _block->exit);
});
}