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Evaluate keccak(a, const) if value at memory location a is known

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Here the value of constant can be at most 32.
This commit is contained in:
hrkrshnn
2021-04-22 18:19:47 +02:00
parent 011f8d3ff7
commit 3bc4f5708a
4 changed files with 103 additions and 4 deletions
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libyul/optimiser/LoadResolver.cpp
+73 -1
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@@ -23,13 +23,23 @@
#include <libyul/optimiser/LoadResolver.h>
#include <libyul/backends/evm/EVMDialect.h>
#include <libyul/backends/evm/EVMMetrics.h>
#include <libyul/optimiser/Semantics.h>
#include <libyul/optimiser/CallGraphGenerator.h>
#include <libyul/SideEffects.h>
#include <libyul/AST.h>
#include <libyul/Utilities.h>
#include <libevmasm/Instruction.h>
#include <libevmasm/GasMeter.h>
#include <libsolutil/Keccak256.h>
#include <limits>
using namespace std;
using namespace solidity;
using namespace solidity::util;
using namespace solidity::evmasm;
using namespace solidity::yul;
void LoadResolver::run(OptimiserStepContext& _context, Block& _ast)
@@ -38,7 +48,8 @@ void LoadResolver::run(OptimiserStepContext& _context, Block& _ast)
LoadResolver{
_context.dialect,
SideEffectsPropagator::sideEffects(_context.dialect, CallGraphGenerator::callGraph(_ast)),
!containsMSize
!containsMSize,
_context.expectedExecutionsPerDeployment
}(_ast);
}
@@ -47,12 +58,17 @@ void LoadResolver::visit(Expression& _e)
DataFlowAnalyzer::visit(_e);
if (FunctionCall const* funCall = std::get_if<FunctionCall>(&_e))
{
for (auto location: { StoreLoadLocation::Memory, StoreLoadLocation::Storage })
if (funCall->functionName.name == m_loadFunctionName[static_cast<unsigned>(location)])
{
tryResolve(_e, location, funCall->arguments);
break;
}
if (funCall->functionName.name == m_dialect.hashFunction({}))
tryEvaluateKeccak(_e, funCall->arguments);
}
}
void LoadResolver::tryResolve(
@@ -76,3 +92,59 @@ void LoadResolver::tryResolve(
if (inScope(*value))
_e = Identifier{locationOf(_e), *value};
}
void LoadResolver::tryEvaluateKeccak(
Expression& _e,
std::vector<Expression> const& _arguments
)
{
// The costs are only correct for hashes of 32 bytes or 1 word (when rounded up).
GasMeter gasMeter{
dynamic_cast<EVMDialect const&>(m_dialect),
!m_expectedExecutionsPerDeployment,
m_expectedExecutionsPerDeployment ? *m_expectedExecutionsPerDeployment : 1
};
bigint costOfKeccak = gasMeter.costs(_e);
bigint costOfLiteral = gasMeter.costs(
Literal{
{},
LiteralKind::Number,
// a dummy 256-bit number to represent the Keccak256 hash.
YulString{numeric_limits<u256>::max().str()},
{}
}
);
// We skip if there are no net gas savings.
// Note that for default `m_runs = 200`, the values are
// `costOfLiteral = 7200` and `costOfKeccak = 9000` for runtime context.
// For creation context: `costOfLiteral = 531` and `costOfKeccak = 90`.
if (costOfLiteral > costOfKeccak)
return;
yulAssert(_arguments.size() == 2, "");
Identifier const* memoryKey = std::get_if<Identifier>(&_arguments.at(0));
Identifier const* length = std::get_if<Identifier>(&_arguments.at(1));
if (!memoryKey || !length)
return;
auto memoryValue = util::valueOrNullptr(m_memory, memoryKey->name);
if (memoryValue && inScope(*memoryValue))
{
optional<u256> memoryContent = valueOfIdentifier(*memoryValue);
optional<u256> byteLength = valueOfIdentifier(length->name);
if (memoryContent && byteLength && *byteLength <= 32)
{
bytes contentAsBytes = toBigEndian(*memoryContent);
contentAsBytes.resize(static_cast<size_t>(*byteLength));
_e = Literal{
locationOf(_e),
LiteralKind::Number,
YulString{u256(keccak256(contentAsBytes)).str()},
m_dialect.defaultType
};
}
}
}