aboutsummaryrefslogtreecommitdiffstats
path: root/libsolidity
diff options
context:
space:
mode:
authorchriseth <chris@ethereum.org>2017-04-29 00:15:18 +0800
committerchriseth <chris@ethereum.org>2017-05-20 01:20:28 +0800
commite2b21e1c9611fd94f8e7b91b0ecbb0ec39145758 (patch)
tree9cc66e31297b7fff84166225d008b23351ba6e06 /libsolidity
parent41ee2cefbbe0009fd7576f3e965b63e076d1ad24 (diff)
downloaddexon-solidity-e2b21e1c9611fd94f8e7b91b0ecbb0ec39145758.tar.gz
dexon-solidity-e2b21e1c9611fd94f8e7b91b0ecbb0ec39145758.tar.zst
dexon-solidity-e2b21e1c9611fd94f8e7b91b0ecbb0ec39145758.zip
Refactor to abstract assembly to JULIA.
Diffstat (limited to 'libsolidity')
-rw-r--r--libsolidity/codegen/CompilerContext.cpp16
-rw-r--r--libsolidity/codegen/ContractCompiler.cpp36
-rw-r--r--libsolidity/inlineasm/AsmCodeGen.cpp143
-rw-r--r--libsolidity/inlineasm/AsmScope.h4
-rw-r--r--libsolidity/inlineasm/AsmStack.h4
5 files changed, 127 insertions, 76 deletions
diff --git a/libsolidity/codegen/CompilerContext.cpp b/libsolidity/codegen/CompilerContext.cpp
index 51dd9fd2..5c4f88c4 100644
--- a/libsolidity/codegen/CompilerContext.cpp
+++ b/libsolidity/codegen/CompilerContext.cpp
@@ -264,7 +264,7 @@ void CompilerContext::appendInlineAssembly(
assembly = &replacedAssembly;
}
- unsigned startStackHeight = stackHeight();
+ int startStackHeight = stackHeight();
assembly::ExternalIdentifierAccess identifierAccess;
identifierAccess.resolve = [&](
@@ -278,26 +278,26 @@ void CompilerContext::appendInlineAssembly(
identifierAccess.generateCode = [&](
assembly::Identifier const& _identifier,
assembly::IdentifierContext _context,
- eth::Assembly& _assembly
+ julia::AbstractAssembly& _assembly
)
{
auto it = std::find(_localVariables.begin(), _localVariables.end(), _identifier.name);
solAssert(it != _localVariables.end(), "");
- unsigned stackDepth = _localVariables.end() - it;
- int stackDiff = _assembly.deposit() - startStackHeight + stackDepth;
+ int stackDepth = _localVariables.end() - it;
+ int stackDiff = _assembly.stackHeight() - startStackHeight + stackDepth;
if (_context == assembly::IdentifierContext::LValue)
stackDiff -= 1;
if (stackDiff < 1 || stackDiff > 16)
BOOST_THROW_EXCEPTION(
CompilerError() <<
- errinfo_comment("Stack too deep, try removing local variables.")
+ errinfo_comment("Stack too deep (" + to_string(stackDiff) + "), try removing local variables.")
);
if (_context == assembly::IdentifierContext::RValue)
- _assembly.append(dupInstruction(stackDiff));
+ _assembly.appendInstruction(dupInstruction(stackDiff));
else
{
- _assembly.append(swapInstruction(stackDiff));
- _assembly.append(Instruction::POP);
+ _assembly.appendInstruction(swapInstruction(stackDiff));
+ _assembly.appendInstruction(Instruction::POP);
}
};
diff --git a/libsolidity/codegen/ContractCompiler.cpp b/libsolidity/codegen/ContractCompiler.cpp
index 34ef13c0..41940e1a 100644
--- a/libsolidity/codegen/ContractCompiler.cpp
+++ b/libsolidity/codegen/ContractCompiler.cpp
@@ -530,7 +530,7 @@ bool ContractCompiler::visit(InlineAssembly const& _inlineAssembly)
return size_t(-1);
return ref->second.valueSize;
};
- identifierAccess.generateCode = [&](assembly::Identifier const& _identifier, assembly::IdentifierContext _context, eth::Assembly& _assembly)
+ identifierAccess.generateCode = [&](assembly::Identifier const& _identifier, assembly::IdentifierContext _context, julia::AbstractAssembly& _assembly)
{
auto ref = _inlineAssembly.annotation().externalReferences.find(&_identifier);
solAssert(ref != _inlineAssembly.annotation().externalReferences.end(), "");
@@ -538,21 +538,25 @@ bool ContractCompiler::visit(InlineAssembly const& _inlineAssembly)
solAssert(!!decl, "");
if (_context == assembly::IdentifierContext::RValue)
{
- int const depositBefore = _assembly.deposit();
+ int const depositBefore = _assembly.stackHeight();
solAssert(!!decl->type(), "Type of declaration required but not yet determined.");
if (FunctionDefinition const* functionDef = dynamic_cast<FunctionDefinition const*>(decl))
{
solAssert(!ref->second.isOffset && !ref->second.isSlot, "");
functionDef = &m_context.resolveVirtualFunction(*functionDef);
- _assembly.append(m_context.functionEntryLabel(*functionDef).pushTag());
+ auto functionEntryLabel = m_context.functionEntryLabel(*functionDef).pushTag();
+ solAssert(functionEntryLabel.data() <= std::numeric_limits<size_t>::max(), "");
+ _assembly.appendLabelReference(size_t(functionEntryLabel.data()));
// If there is a runtime context, we have to merge both labels into the same
// stack slot in case we store it in storage.
if (CompilerContext* rtc = m_context.runtimeContext())
{
- _assembly.append(u256(1) << 32);
- _assembly.append(Instruction::MUL);
- _assembly.append(rtc->functionEntryLabel(*functionDef).toSubAssemblyTag(m_context.runtimeSub()));
- _assembly.append(Instruction::OR);
+ _assembly.appendConstant(u256(1) << 32);
+ _assembly.appendInstruction(Instruction::MUL);
+ auto runtimeEntryLabel = rtc->functionEntryLabel(*functionDef).toSubAssemblyTag(m_context.runtimeSub());
+ solAssert(runtimeEntryLabel.data() <= std::numeric_limits<size_t>::max(), "");
+ _assembly.appendLabelReference(size_t(runtimeEntryLabel.data()));
+ _assembly.appendInstruction(Instruction::OR);
}
}
else if (auto variable = dynamic_cast<VariableDeclaration const*>(decl))
@@ -570,7 +574,7 @@ bool ContractCompiler::visit(InlineAssembly const& _inlineAssembly)
}
else if (m_context.isLocalVariable(decl))
{
- int stackDiff = _assembly.deposit() - m_context.baseStackOffsetOfVariable(*variable);
+ int stackDiff = _assembly.stackHeight() - m_context.baseStackOffsetOfVariable(*variable);
if (ref->second.isSlot || ref->second.isOffset)
{
solAssert(variable->type()->dataStoredIn(DataLocation::Storage), "");
@@ -587,7 +591,7 @@ bool ContractCompiler::visit(InlineAssembly const& _inlineAssembly)
// only slot, offset is zero
if (ref->second.isOffset)
{
- _assembly.append(u256(0));
+ _assembly.appendConstant(u256(0));
return;
}
}
@@ -601,7 +605,7 @@ bool ContractCompiler::visit(InlineAssembly const& _inlineAssembly)
errinfo_comment("Stack too deep, try removing local variables.")
);
solAssert(variable->type()->sizeOnStack() == 1, "");
- _assembly.append(dupInstruction(stackDiff));
+ _assembly.appendInstruction(dupInstruction(stackDiff));
}
else
solAssert(false, "");
@@ -610,11 +614,11 @@ bool ContractCompiler::visit(InlineAssembly const& _inlineAssembly)
{
solAssert(!ref->second.isOffset && !ref->second.isSlot, "");
solAssert(contract->isLibrary(), "");
- _assembly.appendLibraryAddress(contract->fullyQualifiedName());
+ _assembly.appendLinkerSymbol(contract->fullyQualifiedName());
}
else
solAssert(false, "Invalid declaration type.");
- solAssert(_assembly.deposit() - depositBefore == int(ref->second.valueSize), "");
+ solAssert(_assembly.stackHeight() - depositBefore == int(ref->second.valueSize), "");
}
else
{
@@ -626,15 +630,15 @@ bool ContractCompiler::visit(InlineAssembly const& _inlineAssembly)
"Can only assign to stack variables in inline assembly."
);
solAssert(variable->type()->sizeOnStack() == 1, "");
- int stackDiff = _assembly.deposit() - m_context.baseStackOffsetOfVariable(*variable) - 1;
+ int stackDiff = _assembly.stackHeight() - m_context.baseStackOffsetOfVariable(*variable) - 1;
if (stackDiff > 16 || stackDiff < 1)
BOOST_THROW_EXCEPTION(
CompilerError() <<
errinfo_sourceLocation(_inlineAssembly.location()) <<
- errinfo_comment("Stack too deep, try removing local variables.")
+ errinfo_comment("Stack too deep(" + to_string(stackDiff) + "), try removing local variables.")
);
- _assembly.append(swapInstruction(stackDiff));
- _assembly.append(Instruction::POP);
+ _assembly.appendInstruction(swapInstruction(stackDiff));
+ _assembly.appendInstruction(Instruction::POP);
}
};
solAssert(_inlineAssembly.annotation().analysisInfo, "");
diff --git a/libsolidity/inlineasm/AsmCodeGen.cpp b/libsolidity/inlineasm/AsmCodeGen.cpp
index 44e12b3e..a6d7e76b 100644
--- a/libsolidity/inlineasm/AsmCodeGen.cpp
+++ b/libsolidity/inlineasm/AsmCodeGen.cpp
@@ -32,6 +32,8 @@
#include <libevmasm/SourceLocation.h>
#include <libevmasm/Instruction.h>
+#include <libjulia/backends/AbstractAssembly.h>
+
#include <libdevcore/CommonIO.h>
#include <boost/range/adaptor/reversed.hpp>
@@ -48,24 +50,61 @@ using namespace dev::solidity::assembly;
struct GeneratorState
{
- GeneratorState(ErrorList& _errors, AsmAnalysisInfo& _analysisInfo, eth::Assembly& _assembly):
- errors(_errors), info(_analysisInfo), assembly(_assembly) {}
+ GeneratorState(ErrorList& _errors, AsmAnalysisInfo& _analysisInfo):
+ errors(_errors), info(_analysisInfo) {}
+
+ ErrorList& errors;
+ AsmAnalysisInfo info;
+};
- size_t newLabelId()
+class EthAssemblyAdapter: public julia::AbstractAssembly
+{
+public:
+ EthAssemblyAdapter(eth::Assembly& _assembly):
+ m_assembly(_assembly)
+ {
+ }
+ virtual void setSourceLocation(SourceLocation const& _location) override
+ {
+ m_assembly.setSourceLocation(_location);
+ }
+ virtual int stackHeight() const override { return m_assembly.deposit(); }
+ virtual void appendInstruction(solidity::Instruction _instruction) override
+ {
+ m_assembly.append(_instruction);
+ }
+ virtual void appendConstant(u256 const& _constant) override
+ {
+ m_assembly.append(_constant);
+ }
+ /// Append a label.
+ virtual void appendLabel(size_t _labelId) override
+ {
+ m_assembly.append(eth::AssemblyItem(eth::Tag, _labelId));
+ }
+ /// Append a label reference.
+ virtual void appendLabelReference(size_t _labelId) override
+ {
+ m_assembly.append(eth::AssemblyItem(eth::PushTag, _labelId));
+ }
+ virtual size_t newLabel() override
+ {
+ return assemblyTagToIdentifier(m_assembly.newTag());
+ }
+ virtual void appendLinkerSymbol(std::string const& _linkerSymbol) override
{
- return assemblyTagToIdentifier(assembly.newTag());
+ m_assembly.appendLibraryAddress(_linkerSymbol);
}
- size_t assemblyTagToIdentifier(eth::AssemblyItem const& _tag) const
+private:
+ size_t assemblyTagToIdentifier(eth::AssemblyItem const& _tag)
{
u256 id = _tag.data();
solAssert(id <= std::numeric_limits<size_t>::max(), "Tag id too large.");
return size_t(id);
}
- ErrorList& errors;
- AsmAnalysisInfo info;
- eth::Assembly& assembly;
+ eth::Assembly& m_assembly;
};
class CodeTransform: public boost::static_visitor<>
@@ -76,81 +115,84 @@ public:
/// @param _identifierAccess used to resolve identifiers external to the inline assembly
explicit CodeTransform(
GeneratorState& _state,
+ julia::AbstractAssembly& _assembly,
assembly::Block const& _block,
assembly::ExternalIdentifierAccess const& _identifierAccess = assembly::ExternalIdentifierAccess()
- ): CodeTransform(_state, _block, _identifierAccess, _state.assembly.deposit())
+ ): CodeTransform(_state, _assembly, _block, _identifierAccess, _assembly.stackHeight())
{
}
private:
CodeTransform(
GeneratorState& _state,
+ julia::AbstractAssembly& _assembly,
assembly::Block const& _block,
assembly::ExternalIdentifierAccess const& _identifierAccess,
- int _initialDeposit
+ int _initialStackHeight
):
m_state(_state),
+ m_assembly(_assembly),
m_scope(*m_state.info.scopes.at(&_block)),
m_identifierAccess(_identifierAccess),
- m_initialDeposit(_initialDeposit)
+ m_initialStackHeight(_initialStackHeight)
{
- int blockStartDeposit = m_state.assembly.deposit();
+ int blockStartStackHeight = m_assembly.stackHeight();
std::for_each(_block.statements.begin(), _block.statements.end(), boost::apply_visitor(*this));
- m_state.assembly.setSourceLocation(_block.location);
+ m_assembly.setSourceLocation(_block.location);
// pop variables
for (auto const& identifier: m_scope.identifiers)
if (identifier.second.type() == typeid(Scope::Variable))
- m_state.assembly.append(solidity::Instruction::POP);
+ m_assembly.appendInstruction(solidity::Instruction::POP);
- int deposit = m_state.assembly.deposit() - blockStartDeposit;
+ int deposit = m_assembly.stackHeight() - blockStartStackHeight;
solAssert(deposit == 0, "Invalid stack height at end of block.");
}
public:
void operator()(assembly::Instruction const& _instruction)
{
- m_state.assembly.setSourceLocation(_instruction.location);
- m_state.assembly.append(_instruction.instruction);
+ m_assembly.setSourceLocation(_instruction.location);
+ m_assembly.appendInstruction(_instruction.instruction);
checkStackHeight(&_instruction);
}
void operator()(assembly::Literal const& _literal)
{
- m_state.assembly.setSourceLocation(_literal.location);
+ m_assembly.setSourceLocation(_literal.location);
if (_literal.kind == assembly::LiteralKind::Number)
- m_state.assembly.append(u256(_literal.value));
+ m_assembly.appendConstant(u256(_literal.value));
else if (_literal.kind == assembly::LiteralKind::Boolean)
{
if (_literal.value == "true")
- m_state.assembly.append(u256(1));
+ m_assembly.appendConstant(u256(1));
else
- m_state.assembly.append(u256(0));
+ m_assembly.appendConstant(u256(0));
}
else
{
solAssert(_literal.value.size() <= 32, "");
- m_state.assembly.append(u256(h256(_literal.value, h256::FromBinary, h256::AlignLeft)));
+ m_assembly.appendConstant(u256(h256(_literal.value, h256::FromBinary, h256::AlignLeft)));
}
checkStackHeight(&_literal);
}
void operator()(assembly::Identifier const& _identifier)
{
- m_state.assembly.setSourceLocation(_identifier.location);
+ m_assembly.setSourceLocation(_identifier.location);
// First search internals, then externals.
if (m_scope.lookup(_identifier.name, Scope::NonconstVisitor(
[=](Scope::Variable& _var)
{
if (int heightDiff = variableHeightDiff(_var, _identifier.location, false))
- m_state.assembly.append(solidity::dupInstruction(heightDiff));
+ m_assembly.appendInstruction(solidity::dupInstruction(heightDiff));
else
// Store something to balance the stack
- m_state.assembly.append(u256(0));
+ m_assembly.appendConstant(u256(0));
},
[=](Scope::Label& _label)
{
assignLabelIdIfUnset(_label);
- m_state.assembly.append(eth::AssemblyItem(eth::PushTag, _label.id));
+ m_assembly.appendLabelReference(*_label.id);
},
[=](Scope::Function&)
{
@@ -164,14 +206,14 @@ public:
m_identifierAccess.generateCode,
"Identifier not found and no external access available."
);
- m_identifierAccess.generateCode(_identifier, IdentifierContext::RValue, m_state.assembly);
+ m_identifierAccess.generateCode(_identifier, IdentifierContext::RValue, m_assembly);
checkStackHeight(&_identifier);
}
void operator()(FunctionalInstruction const& _instr)
{
for (auto it = _instr.arguments.rbegin(); it != _instr.arguments.rend(); ++it)
{
- int height = m_state.assembly.deposit();
+ int height = m_assembly.stackHeight();
boost::apply_visitor(*this, *it);
expectDeposit(1, height);
}
@@ -184,31 +226,31 @@ public:
}
void operator()(Label const& _label)
{
- m_state.assembly.setSourceLocation(_label.location);
+ m_assembly.setSourceLocation(_label.location);
solAssert(m_scope.identifiers.count(_label.name), "");
Scope::Label& label = boost::get<Scope::Label>(m_scope.identifiers.at(_label.name));
assignLabelIdIfUnset(label);
- m_state.assembly.append(eth::AssemblyItem(eth::Tag, label.id));
+ m_assembly.appendLabel(*label.id);
checkStackHeight(&_label);
}
void operator()(assembly::Assignment const& _assignment)
{
- m_state.assembly.setSourceLocation(_assignment.location);
+ m_assembly.setSourceLocation(_assignment.location);
generateAssignment(_assignment.variableName, _assignment.location);
checkStackHeight(&_assignment);
}
void operator()(FunctionalAssignment const& _assignment)
{
- int height = m_state.assembly.deposit();
+ size_t height = m_assembly.stackHeight();
boost::apply_visitor(*this, *_assignment.value);
expectDeposit(1, height);
- m_state.assembly.setSourceLocation(_assignment.location);
+ m_assembly.setSourceLocation(_assignment.location);
generateAssignment(_assignment.variableName, _assignment.location);
checkStackHeight(&_assignment);
}
void operator()(assembly::VariableDeclaration const& _varDecl)
{
- int height = m_state.assembly.deposit();
+ size_t height = m_assembly.stackHeight();
boost::apply_visitor(*this, *_varDecl.value);
expectDeposit(1, height);
auto& var = boost::get<Scope::Variable>(m_scope.identifiers.at(_varDecl.variable.name));
@@ -217,7 +259,7 @@ public:
}
void operator()(assembly::Block const& _block)
{
- CodeTransform(m_state, _block, m_identifierAccess, m_initialDeposit);
+ CodeTransform(m_state, m_assembly, _block, m_identifierAccess, m_initialStackHeight);
checkStackHeight(&_block);
}
void operator()(assembly::FunctionDefinition const&)
@@ -233,8 +275,8 @@ private:
{
Scope::Variable const& _var = boost::get<Scope::Variable>(*var);
if (int heightDiff = variableHeightDiff(_var, _location, true))
- m_state.assembly.append(solidity::swapInstruction(heightDiff - 1));
- m_state.assembly.append(solidity::Instruction::POP);
+ m_assembly.appendInstruction(solidity::swapInstruction(heightDiff - 1));
+ m_assembly.appendInstruction(solidity::Instruction::POP);
}
else
{
@@ -242,16 +284,16 @@ private:
m_identifierAccess.generateCode,
"Identifier not found and no external access available."
);
- m_identifierAccess.generateCode(_variableName, IdentifierContext::LValue, m_state.assembly);
+ m_identifierAccess.generateCode(_variableName, IdentifierContext::LValue, m_assembly);
}
}
/// Determines the stack height difference to the given variables. Automatically generates
/// errors if it is not yet in scope or the height difference is too large. Returns 0 on
/// errors and the (positive) stack height difference otherwise.
- int variableHeightDiff(Scope::Variable const& _var, SourceLocation const& _location, bool _forSwap)
+ size_t variableHeightDiff(Scope::Variable const& _var, SourceLocation const& _location, bool _forSwap)
{
- int heightDiff = m_state.assembly.deposit() - _var.stackHeight;
+ size_t heightDiff = m_assembly.stackHeight() - _var.stackHeight;
if (heightDiff <= (_forSwap ? 1 : 0) || heightDiff > (_forSwap ? 17 : 16))
{
//@TODO move this to analysis phase.
@@ -268,14 +310,14 @@ private:
void expectDeposit(int _deposit, int _oldHeight)
{
- solAssert(m_state.assembly.deposit() == _oldHeight + _deposit, "Invalid stack deposit.");
+ solAssert(m_assembly.stackHeight() == _oldHeight + _deposit, "Invalid stack deposit.");
}
void checkStackHeight(void const* _astElement)
{
solAssert(m_state.info.stackHeightInfo.count(_astElement), "Stack height for AST element not found.");
solAssert(
- m_state.info.stackHeightInfo.at(_astElement) == m_state.assembly.deposit() - m_initialDeposit,
+ m_state.info.stackHeightInfo.at(_astElement) == m_assembly.stackHeight() - m_initialStackHeight,
"Stack height mismatch between analysis and code generation phase."
);
}
@@ -283,15 +325,16 @@ private:
/// Assigns the label's id to a value taken from eth::Assembly if it has not yet been set.
void assignLabelIdIfUnset(Scope::Label& _label)
{
- if (_label.id == Scope::Label::unassignedLabelId)
- _label.id = m_state.newLabelId();
+ if (!_label.id)
+ _label.id.reset(m_assembly.newLabel());
}
GeneratorState& m_state;
+ julia::AbstractAssembly& m_assembly;
Scope& m_scope;
ExternalIdentifierAccess m_identifierAccess;
- int const m_initialDeposit;
+ int const m_initialStackHeight;
};
eth::Assembly assembly::CodeGenerator::assemble(
@@ -301,8 +344,9 @@ eth::Assembly assembly::CodeGenerator::assemble(
)
{
eth::Assembly assembly;
- GeneratorState state(m_errors, _analysisInfo, assembly);
- CodeTransform(state, _parsedData, _identifierAccess);
+ GeneratorState state(m_errors, _analysisInfo);
+ EthAssemblyAdapter assemblyAdapter(assembly);
+ CodeTransform(state, assemblyAdapter, _parsedData, _identifierAccess);
return assembly;
}
@@ -313,6 +357,7 @@ void assembly::CodeGenerator::assemble(
ExternalIdentifierAccess const& _identifierAccess
)
{
- GeneratorState state(m_errors, _analysisInfo, _assembly);
- CodeTransform(state, _parsedData, _identifierAccess);
+ GeneratorState state(m_errors, _analysisInfo);
+ EthAssemblyAdapter assemblyAdapter(_assembly);
+ CodeTransform(state, assemblyAdapter, _parsedData, _identifierAccess);
}
diff --git a/libsolidity/inlineasm/AsmScope.h b/libsolidity/inlineasm/AsmScope.h
index dd45613d..70786dce 100644
--- a/libsolidity/inlineasm/AsmScope.h
+++ b/libsolidity/inlineasm/AsmScope.h
@@ -23,6 +23,7 @@
#include <libsolidity/interface/Exceptions.h>
#include <boost/variant.hpp>
+#include <boost/optional.hpp>
#include <functional>
#include <memory>
@@ -75,8 +76,7 @@ struct Scope
struct Label
{
- size_t id = unassignedLabelId;
- static const size_t unassignedLabelId = 0;
+ boost::optional<size_t> id;
};
struct Function
diff --git a/libsolidity/inlineasm/AsmStack.h b/libsolidity/inlineasm/AsmStack.h
index 77a7e02a..e223ccc9 100644
--- a/libsolidity/inlineasm/AsmStack.h
+++ b/libsolidity/inlineasm/AsmStack.h
@@ -24,6 +24,8 @@
#include <libsolidity/interface/Exceptions.h>
+#include <libjulia/backends/AbstractAssembly.h>
+
#include <string>
#include <functional>
@@ -51,7 +53,7 @@ struct ExternalIdentifierAccess
/// Resolve a an external reference given by the identifier in the given context.
/// @returns the size of the value (number of stack slots) or size_t(-1) if not found.
Resolver resolve;
- using CodeGenerator = std::function<void(assembly::Identifier const&, IdentifierContext, eth::Assembly&)>;
+ using CodeGenerator = std::function<void(assembly::Identifier const&, IdentifierContext, julia::AbstractAssembly&)>;
/// Generate code for retrieving the value (rvalue context) or storing the value (lvalue context)
/// of an identifier. The code should be appended to the assembly. In rvalue context, the value is supposed
/// to be put onto the stack, in lvalue context, the value is assumed to be at the top of the stack.