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molly-ting avatar molly-ting commented on June 3, 2024

Another similar case is the external call.

Description

    function extcall(uint256 a) public returns(uint256) {
        uint256 res = 0;
        try l.lib(a) returns (uint256 tmp) {
            res=tmp*2;
        } catch {
            res = 1;
        }
        
        return res;
    }

The above function will generate the following opcode.

DUP1 ISZERO PUSH2 0xF5 JUMPI POP PUSH1 0x40 MLOAD ... JUMP JUMPDEST PUSH1 0x1 JUMPDEST PUSH2 0x102 JUMPI PUSH1 0x1 SWAP1 POP

The corresponding assembly code is:

...
tag_11:
      ...
      gas
      call
      swap3
      pop
      pop
      pop
      dup1
      iszero
      tag_12
      jumpi
      pop
      mload(0x40)
      …
      jump	// in
tag_13:
      0x01
tag_12:
        /* "extcall.sol":481:589  try l.lib(a) returns (uint256 tmp) {... */
      tag_15
      jumpi
        /* "extcall.sol":577:578  1 */
      0x01
        /* "extcall.sol":571:578  res = 1 */
      swap1
      pop
        /* "extcall.sol":481:589  try l.lib(a) returns (uint256 tmp) {... */
      jump(tag_19)
...

In tag_11, there is a 'POP' just after 'JUMPI', which means the original value of the DUP is not used. Although this value is used in the other branch(tag_12), this can also be optimized.

Optimization

Specifically, moving tag_12 after tag_15 jumpi would achieve this optimization. The resulting assembly code is as follows:

...
tag_11:
      ...
      gas
      call
      swap3
      pop
      pop
      pop
      iszero
      tag_12
      jumpi
      mload(0x40)
      …
      jump	// in
tag_13:
      0x01
        /* "extcall.sol":481:589  try l.lib(a) returns (uint256 tmp) {... */
      tag_15
      jumpi
tag_12:
        /* "extcall.sol":577:578  1 */
      0x01
        /* "extcall.sol":571:578  res = 1 */
      swap1
      pop
        /* "extcall.sol":481:589  try l.lib(a) returns (uint256 tmp) {... */
      jump(tag_19)
...

solidity/libsolidity/codegen/ExpressionCompiler.cpp

Lines 2854 to 2859 in b849b32

if (_tryCall)
{
m_context << Instruction::DUP1 << Instruction::ISZERO;
m_context.appendConditionalJumpTo(endTag);
m_context << Instruction::POP;
}

solidity/libsolidity/codegen/ExpressionCompiler.cpp

Lines 2923 to 2928 in b849b32

if (_tryCall)
{
// Success branch will reach this, failure branch will directly jump to endTag.
m_context << u256(1);
m_context << endTag;
}

In this function, endTag is tag_12 in this example.

solidity/libsolidity/codegen/ContractCompiler.cpp

Lines 965 to 977 in b849b32

bool ContractCompiler::visit(TryStatement const& _tryStatement)
{
StackHeightChecker checker(m_context);
CompilerContext::LocationSetter locationSetter(m_context, _tryStatement);
compileExpression(_tryStatement.externalCall());
int const returnSize = static_cast<int>(_tryStatement.externalCall().annotation().type->sizeOnStack());
// Stack: [ return values] <success flag>
evmasm::AssemblyItem successTag = m_context.appendConditionalJump();
// Catch case.
m_context.adjustStackOffset(-returnSize);

In this function, successTag is tag_15 in this example. Is it possible to move the conditionalJump in this function to the front of the endTag in the function appendExternalFunctionCall so that we can save one duplication and pop?

from solidity.

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