solidity/docs/analysing-compilation-output.rst

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2021-04-06 16:28:05 +00:00
.. index:: analyse, asm
#############################
Analysing the Compiler Output
#############################
It is often useful to look at the assembly code generated by the compiler. The generated binary,
i.e., the output of ``solc --bin contract.sol``, is generally difficult to read. It is recommended
to use the flag ``--asm`` to analyse the assembly output. Even for large contracts, looking at a
visual diff of the assembly before and after a change is often very enlightening.
Consider the following contract (named, say ``contract.sol``):
.. code-block:: Solidity
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.0 <0.9.0;
contract C {
function one() public pure returns (uint) {
return 1;
}
}
The following would be the output of ``solc --asm contract.sol``
.. code-block:: none
======= contract.sol:C =======
EVM assembly:
/* "contract.sol":0:86 contract C {... */
mstore(0x40, 0x80)
callvalue
dup1
iszero
tag_1
jumpi
0x00
dup1
revert
tag_1:
pop
dataSize(sub_0)
dup1
dataOffset(sub_0)
0x00
codecopy
0x00
return
stop
sub_0: assembly {
/* "contract.sol":0:86 contract C {... */
mstore(0x40, 0x80)
callvalue
dup1
iszero
tag_1
jumpi
0x00
dup1
revert
tag_1:
pop
jumpi(tag_2, lt(calldatasize, 0x04))
shr(0xe0, calldataload(0x00))
dup1
0x901717d1
eq
tag_3
jumpi
tag_2:
0x00
dup1
revert
/* "contract.sol":17:84 function one() public pure returns (uint) {... */
tag_3:
tag_4
tag_5
jump // in
tag_4:
mload(0x40)
tag_6
swap2
swap1
tag_7
jump // in
tag_6:
mload(0x40)
dup1
swap2
sub
swap1
return
tag_5:
/* "contract.sol":53:57 uint */
0x00
/* "contract.sol":76:77 1 */
0x01
/* "contract.sol":69:77 return 1 */
swap1
pop
/* "contract.sol":17:84 function one() public pure returns (uint) {... */
swap1
jump // out
/* "#utility.yul":7:125 */
tag_10:
/* "#utility.yul":94:118 */
tag_12
/* "#utility.yul":112:117 */
dup2
/* "#utility.yul":94:118 */
tag_13
jump // in
tag_12:
/* "#utility.yul":89:92 */
dup3
/* "#utility.yul":82:119 */
mstore
/* "#utility.yul":72:125 */
pop
pop
jump // out
/* "#utility.yul":131:353 */
tag_7:
0x00
/* "#utility.yul":262:264 */
0x20
/* "#utility.yul":251:260 */
dup3
/* "#utility.yul":247:265 */
add
/* "#utility.yul":239:265 */
swap1
pop
/* "#utility.yul":275:346 */
tag_15
/* "#utility.yul":343:344 */
0x00
/* "#utility.yul":332:341 */
dup4
/* "#utility.yul":328:345 */
add
/* "#utility.yul":319:325 */
dup5
/* "#utility.yul":275:346 */
tag_10
jump // in
tag_15:
/* "#utility.yul":229:353 */
swap3
swap2
pop
pop
jump // out
/* "#utility.yul":359:436 */
tag_13:
0x00
/* "#utility.yul":425:430 */
dup2
/* "#utility.yul":414:430 */
swap1
pop
/* "#utility.yul":404:436 */
swap2
swap1
pop
jump // out
auxdata: 0xa2646970667358221220a5874f19737ddd4c5d77ace1619e5160c67b3d4bedac75fce908fed32d98899864736f6c637827302e382e342d646576656c6f702e323032312e332e33302b636f6d6d69742e65613065363933380058
}
Alternatively, the above output can also be obtained from `Remix <https://remix.ethereum.org/>`_,
under the option "Compilation Details" after compiling a contract.
Notice that the ``asm`` output starts with the creation / constructor code. The deploy code is
provided as part of the sub object (in the above example, it is part of the sub-object ``sub_0``).
The ``auxdata`` field corresponds to the contract :ref:`metadata
<encoding-of-the-metadata-hash-in-the-bytecode>`. The comments in the assembly output point to the
source location. Note that ``#utility.yul`` is an internally generated file of utility functions
that can be obtained using the flags ``--combined-json
generated-sources,generated-sources-runtime``.
Similarly, the optimized assembly can be obtained with the command: ``solc --optimize --asm
contract.sol``. Often times, it is interesting to see if two different sources in Solidity result in
the same optimized code. For example, to see if the expressions ``(a * b) / c``, ``a * b / c``
generates the same bytecode. This can be easily done by taking a ``diff`` of the corresponding
assembly output, after potentially stripping comments that reference the source locations.
.. note::
The ``--asm`` output is not designed to be machine readable. Therefore, there may be breaking
changes on the output between minor versions of solc.