99
votes
Accepted
What OPCODES are available for the Ethereum EVM?
All of the opcodes and their complete descriptions are available in the Ethereum Yellow paper. For convenience, though, I've made a handy reference list of them all:
0s: Stop and Arithmetic Operations
...
- 4,621
28
votes
understanding mload assembly function
As others have stated, the x
m:=mload(0x40) instruction reads the 32 bytes of memory starting at position 0x40.
In solidity, the 0x40 slot in memory is special: it contains the "free memory pointer" ...
- 33.9k
21
votes
Accepted
understanding mload assembly function
mload(0xAB) Loads the word (32byte) located at the memory address 0xAB. e.g mload(0x60) loads the word located at 0x60 memory address.
let's code to understand more :
function f ()
{
...
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17
votes
Accepted
What are some examples of how inline assembly benefits smart contract development?
As the author of that module, a few things that came up:
Inline assembly allows you to read entire words (256 bits) from data types like string and bytes in a single operation. Solidity-stringutils ...
- 7,549
17
votes
Accepted
isContract function using EVM assembly to get the address' code size
EDIT: The function will return false if it is invoked from a contract's constructor (because the contract has not been deployed yet).
The code should be used very carefully, if at all, to avoid ...
eth♦
- 77.9k
15
votes
Accepted
Solidity assembly question - mstore
The explanation is quite simple:
In Solidity, bytes is a dynamically-sized byte array:
Variables of type bytes and string are special arrays. A bytes is
similar to byte[], but it is packed ...
- 818
13
votes
Accepted
Understanding solidity inline assembly code
Within the assembly code, _data is the memory address of the start of the array data.
However, the first memory word (32 bytes = 0x20 bytes) is reserved for the length of the array, so we need to ...
- 8,612
11
votes
Accepted
What is the cryptic part at the end of a solidity contract bytecode?
This is the Swarm hash. It is documented at https://solidity.readthedocs.io/en/develop/metadata.html
Extract follows
Contract Metadata
The Solidity compiler automatically generates a JSON file, the
...
- 8,612
11
votes
How could msg.sender != tx.origin AND extcodesize(caller) == 0 be true?
All things must have a beginning
I think you're getting called from a contract constructor before it's committed to the chain.
At this point, it will have a (theoretical) address but no code yet in ...
- 1,308
9
votes
Accepted
What is the benefit of using create2() to create a smart contract?
CREATE opcode is used by default when deploying smart contracts. The deployed contract address is calculated like this.
keccak256(senderAddress, nonce)
CREATE2 opcode is introduced later and allows ...
- 340
8
votes
Accepted
How can a factory contract clone the bytecode of an existing contract?
Martin Holst Swende published an implementation of a cloner contract here! https://gist.github.com/holiman/069de8d056a531575d2b786df3345665
Excerpt:
function clone(address a) returns(address){
/...
- 3,420
8
votes
isContract function using EVM assembly to get the address' code size
This code snippet is correct, although it seems important to note that EXTCODESIZE will return 0 when called within the constructor of a contract, since the contract is not created yet. Hence, if msg....
- 472
8
votes
Accepted
Get address from public key in Solidity
This works:
return address(keccak256(publicKey) & (2**(8*21)-1));
2**(8*21)-1 is just a trick to get 0xFFFFFF... (40 Fs) without typing it. :-)
EDIT
As pointed out by @schnorr, there's no need ...
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7
votes
Accepted
Can I use delegatecall in a transaction?
DELEGATECALL is an instruction interpreted by EVM. So to execute DELEGATECALL you have to have a program (contract) that gets executed on EVM.
DELEGATECALL takes six operands, one of them is the ...
- 706
7
votes
Accepted
How to convert bytes to uint in Solidity
You can use this function to convert bytes to unit:
pragma solidity ^0.4.23;
contract mycontract {
function bytesToUint(bytes b) public returns (uint256){
uint256 number;
for(...
- 7,735
7
votes
Difference in "delegate call via assembly"
OpenZeppelin's one is the most generic and addresses contract upgrade scenario. It forwards msg.data as is to called contracts, and returns what called contract returned.
MakerDAO's is also generic ...
- 6,467
7
votes
Passing constructor arguments to the CREATE assembly instruction in solidity
This worked for CREATE2. Maybe it also works on CREATE?
abi.encodePacked(bytecode, abi.encode(arg1, arg2))
Example (CREATE2)
pragma solidity ^0.5.11;
contract Wallet {
address public owner;
...
- 419
6
votes
Difference between the assembly code generated for uint and uint32?
Others have pointed in the right direction, but let me try to specifically answer the questions.
First, each 256-bit word of contract storage is very expensive. When a contract is using smaller ...
- 8,612
6
votes
Accepted
How does this assembly code create a new "memory end"?
It explained a little later in the same documentation:
Solidity manages memory in a very simple way: There is a “free memory pointer” at position 0x40 in memory. If you want to allocate memory, ...
- 29.3k
6
votes
Accepted
How to get access to the storage mapping through the solidity assembler?
Solution
function getStorageValue(uint num, uint slot) public view returns (address result) {
assembly {
// Store num in memory scratch space (note: lookup "free memory pointer" if you ...
- 1,575
5
votes
Accepted
Help to understand assembly method
This is designed to convert an address to a dynamic bytes type. Addresses are 20 bytes long, and occupy the right-most 20 bytes of a 32-byte word:
...
- 8,612
5
votes
Accepted
Difference between sending a transaction with no to field and create() opcode?
Both create a contract.
An externally owned account (EOA) creates a contract by sending a transaction with no to field.
A contract (account) creates another contract by using the CREATE opcode.
It'...
eth♦
- 77.9k
5
votes
Accepted
Parameters on evm opcode create
This is because memory variables are treated as pointers from in solidity assembly - and that pointer points to the start in memory of the variable. The way arrays are stored in memory in Solidity is ...
- 10.5k
5
votes
Accepted
Instantiating a smart contract in Uniswap using assembly and create2
The lines you showed are using the new CREATE2 opcode, instead of the regular CREATE opcode (which would be invoked by using pair = new UniswapV2Pair()).
The benefit of using CREATE2 is that the ...
- 22.9k
5
votes
Accepted
abi.encode and abi.decode arguments, including variable-sized arrays
I'm not sure what are you trying to achieve but if you are using abi.encode then you can use abi.decode.
// SPDX-License-Identifier: MIT
pragma solidity 0.8.4;
contract A {
function foo() ...
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5
votes
Accepted
Storage and memory layout of strings
Layout of State Variables in Storage > bytes and string
bytes and string are encoded identically. In general, the encoding is similar to bytes1[], in the sense that there is a slot for the array ...
- 1,386
4
votes
What are some examples of how inline assembly benefits smart contract development?
I know this is an old topic, but here's a couple of tutorials I put together to cover assembly in solidity:
Functional Assembly - https://www.youtube.com/watch?v=nkGN6GwkMzU
Instructional Assembly - ...
- 494
4
votes
Accepted
Problem using created array from assembly
The author of the github page that you referenced didn't update the free memory pointer after allocating the memory. Also as Tjaden Hess noted in the comments
return as an opcode and return as a ...
- 10.2k
4
votes
Accepted
How to pass parameters to Contract constructor using when using assembly?
Constructor arguments are appended to the deployment byte code in the standard ABI format.
For example, this contract takes uint constructor argument.
contract Foo {
uint a;
function Foo(...
- 8,612
4
votes
How to convert bytes to uint in Solidity
Slicing an uint out of bytes using inline assembly
function sliceUint(bytes bs, uint start)
internal pure
returns (uint)
{
require(bs.length >= start + 32, "slicing out of range");
...
- 1,258
Only top scored, non community-wiki answers of a minimum length are eligible