Created Understanding and debugging Nimbus EVM JSON tests (markdown)
Mamy Ratsimbazafy
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## Introduction
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Nimbus JSON tests suite is taken from the [official Ethereum tests](https://github.com/ethereum/tests). As of July 2018, only the VMTests are used out of the following list:
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- ABITests
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- BasicTests
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- BlockchainTests
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- GeneralStateTests
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- GenesisTests
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- KeyStoreTests
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- PoWTests
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- RLPTests
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- RPCTests
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- TransactionTests
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- TrieTests
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- VMTests
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The full documentation for the official Ethereum tests is available [here](https://ethereum-tests.readthedocs.io/en/latest/).
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## VMTests
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### JSON structure
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A JSON EVM test has the following structure (example is add0.json).
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```json
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{
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"add0" : {
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"_info" : {
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"comment" : "",
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"filledwith" : "cpp-1.3.0+commit.6e0ce939.Linux.g++",
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"lllcversion" : "Version: 0.4.18-develop.2017.9.25+commit.a72237f2.Linux.g++",
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"source" : "src/VMTestsFiller/vmArithmeticTest/add0Filler.json",
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"sourceHash" : "dcc7fc8aebdc2d7334440cfe6c63172941b4164c1ba8c32897318ca0cdfb7a1c"
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},
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"callcreates" : [
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],
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"env" : {
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"currentCoinbase" : "0x2adc25665018aa1fe0e6bc666dac8fc2697ff9ba",
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"currentDifficulty" : "0x0100",
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"currentGasLimit" : "0x0f4240",
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"currentNumber" : "0x00",
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"currentTimestamp" : "0x01"
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},
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"exec" : {
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"address" : "0x0f572e5295c57f15886f9b263e2f6d2d6c7b5ec6",
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"caller" : "0xcd1722f2947def4cf144679da39c4c32bdc35681",
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"code" : "0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff01600055",
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"data" : "0x",
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"gas" : "0x0186a0",
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"gasPrice" : "0x5af3107a4000",
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"origin" : "0xcd1722f2947def4cf144679da39c4c32bdc35681",
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"value" : "0x0de0b6b3a7640000"
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},
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"gas" : "0x013874",
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"logs" : "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
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"out" : "0x",
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"post" : {
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"0x0f572e5295c57f15886f9b263e2f6d2d6c7b5ec6" : {
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"balance" : "0x0de0b6b3a7640000",
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"code" : "0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff01600055",
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"nonce" : "0x00",
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"storage" : {
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"0x00" : "0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffe"
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}
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}
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},
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"pre" : {
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"0x0f572e5295c57f15886f9b263e2f6d2d6c7b5ec6" : {
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"balance" : "0x0de0b6b3a7640000",
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"code" : "0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff01600055",
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"nonce" : "0x00",
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"storage" : {
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}
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}
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}
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}
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}
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```
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**Important:** As of July 2018, VMTests always use Homestead fork for opcode implementations (notably `CALL`) and gas prices. We should not rely on `currentNumber`.
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The important fields are:
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- `exec` section:
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- `code`: the code being executed in isolation. (In production this is part of a transaction).
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- `gas`: usually 0x0186a0 (100000) the starting gas
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- `gas` field: the remaining gas after code execution. Here 0x013874 (79988)
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- `pre` section: the state before execution of the code by the VM
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- storage: if the account already stores data that we can retrieve with SLOAD or overwrite/reset with SSTORE
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- `post` section: the state after execution of the code by the VM.
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If there is no `post` section, the code is supposed to throw an EVM exception.
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After an EVM exception, gas is consumed but state is reverted.
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Implementation-wise `computation.isError` returns true and an error message is available in `computation.error` field (implementation as of July 2018)
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### Decompiling bytecode.
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You can decompile EVM bytecode using Etherscan: https://etherscan.io/opcode-tool
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Pasting 0x6003600202600055 gives:
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```
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[1] PUSH1 0x03
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[3] PUSH1 0x02
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[4] MUL
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[6] PUSH1 0x00
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[7] SSTORE
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```
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Alternatively you can use the following:
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```Nim
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import ../nimbus/vm/code_stream, strformat
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var c = newCodeStreamFromUnescaped("0x6003600202600055")
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let opcodes = c.decompile()
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for op in opcodes:
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echo &"[{op[0]}]\t{op[1]}\t{op[2]}"
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# [1] PUSH1 0x03
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# [3] PUSH1 0x02
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# [4] MUL
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# [6] PUSH1 0x00
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# [7] SSTORE
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# [-1] STOP
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```
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The number in bracket refers to the position after reading the opcode and its arguments (i.e. the value of the program counter).
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`add0 (0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff01600055)` decompiles to:
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```
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[32] PUSH32 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
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[65] PUSH32 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
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[66] ADD
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[68] PUSH1 0x00
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[69] SSTORE
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```
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