# hera

**Repository Path**: FISCO-BCOS/hera

## Basic Information

- **Project Name**: hera
- **Description**: No description available
- **Primary Language**: Unknown
- **License**: Apache-2.0
- **Default Branch**: master
- **Homepage**: None
- **GVP Project**: No

## Statistics

- **Stars**: 0
- **Forks**: 0
- **Created**: 2019-05-22
- **Last Updated**: 2022-07-11

## Categories & Tags

**Categories**: Uncategorized

**Tags**: None

## README

# Hera ![Status](https://circleci.com/gh/ewasm/hera.svg?style=shield&circle-token=:circle-token)

Hera is an [ewasm](https://github.com/ewasm/design) virtual machine implemented in C++ conforming to [EVMC](https://github.com/ethereum/evmc/).

It can be used with [aleth] and perhaps in the future with other implementations through appropriate bindings.

Currently it uses [Binaryen](https://github.com/webassembly/binaryen)'s interpreter for running WebAssembly bytecode and it should be improved to support [wabt](https://github.com/webassembly/wabt) and [wavm](https://github.com/AndrewScheidecker/WAVM) as backends.

## Build options

- `-DHERA_DEBUGGING=ON` will turn on debugging features and messages
- `-DBUILD_SHARED_LIBS=ON` is a standard CMake option to build libraries as shared. This will build Hera shared library that can be then dynamically loaded by EVMC compatible Clients (e.g. `eth` from [cpp-ethereum]).

### wabt support

*Unfinished support, work in progress.*

- `-DHERA_WABT=ON` will request the compilation of wabt support

### WAVM support

*Unfinished support, work in progress.*

- `-DHERA_WAVM=ON` will request the compilation of WAVM support
- `-DLLVM_DIR=...` one will need to specify the path to LLVM's CMake file. In most installations this has to be within the `lib/cmake/llvm` directory, such as `/usr/local/Cellar/llvm/6.0.1/lib/cmake/llvm` on Homebrew.

## Runtime options

These are to be used via EVMC `set_option`:

- `engine=<engine>` will select the underlying WebAssembly engine, where the only accepted value currently is `binaryen`
- `metering=true` will enable metering of bytecode at deployment using the [Sentinel system contract](https://github.com/ewasm/design/blob/master/system_contracts.md#sentinel-contract) (set to `false` by default)
- `evm2wasm=true` will enable metering of bytecode at deployment using the [EVM Transcompiler](https://github.com/ewasm/design/blob/master/system_contracts.md#evm-transcompiler) (set to `false` by default)
- `fallback=true` will allow EVM bytecode to be passed through to the client for execution (if set to `false`, the default, it will be rejected as invalid) (note: is ignored if `evm2wasm` is `true`)
- `evm2wasm.js=true` will use a `evm2wasm.js` as an external commandline tool instead of the system contract
- `evm2wasm.js-trace=true` will turn on EVM tracing for `evm2wasm.js`
- `evm2wasm.cpp=true` will use a `evm2wasm` as a compiled-in dependency instead of the system contract
- `evm2wasm.cpp-trace=true` will turn on EVM tracing for `evm2wasm.cpp`

## Interfaces

Hera implements two interfaces: [EEI](https://github.com/ewasm/design/blob/master/eth_interface.md) and a debugging module.

### Debugging module

- `debug::print32(value: i32)` - print value
- `debug::print64(value: i64)` - print value
- `debug::printMem(offset: i32, len: i32)` - print memory segment as printable characters
- `debug::printMemHex(offset: i32, len: i32)` - print memory segment as hex
- `debug::printStorage(pathOffset: i32)` - print storage value as printable characters
- `debug::printStorageHex(pathOffset: i32)` - print storage value as hex

These are only enabled if Hera is compiled with debugging on.

### EVM Tracing

- `debug::evmTrace(pc: i32, opcode: i32, cost: i32, sp: i32)`

This is useful to trace the transpiled code from [evm2wasm](https://github.com/ewasm/evm2wasm). This is only enabled if Hera is compiled with debugging on.

**Note:** it is valid to invoke `evmTrace` with a negative value for `sp`.  In this case, no stack values will be printed.

## Caveats

Although Hera enables the execution of ewasm bytecode, there are more elements to ewasm an Ethereum node must be aware of:

- [backwards compatibility](https://github.com/ewasm/design/blob/master/backwards_compatibility.md) provisions
- injecting metering code to ewasm contracts
- transcompiling EVM1 contracts to ewasm if desired

All of the above must be implemented outside of Hera.

## Author(s)

Alex Beregszaszi, Jake Lang

## License

Apache 2.0

[aleth]: https://github.com/ethereum/cpp-ethereum