PVM and the pallet-revive runtime target

The revive compiler targets PolkaVM (PVM) via pallet-revive on Polkadot.

Target CPU configuration

The exact target CPU configuration can be found here.

Note

The PVM linker requires fully relocatable ELF objects.

Why PVM

PVM is a RISC-V based VM designed to overcome the flaws of WebAssebmly (Wasm). Wasm was believed to be a more efficient successor to the rather slow EVM. However, Wasm is far from an ideal target for smart contracts as some of its design decisions are unfavorable for short-lived workloads. The main problem is on-chain Wasm bytecode compilation or interpretation overhead. Prior benchmarks consistently ignoring this overhead seeded the blockchain industry with flawed assumptions: Only when ignoring the startup overhead Wasm is much faster than the slow computing EVM. In practice however, gains are nullified entirely and Wasm loses completely even against very slow VMs like the EVM. Executing Wasm contracts is in fact so inefficient that typical contract workloads are orders of magnitude more expensive than the equivalent EVM variant.

On the other hand, since RISC-V is similar to CPUs found in validator hardware (x86 and ARM), bytecode translation mostly boils down to a linear mapping from one instruction to another. The embedded ISA specification reduces the number of general purpose registers, in turn removing the need for expensive register allocation. This guarantees single-pass O(n) JIT compilation of contract bytecode. The close proximity of PVM bytecode with actual validator CPU bytecode effectively allows to move all expensive compilation workload off-chain. Benchmarks (1, 2) show that with the PVM JIT, sandboxed PVM code executes at around half the speed of native code, which falls into the same ballpark of the state-of-the-art wasmtime Wasm implementation (while EVM sits somewhere around 1/10 to less than 1/100 of native speed). However, the PVM JIT compiler only uses a fraction of the time wasmtime requires to compile the code.

Note

The PVM JIT isn’t available yet in pallet-revive. At the time of writing, the contract code is interpreted, which is orders of magnitude slower than the JIT.

Host environment: pallet-revive

The revive compiler targets the pallet-revive runtime environment.

pallet-revive exposes a syscall like interface for contract interactions with the host environment. This is provided by the revive-runtime-api library.

After the initial launch on the Polkadot Asset Hub blockchain, the runtime API is considered stable and backwards compatible indefinitively.