Performance Engineer, Language Runtime
European Tech Recruit·Senior·On-site · Cambridge
Posted Oct 18, 2026·eu-recruit.com
Performance Engineer, Language Runtime - PAYE contract / C++ / Rust
Our client is looking for a Performance Engineer to make managed language runtimes run measurably faster on ARM64 mobile-class CPUs. You will work across the full runtime stack, from interpreter, JIT and AOT compilers to the garbage collector and runtime libraries, and follow every optimisation down to the generated machine code and the hardware counters that explain it.
The is as inside IR35/PAYE contract, initially for 12 months' and will require 100% onsite presence in Cambridge, UK.
You will sit within an existing CPU team, so what you learn about how runtimes stress the pipeline, caches and branch predictors will feed directly into next-generation core design. You will also work with LLM-agent pipelines that automate parts of the analyse, patch, benchmark and validate loop.
What you will be doing:
- Finding and ranking performance bottlenecks in interpreter, JIT/AOT, GC and runtime library code using sampling profilers, PMU counters, top-down analysis and flame graphs
- Improving interpreter and compiler tiers, including dispatch, inline caches, hidden classes, tier-up and deoptimisation policy, instruction selection, register allocation, inlining heuristics and check elimination
- Reducing startup time through snapshots, AOT images, lazy compilation and warm-up tuning
- Tuning allocation and collection for throughput, pause time and footprint, including barriers, generational and concurrent collectors, object layout and pointer compression
- Explaining runtime behaviour in hardware terms, such as indirect-branch misprediction, I-cache and iTLB pressure and load-to-use latency, and turning findings into software changes and hardware feedback
- Building benchmarking and regression-tracking pipelines with noise control and statistically sound A/B comparison
- Building and using LLM-agent pipelines to analyse profiles, propose patches and validate correctness
- Working with CPU architects on which hardware or ISA features would help runtime workloads
- Landing upstream patches, writing technical reports and contributing to patents and publications where appropriate
Your background:
- Degree in Computer Science, Computer Engineering, Electrical Engineering or a related field, or equivalent practical experience
- Strong C and C++ (or Rust), with the ability to work productively in a large, performance-critical codebase
- Hands-on performance work in a language runtime, VM or compiler backend, with measured speedups you can walk through in detail (for example V8, JavaScriptCore, SpiderMonkey, HotSpot, PyPy, LuaJIT, CPython or an LLVM backend)
- Fluency with profiling tools such as perf, PMU counters and flame graphs, and the ability to read disassembly and relate it to source
- Solid grasp of a modern ISA (ARM64 or x86-64), calling conventions, the memory model and the cost of branches and cache misses
- Measurement rigour: you design experiments, control for noise and report results with appropriate statistics
Desirable skills:
- Working knowledge of CPU microarchitecture, including out-of-order pipelines, branch prediction, caches, TLBs and prefetching
- ARM64 experience, particularly on mobile or embedded SoCs
- Contributions to an open-source runtime, compiler or GC
- Experience with LLM agents or ML-assisted tooling for code optimisation
- Familiarity with simulators such as gem5
- Publications or patents in runtimes, compilers or computer architecture
In accordance with local employment laws, applicants must have current, valid authorisation to work in the UK at the time of application. We are unable to sponsor employment visas for this role.
