Add Apple Metal backend support

[ArturSkowronski]

Add Apple Metal backend support to the llama-tornado launcher, enabling GPULlama3 to run on macOS with TornadoVM's native Metal driver (shipped in TornadoVM 4.0+).

Changes (launcher only, no Java code changes):

• Add METAL variant to the Backend enum
• Add --metal CLI flag for backend selection
• Configure Metal-specific module path (tornado.drivers.metal) and export list (metal-exports)

The TornadoVM API is backend-agnostic, so the Java inference code works without modification - only the launcher needed updating.

Motivation

TornadoVM 4.0 shipped a Metal backend (PR #796), but llama-tornado only supported --opencl and --ptx. The GPULlama3 README already notes:

"TornadoVM does not have a Metal backend yet [...] until we add a Metal backend to TornadoVM and start optimizing it."

TornadoVM 4.0 has now added it - this PR enables GPULlama3 to use it 😊

Benchmark Results

Tested on Apple M1 Pro (macOS, ARM64) with Llama-3.2-1B-Instruct-f16.gguf.

LLM Inference (GPULlama3)

Backend	TornadoVM	JDK	tok/s	Tokens	Time (s)
OpenCL	2.2.0	21 (GraalVM CE)	6.35	47	7.40
OpenCL	3.0.0	25 (Temurin)	6.87	47	6.84
OpenCL	4.0.0	21 (GraalVM CE)	6.48	57	8.79
Metal	4.0.0	21 (GraalVM CE)	0.23	44	189.85

VectorAdd (10M elements)

Backend	TornadoVM	Best (ms)	Throughput (GB/s)
OpenCL	2.2.0	2.704	41.34
OpenCL	3.0.0	2.427	46.04
OpenCL	4.0.0	2.695	41.47
Metal	4.0.0	2.392	46.72

Analysis

VectorAdd: Metal is competitive with OpenCL (~46 GB/s), slightly faster on simple parallel kernels. This matches expectations - the Metal backend handles straightforward array operations well.

LLM inference: Metal is ~28x slower than OpenCL (0.23 vs 6.48 tok/s). This is consistent with the known state of the Metal backend:

• TornadoVM PR #796 notes that MSL-specific optimizations (threadgroup memory, SIMD shuffle, async copies) are not yet implemented
• The Metal backend test pass rate does not yet match OpenCL/PTX/SPIR-V
• LLM inference involves complex control flow and frequent CPU ↔ GPU transfers - precisely the workloads where the immature Metal backend struggles

[CLAassistant]

All committers have signed the CLA.

[ArturSkowronski]

Hey @mikepapadim - sharing my numbers and analysis 😁 I wanted to test it for the new JVM Weekly, here are my results.

I will update also my repo with new TornadoVM: https://github.com/ArturSkowronski/conference-jvm-in-age-ai-2026

[mikepapadim]

Hello @ArturSkowronski , thank you for your contribution! Thats great actually.

Can you let me know which models you tested with the metal backend?
I need to add into the CI workflow otherwise I am quite happy to merge.

Also, can you please sign the CLA?

[ArturSkowronski]

@mikepapadim - Here you will find the whole "benchmark" I use 😊

ArturSkowronski/conference-jvm-in-age-ai-2026#13

Model under test from my side: Llama-3.2-1B-Instruct-f16.gguf