XLA Terminology

There are several terms that are used in the context of XLA, MLIR, LLVM, and other related technologies. Below is a partial list of these terms and their definitions.

• OpenXLA
  • OpenXLA is an open ecosystem of performant, portable, and extensible machine learning (ML) infrastructure components that simplify ML development by defragmenting the tools between frontend frameworks and hardware backends. It includes the XLA compiler, StableHLO, VHLO, PJRT and other components.
• XLA
  • XLA (Accelerated Linear Algebra) is an open source compiler for machine learning. The XLA compiler takes models from popular frameworks such as PyTorch, TensorFlow, and JAX, and optimizes the models for high-performance execution across different hardware platforms including GPUs, CPUs, and ML accelerators. The XLA compiler outputs some code to LLVM, some to "standard" MLIR, and some to Triton MLIR that is processed by (MLIR-based) OpenAI Triton compiler.
• PJRT
  • PJRT is a uniform Device API that simplifies the growing complexity of ML workload execution across hardware and frameworks. It provides a hardware and framework independent interface for compilers and runtimes.
• StableHLO
  • StableHLO is the public interface to OpenXLA, it is a standardized MLIR dialect that may be used by different frameworks and compilers in the OpenXLA ecosystem. XLA supports StableHLO, and immediately converts it to HLO on the input. There are some StableHLO to StableHLO passes implemented using the MLIR framework. It is also possible to convert StableHLO to other compilers' IR without using HLO, for example in cases where an existing IR is more appropriate.
• CHLO
  • CHLO is a collection of higher level operations which are optionally decomposable to StableHLO.
• VHLO
  • The VHLO Dialect is a MLIR dialect that is a compatibility layer on top of StableHLO. It provides a snapshot of the StableHLO dialect at a given point in time by versioning individual program elements, and is used for serialization and stability.
• MHLO
  • MHLO is a standalone MLIR-based representation of XLA's HLO IR. The dialect is being evaluated for deprecation, and new users of the dialect should prefer to use StableHLO instead.
• HLO
  • HLO (High Level Optimizer) is an internal graph representation (IR) for the XLA compiler (and also supported input). It is not based on MLIR, and has its own textual syntax and binary (protobuf based) representation.
• MLIR
  • MLIR is a hybrid IR infrastructure that allows users to define "dialects" of operations at varying degrees of abstraction, and gradually lower between these opsets, performing transformations at each level of granularity. StableHLO and CHLO are two examples of MLIR dialects.
• LLVM
  • LLVM is a compiler backend, and a language that it takes as an input. Many compilers generate LLVM code as a first step, and then LLVM generates machine code from it. This allows developers to reuse code that is similar in different compilers, and also makes supporting different target platforms easier. XLA:GPU and CPU backends have LLVM IR emitters for targeting specific hardware.