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MLIR dialects

Overview

To separate different hardware and software targets, MLIR has “dialects”, including:

  • TensorFlow IR, which represents all things possible in TensorFlow graphs.
  • XLA HLO IR, which is designed to take advantage of XLA’s compilation abilities (with output to, among other things, TPUs).
  • An experimental affine dialect, which focuses on polyhedral representations and optimizations.
  • LLVM IR, which has a 1:1 mapping between it and LLVM’s own representation, allowing MLIR to emit GPU and CPU code through LLVM.
  • TensorFlow Lite, which will translate to running code on mobile platforms.

Each dialect consists of a set of defined operations which have invariants placed on them, like: “This is a binary operator, and the inputs and outputs have the same types.”

Adding to MLIR

MLIR has no fixed/built-in list of globally known operations (no “intrinsics”). Dialects can define entirely custom types, which is how MLIR can model things like the LLVM IR type system (which has first class aggregates), domain abstractions important for ML-optimized accelerators like quantized types, and even the Swift or Clang type systems (which are built around Swift/Clang declaration nodes) in the future.

If you want to connect a new low-level compiler, you would create a new dialect and the lowerings between the TensorFlow Graph dialect and your dialect. This smooths the path for hardware and compiler makers. You can even target dialects at different levels in the same model; the higher-level optimizers will respect the unfamiliar parts of the IR and wait for a lower level to handle it.