llvm/docs/GlobalISel/RegBankSelect.rst

.. _regbankselect:

RegBankSelect
-------------

This pass constrains the :ref:`gmir-gvregs` operands of generic
instructions to some :ref:`gmir-regbank`.

It iteratively maps instructions to a set of per-operand bank assignment.
The possible mappings are determined by the target-provided
:ref:`RegisterBankInfo <api-registerbankinfo>`.
The mapping is then applied, possibly introducing ``COPY`` instructions if
necessary.

It traverses the ``MachineFunction`` top down so that all operands are already
mapped when analyzing an instruction.

This pass could also remap target-specific instructions when beneficial.
In the future, this could replace the ExeDepsFix pass, as we can directly
select the best variant for an instruction that's available on multiple banks.

.. _api-registerbankinfo:

API: RegisterBankInfo
^^^^^^^^^^^^^^^^^^^^^

The ``RegisterBankInfo`` class describes multiple aspects of register banks.

* **Banks**: ``addRegBankCoverage`` --- which register bank covers each
  register class.

* **Cross-Bank Copies**: ``copyCost`` --- the cost of a ``COPY`` from one bank
  to another.

* **Default Mapping**: ``getInstrMapping`` --- the default bank assignments for
  a given instruction.

* **Alternative Mapping**: ``getInstrAlternativeMapping`` --- the other
  possible bank assignments for a given instruction.

``TODO``:
All this information should eventually be static and generated by TableGen,
mostly using existing information augmented by bank descriptions.

``TODO``:
``getInstrMapping`` is currently separate from ``getInstrAlternativeMapping``
because the latter is more expensive: as we move to static mapping info,
both methods should be free, and we should merge them.

.. _regbankselect-modes:

RegBankSelect Modes
^^^^^^^^^^^^^^^^^^^

``RegBankSelect`` currently has two modes:

* **Fast** --- For each instruction, pick a target-provided "default" bank
  assignment.  This is the default at -O0.

* **Greedy** --- For each instruction, pick the cheapest of several
  target-provided bank assignment alternatives.

We intend to eventually introduce an additional optimizing mode:

* **Global** --- Across multiple instructions, pick the cheapest combination of
  bank assignments.

``NOTE``:
On AArch64, we are considering using the Greedy mode even at -O0 (or perhaps at
backend -O1):  because :ref:`gmir-llt` doesn't distinguish floating point from
integer scalars, the default assignment for loads and stores is the integer
bank, introducing cross-bank copies on most floating point operations.