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.. _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.
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