1.. _regbankselect:
2
3RegBankSelect
4-------------
5
6This pass constrains the :ref:`gmir-gvregs` operands of generic
7instructions to some :ref:`gmir-regbank`.
8
9It iteratively maps instructions to a set of per-operand bank assignment.
10The possible mappings are determined by the target-provided
11:ref:`RegisterBankInfo <api-registerbankinfo>`.
12The mapping is then applied, possibly introducing ``COPY`` instructions if
13necessary.
14
15It traverses the ``MachineFunction`` top down so that all operands are already
16mapped when analyzing an instruction.
17
18This pass could also remap target-specific instructions when beneficial.
19In the future, this could replace the ExeDepsFix pass, as we can directly
20select the best variant for an instruction that's available on multiple banks.
21
22.. _api-registerbankinfo:
23
24API: RegisterBankInfo
25^^^^^^^^^^^^^^^^^^^^^
26
27The ``RegisterBankInfo`` class describes multiple aspects of register banks.
28
29* **Banks**: ``addRegBankCoverage`` --- which register bank covers each
30  register class.
31
32* **Cross-Bank Copies**: ``copyCost`` --- the cost of a ``COPY`` from one bank
33  to another.
34
35* **Default Mapping**: ``getInstrMapping`` --- the default bank assignments for
36  a given instruction.
37
38* **Alternative Mapping**: ``getInstrAlternativeMapping`` --- the other
39  possible bank assignments for a given instruction.
40
41``TODO``:
42All this information should eventually be static and generated by TableGen,
43mostly using existing information augmented by bank descriptions.
44
45``TODO``:
46``getInstrMapping`` is currently separate from ``getInstrAlternativeMapping``
47because the latter is more expensive: as we move to static mapping info,
48both methods should be free, and we should merge them.
49
50.. _regbankselect-modes:
51
52RegBankSelect Modes
53^^^^^^^^^^^^^^^^^^^
54
55``RegBankSelect`` currently has two modes:
56
57* **Fast** --- For each instruction, pick a target-provided "default" bank
58  assignment.  This is the default at -O0.
59
60* **Greedy** --- For each instruction, pick the cheapest of several
61  target-provided bank assignment alternatives.
62
63We intend to eventually introduce an additional optimizing mode:
64
65* **Global** --- Across multiple instructions, pick the cheapest combination of
66  bank assignments.
67
68``NOTE``:
69On AArch64, we are considering using the Greedy mode even at -O0 (or perhaps at
70backend -O1):  because :ref:`gmir-llt` doesn't distinguish floating point from
71integer scalars, the default assignment for loads and stores is the integer
72bank, introducing cross-bank copies on most floating point operations.
73
74