forked from OSchip/llvm-project
74 lines
2.5 KiB
ReStructuredText
74 lines
2.5 KiB
ReStructuredText
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.. _regbankselect:
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RegBankSelect
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-------------
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This pass constrains the :ref:`gmir-gvregs` operands of generic
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instructions to some :ref:`gmir-regbank`.
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It iteratively maps instructions to a set of per-operand bank assignment.
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The possible mappings are determined by the target-provided
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:ref:`RegisterBankInfo <api-registerbankinfo>`.
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The mapping is then applied, possibly introducing ``COPY`` instructions if
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necessary.
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It traverses the ``MachineFunction`` top down so that all operands are already
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mapped when analyzing an instruction.
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This pass could also remap target-specific instructions when beneficial.
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In the future, this could replace the ExeDepsFix pass, as we can directly
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select the best variant for an instruction that's available on multiple banks.
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.. _api-registerbankinfo:
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API: RegisterBankInfo
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^^^^^^^^^^^^^^^^^^^^^
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The ``RegisterBankInfo`` class describes multiple aspects of register banks.
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* **Banks**: ``addRegBankCoverage`` --- which register bank covers each
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register class.
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* **Cross-Bank Copies**: ``copyCost`` --- the cost of a ``COPY`` from one bank
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to another.
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* **Default Mapping**: ``getInstrMapping`` --- the default bank assignments for
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a given instruction.
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* **Alternative Mapping**: ``getInstrAlternativeMapping`` --- the other
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possible bank assignments for a given instruction.
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``TODO``:
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All this information should eventually be static and generated by TableGen,
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mostly using existing information augmented by bank descriptions.
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``TODO``:
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``getInstrMapping`` is currently separate from ``getInstrAlternativeMapping``
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because the latter is more expensive: as we move to static mapping info,
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both methods should be free, and we should merge them.
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.. _regbankselect-modes:
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RegBankSelect Modes
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^^^^^^^^^^^^^^^^^^^
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``RegBankSelect`` currently has two modes:
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* **Fast** --- For each instruction, pick a target-provided "default" bank
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assignment. This is the default at -O0.
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* **Greedy** --- For each instruction, pick the cheapest of several
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target-provided bank assignment alternatives.
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We intend to eventually introduce an additional optimizing mode:
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* **Global** --- Across multiple instructions, pick the cheapest combination of
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bank assignments.
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``NOTE``:
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On AArch64, we are considering using the Greedy mode even at -O0 (or perhaps at
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backend -O1): because :ref:`gmir-llt` doesn't distinguish floating point from
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integer scalars, the default assignment for loads and stores is the integer
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bank, introducing cross-bank copies on most floating point operations.
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