This pass constrains the Generic Virtual Registers operands of generic instructions to some Register Bank.

It iteratively maps instructions to a set of per-operand bank assignment. The possible mappings are determined by the target-provided 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

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 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 Low Level Type 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.