New register bank select for AMDGPU will be split in two passes:
- AMDGPURegBankSelect: select banks based on machine uniformity analysis
- AMDGPURegBankLegalize: lower instructions that can't be inst-selected
with register banks assigned by AMDGPURegBankSelect.
AMDGPURegBankLegalize is similar to legalizer but with context of
uniformity analysis. Does not change already assigned banks.
Main goal of AMDGPURegBankLegalize is to provide high level table-like
overview of how to lower generic instructions based on available target
features and uniformity info (uniform vs divergent).
See RegBankLegalizeRules.
Summary of new features:
At the moment register bank select assigns register bank to output
register using simple algorithm:
- one of the inputs is vgpr output is vgpr
- all inputs are sgpr output is sgpr.
When function does not contain divergent control flow propagating
register banks like this works. In general, first point is still correct
but second is not when function contains divergent control flow.
Examples:
- Phi with uniform inputs that go through divergent branch
- Instruction with temporal divergent use.
To fix this AMDGPURegBankSelect will use machine uniformity analysis
to assign vgpr to each divergent and sgpr to each uniform instruction.
But some instructions are only available on VALU (for example floating
point instructions before gfx1150) and we need to assign vgpr to them.
Since we are no longer propagating register banks we need to ensure that
uniform instructions get their inputs in sgpr in some way.
In AMDGPURegBankLegalize uniform instructions that are only available on
VALU will be reassigned to vgpr on all operands and read-any-lane vgpr
output to original sgpr output.
This reverts commit e9c49901a43f5b16c3df416460b7e4dbdd24ce03.
Current AMDGPURegBankSelect does nothing different then RegBankSelect.
Revert to using generic RegBankSelect in preparation for adding new
regbankselect passes. New AMDGPURegBankSelect, that will use uniformity
analysis for regbank select decisions, will not subclass RegBankSelect.
Revert regression tests to use regbankselect since amdgpu-regbankselect
will be used by new pass and behavior will be different.
Summary:
- Remove wrappers in `MachineDominatorTree`.
- Remove `MachineDominatorTree` update code in
`MachineBasicBlock::SplitCriticalEdge`.
- Use `MachineDomTreeUpdater` in passes which call
`MachineBasicBlock::SplitCriticalEdge` and preserve
`MachineDominatorTreeWrapperPass` or CFG analyses.
Commit abea99f65a97248974c02a5544eaf25fc4240056 introduced related
methods in 2014. Now we have SemiNCA based dominator tree in 2017 and
dominator tree updater, the solution adopted here seems a bit outdated.
Prepare for new pass manager version of `MachineDominatorTreeAnalysis`.
We may need a machine dominator tree version of `DomTreeUpdater` to
handle `SplitCriticalEdge` in some CodeGen passes.
Uniformity analysis needs to be the fundamental basis for
regbank decisions. The considerations of the default pass
are secondary, but potentially useful for some edge cases (e.g.
selecting AGPRs when arbitrary loads and stores can directly use
them). This needs to be a separate pass since it requires new
analysis dependencies.
Boilerplate to subclass the existing pass which does nothing
different.
