[llvm-dev] Machine verification and LiveIntervals

[Jay Foad via llvm-dev]

Hi,

I've been working on two related codegen cleanups: enabling the
machine verifier at more places in the codegen pass pipeline, and
finishing the transition of TwoAddressInstructionPass from
LiveVariables to LiveIntervals. I'd appreciate help with either or
both of them: code reviews, comments, patches, advice, testing etc.

1. Enabling the machine verifier at more places in the codegen pass pipeline

Currently verification is disabled (by passing false as a second
argument to TargetPassConfig::addPass) in various places, mostly
around register allocation and after addPreEmitPass. I think we should
strive to have it enabled everywhere.

My current work in progress branch is here:
https://github.com/jayfoad/llvm-project/tree/enable-machine-verifier

The outstanding patches are:
https://reviews.llvm.org/D110689 [DetectDeadLanes] Enable machine
verification after this pass
https://reviews.llvm.org/D110695 [ProcessImplicitDefs] Enable machine
verification after this pass
https://reviews.llvm.org/D110697 [UnreachableMachineBlockElim] Enable
machine verification after this pass
https://reviews.llvm.org/D110700 [LiveVariables] Skip verification of
kills inside bundles
https://reviews.llvm.org/D110703 [MachineLoopInfo] Enable machine
verification after this pass
https://reviews.llvm.org/D110829 [X86] Copy registers in reverse order
in convertToThreeAddress
https://reviews.llvm.org/D110834 [PHIElimination] Account for
INLINEASM_BR when inserting kills
https://reviews.llvm.org/D110848 [TwoAddressInstruction] Fix
ReplacedAllUntiedUses in processTiedPairs

The status of that branch, based on running llvm-check-codegen with
LLVM_ENABLE_EXPENSIVE_CHECKS on, is:
- verification is still disabled after addPreEmitPass, but only
because of problems with the Hexagon VLIW packetizer.
- it is still disabled after TwoAddressInstructionPass, but only
because of an AMDGPU subreg problem that I haven't understood yet.
- it is still disabled after PHIElimination because of a problem with
updating LiveVariables that shows up on ARM and Thumb2 tests: "Block
should not be in AliveBlocks" / "Block missing from AliveBlocks"

2. Finishing the transition of TwoAddressInstructionPass from
LiveVariables to LiveIntervals

This is work that was started by Jakob Stoklund Olesen and Cameron
Zwarich back in 2013 (thread at
https://lists.llvm.org/pipermail/llvm-dev/2013-February/059261.html)
but never completed. TwoAddressInstructionPass is almost(*) the only
user of LiveVariables. It already has some support for LiveIntervals.
When that is completed, we can switch to using LiveIntervals
throughout and remove LiveVariables from the codebase. There is a
command line flag -early-live-intervals that you can use to test the
switchover.

Apart from paying off technical debt, the advantage of switching to
LiveIntervals is that it gives you accurate subregister liveness
information. With LiveVariables you have to rely on kill flags, which
apply to the whole "super" register not to individual subregs. (There
may be other advantages but that is the one I'm interested in.)

My current work in progress branch is here:
https://github.com/jayfoad/llvm-project/tree/repair-live-intervals

The outstanding patches are:
https://reviews.llvm.org/D110411 [LiveIntervals] Update subranges in
processTiedPairs
https://reviews.llvm.org/D110182 [LiveIntervals] Find better anchoring
end points when repairing ranges

The status of that branch, based on running the ~18500 tests in
llvm-check-codegen with -early-live-intervals forced on, is:
- 76 tests crash, fail assertions, or fail machine verification
- 215 other tests fail, but //hopefully// only due to minor
differences in the generated code

(*) Actually the AMDGPU target has a pass that uses LiveVariables, but
that was introduced quite recently and should be easy to convert to
LiveIntervals.

Thanks!
Jay.