[llvm-dev] Using the new pass manager for CodeGen

[Fedor Sergeev via llvm-dev]

Hi, Charles,

On 6/13/19 11:48 PM, via llvm-dev wrote:
> Hi,
>
> I am looking into potentially porting CodeGen and some of the passes 
> to using the new pass manager. Two questions:
>
> 1. Has anybody made an attempt in this direction already? Maybe I am 
> missing a branch out there that has started on this already. From what 
> I can tell, there is no work in-tree in CodeGen that is aware of the 
> new pass manager besides some assembly utilities in BackendUtils.cpp.
You are right, there is no activity in-tree in that direction to the 
best of my knowledge.
And every time I was asking on LLVM devmtg if anybody working on that 
the answer was - "nay".
> My initial thoughts in this direction were that there would need to be 
> some new pass manager machinery, for example, things like 
> FunctionToMachineFunctionPassAdaptor that handles what 
> MachineFunctionPass::runOnFunction used to do, along with the 
> associated AnalysisManager plumbing to understand units of MIR.
Generally, what you describe is indeed a proper first step towards 
getting CodeGen on New-PM rails:
  - introduce IRUnits for Machine IR
  - provide generic handling for those new units everywhere in New-PM 
framework

The next step would be to start moving at least one backend towards it, 
and thats where the initiative kinda stalls.
I believe people working on CodeGen just do not find enough incentive to 
start that work.

>
> 2. Are there any compile time improvements expected from porting 
> CodeGen passes in the first place? As far as I can tell, the llc pass 
> pipeline seems to be fairly well fixed in terms of phase ordering, and 
> all the analyses are thrown out from opt anyway.
I would expect real savings just by reusing non-Machine-IR analyses.
As soon as you use the same llvm state/module/etc for both Opt and 
CodeGen purposes
(thats what I believe many do, at least our JIT compiler does that). 
That will allow you
to reuse analysis manager and thus keep every cached analysis there.

I would love to see Opt and CodeGen using the same pass manager (as my 
downstream project has already switched to NewPM in Opt).

Said that, perhaps analyses are not a current performance bottleneck in 
CodeGen (I bet instruction selection will always trump anything else).
So this is more about overall consistency of operations rather than 
crazy performance gains.

regards,
   Fedor.

> Thanks,
> Charles
>
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