llvm dev - Jan 2021 - AMDGPU and support for the new pass manager

1) Unfortunately LegacyDivergenceAnalysis has known bugs (e.g.
https://bugs.llvm.org/show_bug.cgi?id=42741) but DivergenceAnalysis
does not work on irreducible control flow (it comes with a wrapper
that falls back to LegacyDivergenceAnalysis if it detects irreducible
control flow). So neither is perfect. Does the new pass manager
currently support either of them?

I think Sameer has looked into
https://bugs.llvm.org/show_bug.cgi?id=42741 in some detail. I'm not
sure if there are other known bugs.

Jay.

Jay.

On Mon, 4 Jan 2021 at 22:16, Arthur Eubanks via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> I've ported most of the IR passes and added them to AMDGPU's opt
pipeline.
> There are 2 issues remaining:
> 1) LegacyDivergenceAnalysis is used in LoopUnswitch to avoid unswitching
loops with divergent condition Values. I'm not sure what the state of the
LegacyDivergenceAnalysis vs DivergenceAnalysis is. (Also the new PM only has
SimpleLoopUnswitch instead of LoopUnswitch).
> Transforms/LoopUnswitch/AMDGPU/divergent-unswitch.ll.
> 2) The AMDGPU backend has its own inliner (set here). Any ideas for how to
do custom inliner cost modeling in the new PM pipelines? Allow targets to
override an analysis pass that the inliner uses to get an InlineAdvisor?
> CodeGen/AMDGPU/amdgpu-inline.ll
>
> On Fri, Dec 4, 2020 at 4:44 PM Arthur Eubanks <aeubanks at
google.com> wrote:
>>
>> Some indicative failing tests are under the llvm/test/CodeGen/AMDGPU
directory when opt's -enable-new-pm flag is set to true by default. They
should also be repro'able by adding a corresponding NPM RUN line, e.g.
"opt -S -O1 -mtriple=amdgcn-- ..." -> "opt -S
-passes='default<O1>' -mtriple=amdgcn-- ...".
>>
>> I see the following AMDGPU-specific failures:
>>   LLVM :: CodeGen/AMDGPU/amdgpu-inline.ll
>>   LLVM :: CodeGen/AMDGPU/infer-addrpace-pipeline.ll
>>   LLVM :: CodeGen/AMDGPU/internalize.ll
>>   LLVM :: CodeGen/AMDGPU/llvm.amdgcn.wavefrontsize.ll
>>   LLVM :: CodeGen/AMDGPU/opt-pipeline.ll
>>   LLVM :: CodeGen/AMDGPU/propagate-attributes-clone.ll
>>   LLVM :: CodeGen/AMDGPU/propagate-attributes-single-set.ll
>>   LLVM :: CodeGen/AMDGPU/simplify-libcalls.ll
>>   LLVM :: CodeGen/AMDGPU/sroa-before-unroll.ll
>>   LLVM :: Transforms/LoopUnswitch/AMDGPU/divergent-unswitch.ll
>>
>> (Also I took a closer look, actually it looks like NVPTX also needs
updating, but NVPTXTargetMachine only adds two passes)
>>
>> On Fri, Dec 4, 2020 at 3:48 PM Arthur Eubanks <aeubanks at
google.com> wrote:
>>>
>>> Currently AMDGPU is the last target which injects passes into the
pipeline but hasn't been updated to work with the new pass manager.
>>>
>>> Recently there was added support for the NPM's equivalent of
TargetMachine::adjustPassManager(). (Bug, Phab). BPF and Hexagon have their NPM
equivalents implemented already, but AMDGPU has a lot of custom passes added by
adjustPassManager and hasn't been updated to work with the NPM.
>>>
>>> Could the maintainers of AMDGPU port the necessary passes to the
new pass manager add them to
AMDGPUTargetMachine::registerPassBuilderCallbacks()? I'm happy to provide
any guidance if necessary. Here's an example of porting a pass and adding it
to the pipeline.
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Adding Simon Moll. Also replaced my work ID with my personal ID because Outlook
at work doesn't seem to play well with llvm-dev.

---- On Mon, 11 Jan 2021 22:11:22 +0530 Jay Foad via llvm-dev <llvm-dev at
lists.llvm.org> wrote ----

 > On Mon, 4 Jan 2021 at 22:16, Arthur Eubanks via llvm-dev
 > <llvm-dev at lists.llvm.org> wrote:
 > >
 > > I've ported most of the IR passes and added them to AMDGPU's
opt pipeline.
 > > There are 2 issues remaining:
 > > 1) LegacyDivergenceAnalysis is used in LoopUnswitch to avoid
unswitching loops with divergent condition Values. I'm not sure what the
state of the LegacyDivergenceAnalysis vs DivergenceAnalysis is. (Also the new PM
only has SimpleLoopUnswitch instead of LoopUnswitch).
 > > Transforms/LoopUnswitch/AMDGPU/divergent-unswitch.ll.

Here's what I understood so far from a general eyeballing of the code:

1. Legacy DA is a wrapper around GPU DA. I could not find any direct use of GPU
DA.
2. It is likely that AMDGPU no longer "requires" Legacy DA. The
advantage of the Legacy DA is that it can handle irreducible regions, but we
usually convert them into loops for AMDGPU anyway. I don't know if we have
reached a point where we don't care about legacy DA in this respect.
3. StructurizeCFG under the new pass manager simply skips DA, and consequently
cannot skip uniform regions. This essentially disables an optimization when
moving to the new pass manager.
4. Similarly, loop unswitching is an optimization and available in its simple
form with the new pass manager. But I don't know for sure if it *must* skip
loops with divergent values.

So we (the AMDGPU folks) need to figure out how much is our dependency on any DA
in the new pass manager. Besides the above optimizations, isn't it required
for later target optimizations around SGPR usage? Is it possible to unblock the
transition to the new pass manager, and then later restore these optimizations?
Also, I am wondering if can focus on making only the new GPU DA available
subject to #2 above.

Sameer.