[llvm-dev] [RFC] Switching to MemorySSA-backed Dead Store Elimination (aka cross-bb DSE)

Fri Sep 4 14:14:02 PDT 2020

Just another quick update: the patch to flip the switch is now up as well https://reviews.llvm.org/D87163 <https://reviews.llvm.org/D87163>

I plan on flipping the switch once the review gets approved, unless there are any remaining major concerns. Please let me know if you see any regressions/problems after the switch.

Cheers,
Florian

> On Sep 3, 2020, at 16:08, Florian Hahn via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> 
>> On Sep 1, 2020, at 23:03, Alina Sbirlea <alina.sbirlea at gmail.com <mailto:alina.sbirlea at gmail.com>> wrote:
>> 
>> Hi Florian,
>> 
>> Following up on D86967, I missed that all the timings were using the legacy pass manager.
>> Did you do any testing on the compile and run time impact for the new pass manager?
> 
> 
> All numbers shared earlier where indeed with the default configuration/LPM.
> 
> I did some testing with the new PM today (using ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER). In terms of removed stores, the improvements are very similar to the legacy pass manager.
> 
> In terms of compile-time, the impact is a bit bigger unfortunately, due to the fact that we need more extra computations of MemorySSA. For example, in the regular pipeline, DSE is scheduled just before LICM, both of which use MemorySSA. With the LPM, if I understand correctly, LICM's loop pass manager constructs MemorySSA even if there is no loop in a function. So in the LPM, the number of times MemorySSA is computed stays the same.
> 
> With the NPW, it seems like LICM’s loop pass manager does only construct MemorySSA if there are actual loops in a function. This is great, but unfortunately means that using DSE + MemorySSA introduces an extra MemorySSA construction for each function without loops. For tramp3d-v4  this introduces 3x more MemorySSA constructions when running DSE + MemorySSA followed by LICM on the LTO module, for example.
> 
> I do not think there are any short-term solutions for this in the NPM, but once more passes, e.g. NewGVN or MemCpyOptimizer, are using MemorySSA, the cost should be somewhat amortized.
> 
> So here are the latest CTMark numbers with DSE + MemorySSA enabled. Note that they those measurements do not include the compile-time improvements made recently to MemDepAnalysis a week ago (https://github.com/llvm/llvm-project/commit/3a54b6a4b71c21cf3bab4f132cbc2904fb9d997e <https://github.com/llvm/llvm-project/commit/3a54b6a4b71c21cf3bab4f132cbc2904fb9d997e>). I think this highlights the benefits of switching as soon as possible, so the DSE + MemorySSA implementation can benefit from more additional and fresh eyes with focus on compile-time. 
> 
>                                      New Pass Manager
>                                     exec instrs.   size-text 
> O3                                 + 0.95%        - 0.25%
> ReleaseThinLTO           + 1.28%        - 0.41%
> ReleaseLTO-g.              + 1.64%        - 0.35%
> RelThinLTO (link only)   + 0.93%        - 0.41%
> RelLO-g (link only).        + 2.09%       - 0.35%
> 
> Details:
> http://195.201.131.214:8000/compare.php?from=7fa5828950d060a70eb57e721765da7cc67c5695&to=b5e5f6ed0f583ddd7a032c274e013335216d6372&stat=instructions <http://195.201.131.214:8000/compare.php?from=7fa5828950d060a70eb57e721765da7cc67c5695&to=b5e5f6ed0f583ddd7a032c274e013335216d6372&stat=instructions>
> 
>                                      Legacy Pass Manager
>                                     exec instrs.   size-text 
> O3                                 + 0.67%        - 0.27%
> ReleaseThinLTO           + 1.07%        - 0.42%
> ReleaseLTO-g.              + 0.81%        - 0.33%
> RelThinLTO (link only)   + 0.94%        - 0.42%
> RelLO-g (link only).        + 0.78%       - 0.33%
> 
> Details:
> http://llvm-compile-time-tracker.com/compare.php?from=7fa5828950d060a70eb57e721765da7cc67c5695&to=b5e5f6ed0f583ddd7a032c274e013335216d6372&stat=instructions <http://llvm-compile-time-tracker.com/compare.php?from=7fa5828950d060a70eb57e721765da7cc67c5695&to=b5e5f6ed0f583ddd7a032c274e013335216d6372&stat=instructions>
> 
> There currently are 2 patches pending until we should be ready to flip the switch:
> 
> https://reviews.llvm.org/D86651 <https://reviews.llvm.org/D86651> [MemCpyOpt] Preserve MemorySSA.
> https://reviews.llvm.org/D86815 <https://reviews.llvm.org/D86815> [LangRef] Adjust guarantee for llvm.memcpy to also allow equal arguments.
> 
> Cheers,
> Florian
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