[llvm-dev] llvm is getting slower, January edition

Tue Jan 17 23:49:49 PST 2017

Hi Mikhail,

> On Jan 17, 2017, at 6:02 PM, Mikhail Zolotukhin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> Continuing recent efforts in understanding compile time slowdowns, I looked at some historical data: I picked one test and tried to pin-point commits that affected its compile-time. The data I have is not 100% accurate, but hopefully it helps to provide an overview of what's going on with compile time in LLVM and give a better understanding of what changes usually impact compile time.
> 
> Configuration:
> The test I used is tramp3d-v4 from LLVM testsuite. It consists of a single source file, but still takes a noticeable time to compile, which makes it very convenient for this kind of experiments. The file was compiled with Os for arm64 on x86 host.
> 
> Results:
> The attached PDF has a compile time graph, on which I marked points where compile time changed with a list of corresponding commits. A textual version of the list is available below, but I think it might be much harder to comprehend the data without the graph. A number in the end shows compile time change after the given commit:
> 
> 1. r239821: [InstSimplify] Allow folding of fdiv X, X with just NaNs ignored. +1%
> 2. r241886: [InstCombine] Employ AliasAnalysis in FindAvailableLoadedValue. +1%
> 3. r245118: [SCEV] Apply NSW and NUW flags via poison value analysis for sub, mul and shl. +2%
> 4. r246694: [RemoveDuplicatePHINodes] Start over after removing a PHI. -1%
> 5. r247269: [ADT] Rewrite the StringRef::find implementation to be simpler... +1%
>   r247240: [LPM] Use a map from analysis ID to immutable passes in the legacy pass manager... +3%
>   r247264: Enable GlobalsAA by default. +1%
> 6. r247674: [GlobalsAA] Disable globals-aa by default. -1%
> 7. r248638: [SCEV] Reapply 'Teach isLoopBackedgeGuardedByCond to exploit trip counts'. +2%
> 8. r249802: [SCEV] Call `StrengthenNoWrapFlags` after `GroupByComplexity`; NFCI. +4%
> 9. r250157: [GlobalsAA] Turn GlobalsAA on again by default. +1%
> 10. r251049: [SCEV] Mark AddExprs as nsw or nuw if legal. +23%
> 11. No data
> 12. r259252: AttributeSetImpl: Summarize existing function attributes in a bitset. -1%
>    r259256: Add LoopSimplifyCFG pass. -2%
> 13. r262250: Enable LoopLoadElimination by default. +3%
> 14. r262839: Revert "Enable LoopLoadElimination by default". -3%
> 15. r263393: Remove PreserveNames template parameter from IRBuilder. -3%
> 16. r263595: Turn LoopLoadElimination on again. +3%
> 17. r267672: [LoopDist] Add llvm.loop.distribute.enable loop metadata. +4%
> 18. r268509: Do not disable completely loop unroll when optimizing for size. -34%
> 19. r269124: Loop unroller: set thresholds for optsize and minsize functions to zero. +50%
> 20. r269392: [LoopDist] Only run LAA for loops with the pragma. -4%
> 21. r270630: Re-enable "[LoopUnroll] Enable advanced unrolling analysis by default" one more time. -28%
> 22. r270881: Don't allocate in APInt::slt.  NFC. -2%
>    r270959: Don't allocate unnecessarily in APInt::operator[+-].  NFC. -1%
>    r271020: Don't generate unnecessary signed ConstantRange during multiply.  NFC. -3%
> 23. r271615: [LoopUnroll] Set correct thresholds for new recently enabled unrolling heuristic. +22%
> 24. r276942: Don't invoke getName() from Function::isIntrinsic(). -1%
>    r277087: Revert "Don't invoke getName() from Function::isIntrinsic().", rL276942. +1%
> 25. r279585: [LoopUnroll] By default disable unrolling when optimizing for size.
> 26. r286814: [InlineCost] Remove skew when calculating call costs. +3%
> 27. r289755: Make processing @llvm.assume more efficient by using operand bundles. +6%
> 28. r290086: Revert @llvm.assume with operator bundles (r289755-r289757). -6%
> <CompileTime.pdf>

This is an amazing set of data!

> Disclaimer:
> The data is specific for this particular test, so I could have skipped some commits affecting compile time on other workloads/configurations.
> The data I have is not perfect, so I could have skipped some commits, even if they impacted compile-time on this test case.
> Same commits might have a different impact on a different test/configuration, up to the opposite to the one listed.
> I didn't mean to label any commits as 'good' or 'bad' by posting these numbers. It's expected that some commits increase compile time, we just need to be aware of it and avoid unnecessary slowdowns.
> 
> Conclusions:
> Changes in optimization thresholds/cost-models usually have the biggest impact on compile time. However, usually they are well-assessed and trade-offs are discussed and agreed on.

My impression is that most of the time, they are well-assessed, discussed, and agreed on, bases solely on the “performance” expectation, without necessarily looking at the compile time impact.
For example, a change in a threshold in the loop unrolled may trigger a pattern that makes SCEV blowup later. Looking at this only from the "performance of the generated code” point of view is a mistake in my opinion, and hopefully a closer tracking like you’ve been doing will help preventing these situations. So thanks a lot for this!

> Introducing a pass doesn't necessarily mean a compile time slowdown. Sometimes the total compile time might decrease because we're saving some work for later passes.
> There are many commits, which individually have a low compile time impact, but together sum up to a noticeable slowdown.
> Conscious efforts on reducing compile time definitely help - thanks everyone who's been working on this!
> 
> Thanks for reading, any comments or suggestions on how to make LLVM faster are welcome! I hope we'll see this graph going down this year :-)

Looking forward for this!
Do you plan to generate a report like that frequently (weekly? Whenever you notice a regression?)

Thanks,

— 
Mehdi