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

Michael Zolotukhin via llvm-dev
Wed, 18 Jan 2017 22:51:09 -0000

Hi Mehdi,
> On Jan 17, 2017, at 11:49 PM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
>
> 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!
Thanks for the interest in this!
>
>> 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.
Runtime performance has definitely been getting more attention, but I think
people who changed heuristics usually looked at compile time too. In fact, I
think changes that are not expected to affect performance much are more likely
to go in without thorough compile time testing. Hopefully, improved regular
tracking will help to detect such undesired side-effects in future.

> 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.
That’s true, but I think it’s an accepted requirement for such sort of changes
to provide compile-time testing results as well.

> 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?)
I didn’t plan to send such a report regularly, but if I find something
interesting, I’ll definitely share it. Also, it will make sense to compare
releases, so that’s what I’ll probably do as well.

Michael

>
> Thanks,
>
> —
> Mehdi

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