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

Davide Italiano via llvm-dev
Wed, 18 Jan 2017 22:55:17 -0000

On Tue, Jan 17, 2017 at 6:02 PM, Mikhail Zolotukhin
<mzolotukhin at apple.com> 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%
>
>
> 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.
> 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 :-)
>
> Michael
>

This is great, thanks for the January update :)
Do you mind to share how you collected the numbers (script etc.. and
how you plotted the graph so I can try repeating at home with my
testcases?)

Thanks,

--
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare