# \`log\` performance regression and JET failures

**URL:** <https://discourse.julialang.org/t/log-performance-regression-and-jet-failures/78146>\
**Category:** Performance\
**Created:** [March 19, 2022, 11:21pm UTC](https://discourse.julialang.org/t/log-performance-regression-and-jet-failures/78146 "2022-03-19T23:21:36Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![cscherrer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cscherrer/32/7631_2.png) [@cscherrer](https://discourse.julialang.org/u/cscherrer)\
**Post date:** [March 19, 2022, 11:21pm UTC](https://discourse.julialang.org/t/log-performance-regression-and-jet-failures/78146/1 "2022-03-19T23:21:36Z")

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Hi, I’m seeing some surprising results for `log` in `1.8.0-beta1`. First, 1.7:

```julia
julia> versioninfo()
Julia Version 1.7.2
Commit bf53498635 (2022-02-06 15:21 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)

julia> @btime log(x) setup=(x=rand());
  5.371 ns (0 allocations: 0 bytes)

julia> JET.@test_opt log(2.1)
Test Passed
  Expression: #= REPL[17]:1 =# JET.@test_call analyzer = JET.OptAnalyzer log(2.1)

```

Then here’s 1.8:

```julia
julia> versioninfo()
Julia Version 1.8.0-beta1
Commit 7b711ce699 (2022-02-23 15:09 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: 8 × Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-13.0.1 (ORCJIT, skylake)
  Threads: 1 on 8 virtual cores

julia> @btime log(x) setup=(x=rand());
  6.548 ns (0 allocations: 0 bytes)

julia> @test_opt log(2.1)
JET-test failed at REPL[16]:1
  Expression: #= REPL[16]:1 =# JET.@test_call analyzer = JET.OptAnalyzer log(2.1)
  ═════ 1 possible error found ═════
  ┌ @ special/log.jl:257 Base.Math.Val(:ℯ)
  │┌ @ essentials.jl:714 %1()
  ││ runtime dispatch detected: %1::Type{Val{_A}} where _A()
  │└─────────────────────
  
ERROR: There was an error during testing

```

It’s measurably slower, and fails `JET.@test_opt`. Is this a real problem, or maybe just a symptom of other changes or things being measured differently?

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<div class="post-metadata">

**Author:** ![aviatesk](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aviatesk/32/7610_2.png) [@aviatesk](https://discourse.julialang.org/u/aviatesk)\
**Post date:** [March 20, 2022, 12:03am UTC](https://discourse.julialang.org/t/log-performance-regression-and-jet-failures/78146/2 "2022-03-20T00:03:57Z")

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It seems to be a bug of JET that `@test_opt` fails on this example. Especially JET still isn’t fully updated to account for the new [effect system](https://github.com/JuliaLang/julia/pull/43852) that will come with 1.8 – The call of `Val(:ℯ)` should be folded as a constant and so JET shouldn’t report any dynamic dispatch within it.

As for the performance, I couldn’t confirm measurable runtime regression on my local machine.

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<div class="post-metadata">

**Author:** ![aviatesk](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/aviatesk/32/7610_2.png) [@aviatesk](https://discourse.julialang.org/u/aviatesk)\
**Post date:** [March 20, 2022, 12:07am UTC](https://discourse.julialang.org/t/log-performance-regression-and-jet-failures/78146/3 "2022-03-20T00:07:25Z")

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FWIW, I filed this issue [here](https://github.com/aviatesk/JET.jl/issues/334).

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<div class="post-metadata">

**Author:** ![cscherrer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cscherrer/32/7631_2.png) [@cscherrer](https://discourse.julialang.org/u/cscherrer)\
**Post date:** [March 20, 2022, 12:33am UTC](https://discourse.julialang.org/t/log-performance-regression-and-jet-failures/78146/4 "2022-03-20T00:33:32Z")

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Great, thanks @aviatesk for the quick response!