This recent GitHub reads like Julia doesn’t do common subexpression elimination at all, is this a correct assumption? Here’s the relevant bits from the issue: using BenchmarkTools f(x) = exp(x)/(1+exp(x)) function g(x) e = exp(x) e/(1 + e) end h!(f::F, v) where {F} = map!(f, v, v) @btime h!…

Yes, as you have very clearly demonstrated, and as is somewhat clearly stated in the issue you linked, julia does not typically do common subexpression elimination. There are some cases where it can do subespression elimination, but it’s probably best to just assume it never happens.

The reason why I was asking was that I really wasn’t expecting it to not happen at all. It’s such a basic optimization that I assumed it generally happens but this example is somehow “pathological”.

It’s not that Julia never does CSE. But it’s not very good at it right now. julia> code_llvm(x -> exp(abs(x)) / (1+exp(abs(x))), Tuple{Float64}; debuginfo=:none) define double @"julia_#36_2009"(double %0) #0 { top: %1 = call double @llvm.fabs.f64(double %0) %2 = call double @j_exp_2011(double %…

Oh yeah I just included the code_lowered bits from the original because I did a lazy cut’n’paste :sweat_smile: It’s interesting that exp doesn’t get CSE’d but abs does despite the inferred effects being the same. I wonder what the difference is? But in any case the takeaway here is, as @Mason said…

Julia has two optimization stages. The first one implemented in Julia and does things like type-inference and some optimizations like scalar-replacement-of-aggregates (SROA). This is also the level effect analysis happens on. The second level is LLVM based and does a lot more optimization like CSE …

Does Julia do common subexpression elimination?

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Does Julia do common subexpression elimination?

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