# What is \`slt\_int\`

**URL:** <https://discourse.julialang.org/t/what-is-slt-int/70157>\
**Category:** Internals & Design\
**Created:** [October 21, 2021, 2:48pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157 "2021-10-21T14:48:05Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [October 21, 2021, 2:48pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/1 "2021-10-21T14:48:05Z")

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What is `Core.Intrinsics.slt_int`? As seen in [base/sort.jl](https://github.com/JuliaLang/julia/blob/b2d15f05698ba78ac2493ec2624a0ee1c9a042a3/base/sort.jl#L1131), added [2013](https://github.com/JuliaLang/julia/commit/69b21e1c1c210e91ef3ebb278ea8946df90111e1).

Also, how can I find out the answer to this sort of question in general? I tried `@edit`, `?`, web searches, and even git blame (a much less ideal solution), but none came up with anything.

Emperically:

```julia
using Test

values = Float64[
    0.0, -0.0, 1.0, -1.0, pi, 1/pi, Inf, -Inf, NaN, -NaN, 
    rand(), 1/rand(), -rand(), -1/rand(), 
    reinterpret(Float64, reinterpret(UInt64, NaN) | 78782),
    -reinterpret(Float64, reinterpret(UInt64, NaN) | 78782)]

@testset "slt_int == isless(reinterpret(Int, ⋅))" begin
    for a in values
        for b in values
            @test Core.Intrinsics.slt_int(a, b) == 
                  isless(reinterpret(Int64, a), reinterpret(Int64, b))
        end
    end
end

```

```julia
Test Summary: | Pass Total
slt_int == isless(reinterpret(Int, ⋅)) | 256 256

```

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<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [October 21, 2021, 2:58pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/2 "2021-10-21T14:58:39Z")

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It is an intrinsic that implements signed less than for integers. That particular line is a bit tricky since it applies integer intrinsics directly to floating-point values. Normally, I’d express that by reinterpreting to an integer type with the same size and then doing the normal `isless` comparison, but this tricky way to do it leverages the fact that intrinsics generate the right sized instructions based on the storage size of the arguments they get. That test you referenced verifies that `slt_int` has the same effect as that reinterpretation and `isless` comparison for floating-point values.

Intrinsics are an internal language implementation detail, not part of the official surface API, so they’re not documented. Code generation for `slt_int` is defined in this line:

[https://github.com/JuliaLang/julia/blob/628209c1f2f746e3fc21ccd7cb34e67289403d44/src/intrinsics.cpp#L1339](https://github.com/JuliaLang/julia/blob/628209c1f2f746e3fc21ccd7cb34e67289403d44/src/intrinsics.cpp#L1339)

You can see that it calls the LLVM method, `CreateICmpSLT`, which is documented [here](https://llvm.org/doxygen/classllvm_1_1IRBuilderBase.html#a6a2a72ba13b8ceb231afa29dca55cf62) the meanings of which are documented [here](https://llvm.org/doxygen/classllvm_1_1CmpInst.html#a2be3583dac92a031fa1458d4d992c78ba15ae464950ac676919c2f0c7aafc706c).

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<div class="post-metadata">

**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [October 21, 2021, 3:49pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/3 "2021-10-21T15:49:25Z")

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Thanks! It’s nice to know for sure what is going on.

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<div class="post-metadata">

**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [October 21, 2021, 4:09pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/4 "2021-10-21T16:09:02Z")

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Is there any reason we define

```julia
struct Right <: Ordering end
right(::DirectOrdering) = Right()
lt(::Right, x::T, y::T) where {T<:Floats} = slt_int(x, y)

```

In [https://github.com/JuliaLang/julia/blob/b2d15f05698ba78ac2493ec2624a0ee1c9a042a3/base/sort.jl#L1123](https://github.com/JuliaLang/julia/blob/b2d15f05698ba78ac2493ec2624a0ee1c9a042a3/base/sort.jl#L1123)  
instead of

```julia
right(::DirectOrdering) = Reverse(Left())

```

?

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<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [October 21, 2021, 9:44pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/5 "2021-10-21T21:44:01Z")

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Not sure, I didn’t write that code.

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**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [October 21, 2021, 9:53pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/6 "2021-10-21T21:53:16Z")

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You didn’t?  
[https://github.com/JuliaLang/julia/commit/69b21e1c1c210e91ef3ebb278ea8946df90111e1](https://github.com/JuliaLang/julia/commit/69b21e1c1c210e91ef3ebb278ea8946df90111e1)

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<div class="post-metadata">

**Author:** ![Sukera](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@Sukera](https://discourse.julialang.org/u/Sukera)\
**Post date:** [October 21, 2021, 10:05pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/7 "2021-10-21T22:05:02Z")

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That code is 9 years old, I think it’s fine to not quite remember why it was written this exact way 🙂 If it wasn’t specially documented, there probably was not a specific reason (though that’s a very fuzzy indicator, so if it there was a reason and it can be documented now, that’d be good too - and even more amazing if future additions have a nice comment/git log trail).

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<div class="post-metadata">

**Author:** ![StefanKarpinski](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stefankarpinski/32/24_2.png) [@StefanKarpinski](https://discourse.julialang.org/u/StefanKarpinski)\
**Post date:** [October 21, 2021, 11:05pm UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/8 "2021-10-21T23:05:00Z")

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That doesn’t include anything called `Right` so I can’t speak to why that is the way it is.

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**Author:** ![Lilith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lilith/32/27492_2.png) [@Lilith](https://discourse.julialang.org/u/Lilith)\
**Post date:** [October 22, 2021, 12:01am UTC](https://discourse.julialang.org/t/what-is-slt-int/70157/9 "2021-10-22T00:01:09Z")

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@Sukera is right, and the way it’s written isn’t posing a problem, so I’m happy to move on. This is the history, though:

Rename from `FpRev` to `Rev`  
[https://github.com/JuliaLang/julia/commit/25919dc9913f6d370776413625e1b2a342fcdccc#diff-7af133c263575ae298b5b0747b2ee9593e87eef99222ed17bfe458520a75e709L276](https://github.com/JuliaLang/julia/commit/25919dc9913f6d370776413625e1b2a342fcdccc#diff-7af133c263575ae298b5b0747b2ee9593e87eef99222ed17bfe458520a75e709L276)

Rename from `Rev` to `Right`  
[https://github.com/JuliaLang/julia/commit/17c619b214791bc3401b9171366b848bd45c3e9c](https://github.com/JuliaLang/julia/commit/17c619b214791bc3401b9171366b848bd45c3e9c)

In digging through the history, I answered my own question. When `Right` was originally written, `Reverse` wasn’t parametric. You only added that here  
[https://github.com/JuliaLang/julia/commit/3326e0ce5b87d0162d9594f404526f55e33b8094](https://github.com/JuliaLang/julia/commit/3326e0ce5b87d0162d9594f404526f55e33b8094)

And nobody got around to using `Reverse` in the `Sort.Float` module. This raises the question: should we change it now? I could make a PR if appropriate.