From 2ffff67f40ee8a32fbf8bc0f64f2675f8051fec3 Mon Sep 17 00:00:00 2001
From: Evan Patterson <evan@epatters.org>
Date: Sun, 18 Oct 2020 17:43:27 -0700
Subject: [PATCH] Cache results of `subtypes` to speed up `dispatch` function.

---
 src/Utilities/Selectors.jl | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/Utilities/Selectors.jl b/src/Utilities/Selectors.jl
index 7ac79bf056..c90b500626 100644
--- a/src/Utilities/Selectors.jl
+++ b/src/Utilities/Selectors.jl
@@ -162,7 +162,10 @@ Selectors.dispatch(MySelector, args...)
 ```
 """
 function dispatch(::Type{T}, x...) where T <: AbstractSelector
-    for t in (sort(subtypes(T); by = order))
+    types = get!(selector_subtypes, T) do
+        sort(subtypes(T); by = order)
+    end
+    for t in types
         if !disable(t) && matcher(t, x...)
             runner(t, x...)
             strict(t) && return
@@ -171,4 +174,9 @@ function dispatch(::Type{T}, x...) where T <: AbstractSelector
     runner(T, x...)
 end
 
+# Under certain circumstances, the function `subtypes` can be very slow
+# (https://github.com/JuliaLang/julia/issues/38079), so to ensure that
+# `dispatch` remains fast we cache the results of `subtypes` here.
+const selector_subtypes = Dict{Type,Vector}()
+
 end