Optimize variable unificationemj/optimize-unification

Avoids building large chains of variable references by always resolving
to the last variable.

Instead of building a chain of references:

    var1 -> var2 -> ... -> varN -> type()

We now create a direction reference:

    var1 -> varN -> type()

This change vastly improves the speed of checking functions with high
complexity and many variables.

1 files changed, 14 insertions, 2 deletions

diff --git a/lib/elixir/lib/module/types/infer.ex b/lib/elixir/lib/module/types/infer.ex
index 0a3bf7995..a84c297bb 100644
--- a/lib/elixir/lib/module/types/infer.ex
+++ b/lib/elixir/lib/module/types/infer.ex
@@ -32,11 +32,23 @@ defmodule Module.Types.Infer do
   end
 
   defp do_unify(type, {:var, var}, stack, context) do
-    unify_var(var, type, stack, context, _var_source = false)
+    case Map.fetch!(context.types, var) do
+      {:var, var_type} ->
+        do_unify(type, {:var, var_type}, stack, context)
+
+      _other ->
+        unify_var(var, type, stack, context, _var_source = false)
+    end
   end
 
   defp do_unify({:var, var}, type, stack, context) do
-    unify_var(var, type, stack, context, _var_source = true)
+    case Map.fetch!(context.types, var) do
+      {:var, var_type} ->
+        do_unify({:var, var_type}, type, stack, context)
+
+      _other ->
+        unify_var(var, type, stack, context, _var_source = true)
+    end
   end
 
   defp do_unify({:tuple, sources}, {:tuple, targets}, stack, context)