When an object becomes "too complex" (in other words it has too many variations in the shape tree), we transition it to use a "too complex" shape and use a hash for storing instance variables. Without this , there were rare cases where shape tree growth could "explode" and cause performance degradation on what would otherwise have been cached fast paths. This puts a limit on shape tree growth, and gracefully degrades in the rare case where there could be a factorial growth in the shape tree. For example: ```ruby class NG; end HUGE_NUMBER.times do NG.new.instance_variable_set(:"@unique_ivar_#{_1}", 1) end ``` We consider objects to be "too complex" when the object's class has more than SHAPE_MAX_VARIATIONS (currently 8) leaf nodes in the shape tree and the object introduces a new variation (a new leaf node) associated with that class. For example, new variations on instances of the following class would be considered "too complex" because those instances create more than 8 leaves in the shape tree: ```ruby class Foo; end 9.times { Foo.new.instance_variable_set(":@uniq_#{_1}", 1) } ``` However, the following class is *not* too complex because it only has one leaf in the shape tree: ```ruby class Foo def initialize @a = @b = @c = @d = @e = @f = @g = @h = @i = nil end end 9.times { Foo.new } `` This case is rare, so we don't expect this change to impact performance of most applications, but it needs to be handled. Co-Authored-By: Aaron Patterson <[email protected]>