Hi, We did (im Pharo12) a simplification of how method trailers are handled. Below is the class comment of the now Deprecated and empty class CompiledMethodTrailer. The issue https://github.com/pharo-project/pharo/issues/13512 has a list of all the intermediate steps done. ========== CompiledMethodTrailer was removed in Pharo12 It was responsible for encoding and decoding various kinds of compiled method trailer data, but that had many downsides: ⢠variable length trailers forced often called methods like CompiledMethod>>#endPC to instantiate an instance ⢠setting a sourcePointer that required a different length trailer (e.g embedding sources) needed to create a new compiled method and do a become ⢠to avoid that, the compiler supported to use an existing trailer and delegated instance creation of CompiledMethod to it. ⢠using 1 byte for the trailer kind lead to the need use Variable length encoding already for our ~40MB source file for some methods. All in all, the CompiledMethodTrailer added a lot of complexity for nothing. What do we do now? ⢠use use a fixed-length trailer (see #trailerSize) ⢠writing and reading that is not that complex (see #setSourcePointer, #sourcePointer:) Q: I used to hand a trailer to the compiler, what should I do now? The compiler now creates always CompiledMethods with a fixed, but empty trailer. Just use #sourcePointer to set it after. Q: I used to clear the trailer using #copyWithTrailerBytes:, what should I do now? There is now #cearSourcePointer. Just use #copy before if you want a copy. Q: I want to have a non-installed method, but with source code. The compiler now sets a #source property (and setting a source pointer via #sourcePointer: removes it). If you use the compiler, you need to do nothing special anymore! All methods come with source via the property by default.