Hi, After simplifying the CompiledMethodTrailer, let's look at what is stored there. It is an integer that encodes 1) which file? .sources or .changes ? and 2) the position in the file. So for two files we need one bit. So we could just shift the position by one and then use the last bit, if it is 1, we are .changes. But if you look at the code... it looks strange. I think this is for compatibility, without a bootstrap, changes like these are hard to do. So this is the old code: fileIndexFromSourcePointer: anInteger "Return the index of the source file which contains the source chunk addressed by anInteger" ^ (anInteger bitAnd: 16r1000000) ~= 0 ifTrue: [ 1 "sources file" ] ifFalse: [ anInteger >= 16r1000000 ifTrue: [ 2 "changes file" ] ifFalse: [ 0 "compatibility with StandardSourceFileArray" ] ] filePositionFromSourcePointer: anInteger "Return the position of the source chunk addressed by anInteger" | hi lo | hi := anInteger // 33554432. lo := anInteger \\ 16777216. ^ ((anInteger bitAnd: 16777216) ~= 0 or: [ anInteger < 16777216 "compatibility with StandardSourceFileArray" ]) ifTrue: [ hi * 16777216 + lo "sources file" ] ifFalse: [ (hi - 1) * 16777216 + lo "changes file" ] sourcePointerFromFileIndex: index andPosition: position "Return a sourcePointer encoding the given file index and position" | hi lo | (index = 1 or: [index = 2]) ifFalse: [self error: 'invalid source file index']. position < 0 ifTrue: [self error: 'invalid source code pointer']. hi := position // 16r1000000 *2 + index. lo := position \\ 16r1000000. ^ hi * 16r1000000 + lo While this is the "just shift by one and use the last bit" version: fileIndexFromSourcePointer: anInteger "Return the index of the source file which contains the source chunk addressed by anInteger" "1 is sources file, 2 is changes file" ^ (anInteger bitAnd: 1) + 1 filePositionFromSourcePointer: anInteger "Return the position of the source chunk addressed by anInteger, just shift by one" ^ anInteger >> 1 sourcePointerFromFileIndex: index andPosition: position "Return a sourcePointer encoding the given file index and position" (index = 1 or: [index = 2]) ifFalse: [self error: 'invalid source file index']. position < 0 ifTrue: [self error: 'invalid source code pointer']. "we shift the position by one and set the last bit for .changes" ^ (position << 1) bitOr: index - 1 The PR https://github.com/pharo-project/pharo/pull/13583 was merged in Pharo12 last week. Marcus