It is not documented as part of the languages and compilers will use this to optimize the stack layout if they see a benefit of it.
Sure, stack space is precious, aligning and packing data is beneficial and these optimisations are to be expected, I have no issue with this. The problem is my data is already aligned and optimised for cache access, but the compiler is changing my declared order for no reason, while also undoing my optimisations.
Data alignment is documented, because there are instructions that expect certain alignment. However the placement inside the stackframe or any padding between variables on the stack is not documented.
Well, on Intel at least, only some SIMD vector loads require alignment and I'm pretty sure these aren't used by FPC. They're hardly used by anyone these days as they have very little benefit on anything but old CPUs and might raise an exception. Aligning data is still very benerficial for performance regardless of instructions that require it of course, but it's not the only concern and the compiler doesn't know what actual memory access patterns of larger data might be.
I'm glad at least class field re-ordering is optional as this would break so much of my code, but I can't say I'm happy the compiler is making me jump through hoops with ugly code and abitrarily undoing my optimised data layouts.
Delphi / Pascal's lack of C style strict aliasing is definitely one of its biggest stengths and reasons I love the language. For someone like me who does important low level optimisations around this rather than caring about some minor ineffectual compiler optimisation it's huge. Obviously tight control over data layouts is critical for this sort of thing to work.
I realise my usage of the language is somewhat atypical and low level optimisation is something programmers concern themselves with less and less. I'm in the process of learning to use FPC so I ask annoying questions and bring up what I consider to be issues that maybe are not a concern to others, but I am loving FPC so far and really appreciate many of its features. I first took interest in it several years ago, but it lacked features I rely on. I am impressed with the progress that has been done since.