C compiler structure optimisation

Question

The C standard does not allow certain optimisations of structures: for example, rearrangement of fields, merging fields, discarding fields that are never read from, hoisting fields out of the structure if they can be turned into auto variables, etc. This is needed for various reasons, including consistent structure layouts across compilation units and allowing cast-compatible structures.

Do any modern compilers (e.g. gcc, clang, Visual C) support extensions that allow me to tell it that it is okay to do these optimisations?

Naturally, they'd only make sense for definitions that were local to a single compilation unit, so that the compiler could see all possible uses of the structure; and certain things (like the aforesaid cast-compatible structure definitions) would become unusable. But for certain tasks this could be a very valuable optimisation.

I do know that gcc used to have a -fipa-struct-reorg option to allow precisely this, but it never worked very well and bit rotted, and was eventually taken out. But I don't know if it's been replaced by anything. And I haven't been able to find anything in clang, which surprises me because I would think that this is precisely the kind of optimisation that clang would be all over...

Answer 1

No. There is no reason for such a thing to be supplied.

1. You can't do it where the structure's address is taken and sent anywhere, as it might be aliased anyway. That pretty much rules out anything outside of a single function.

2. If you can go through and do the analysis required to flag structure members that "this can be optimised away if not used" (beware funky offset calculating macros) then you can see for yourself if it is needed or not, and take it out yourself.

3. If unsure, just comment it out and see if you get a compile error.