Last I checked, field declaration order still mattered to structure size and cache usage because the defacto packing and padding rules preserve the order. I admit that I haven't done any C optimization lately so I am curious. It is possible to make this optimization, but it may disrupt codebases that take shortcuts based on an assumed order. And it would be especially difficult to do the analysis of what should take precedence in the cache.

Could you link an example of compilers accommodating these optimizations?