Can a C compiler rearrange stack variables?

Question

I have worked on projects for embedded systems in the past where we have rearranged the order of declaration of stack variables to decrease the size of the resulting executa

Answer 1

As there is nothing in the standard prohibiting that for C or C++ compilers, yes, the compiler can do that.

It is different for aggregates (i.e. structs), where the relative order must be maintained, but still the compiler may insert pad bytes to achieve preferable alignment.

IIRC newer MSVC compilers use that freedom in their fight against buffer overflows of locals.

As a side note, in C++, the order of destruction must be reverse order of declaration, even if the compiler reorders the memory layout.

(I can't quote chapter and verse, though, this is from memory.)