Capital S will run the pre processor first.

Edit:

From manpage:

    file.s
        Assembler code.
    file.S
    file.sx
        Assembler code that must be preprocessed.

IIRC, I believe that the difference traditionally was whether to pass the input through the preprocessor (.S), or not (.s).

Not sure if it makes a difference on modern toolchains.

The convention I'm used to uses .S to denote hand-written assembly files (usually tracked in git) vs .s for machine generated assembly which can be overwritten as needed.

GCC et al won't overwrite a .S but if you ask it to gen ASM (e.g. gcc -S xyz.c) it will overwrite a .s