DATA_ALIGNMENT vs. DECL_USER_ALIGNMENT

[Richard Kenner]

    1) If the front-end has to specify USER_ALIGN so that the alignment is
    static and the backend will not modify the alignment,

    2) and the front-end is allowed to align the type more strictly,

    3) and the target has minimum alignment requirements,

    then it seems that the front-end needs to know the minimum alignment so
    that it does not specify USER_ALIGN which conflicts with the target
    requirements.  The front-end will use USER_ALIGN and will specify an exact
    alignment, but the alignment will be compatible with the target's own
    minimum alignment requirements.

I don't understand what a "minimum alignment requirement" might mean.
Would it apply to types or only to objects?  

If only to objects, saying that an object is 1-byte aligned allows
*any* alignment.

But I can't find a macro that specifies an minimum alignment.  What is
it called?