report

Consider this code:

struct foo {};
int id_foo(struct foo bar, int x) {
    return x;
}
int id(int x) {
    return x;
}

This link shows the assembly generated for MSP430, MIPS64el, PPC32 and PPC64 (https://godbolt.org/z/yOCJ-z), reproduced here for completeness:

;; MIPS64:
id_foo:
        j       $31
        move    $2,$4

id:
        j       $31
        move    $2,$4

;; MSP430:
id_foo:
        MOV.W   R13, R12
        RET
id:
        RET

;; POWERPC64LE
id_foo:
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
id:
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0

;; POWERPC
id_foo:
        mr 3,4
        blr
id:
        blr


Notice how MSP430 and POWERPC passes ZSTs in the calling convention, while MIPS64 and POWERPC64LE ignore them. 

I can't find an ABI specification document for the MSP430 and POWERPC targets, so I was wondering whether this is 
a bug in the GCC implementation of the ABI for these targets. And if not, then why do these targets care
about passing zero-sized types in their calling convention? Is this documented anywhere?