So, based on the yesterday's discussions, similarly to powerpc64le-linux
I've done some testing for s390x-linux too.

First of all, I found a bug in my patch from yesterday, it was printing
the wrong type like 'double' etc. rather than the class that contained such
the element.  Fix below.

For s390x-linux, I was using
struct X { };
struct Y { int : 0; };
struct Z { int : 0; Y y; };
struct U : public X { X q; };
struct A { double a; };
struct B : public X { double a; };
struct C : public Y { double a; };
struct D : public Z { double a; };
struct E : public U { double a; };
struct F { [[no_unique_address]] X x; double a; };
struct G { [[no_unique_address]] Y y; double a; };
struct H { [[no_unique_address]] Z z; double a; };
struct I { [[no_unique_address]] U u; double a; };
struct J { double a; [[no_unique_address]] X x; };
struct K { double a; [[no_unique_address]] Y y; };
struct L { double a; [[no_unique_address]] Z z; };
struct M { double a; [[no_unique_address]] U u; };
 #define T(S, s) extern S s; extern void foo##s (S); int bar##s () { foo##s (s); return 0; }
T (A, a)
T (B, b)
T (C, c)
T (D, d)
T (E, e)
T (F, f)
T (G, g)
T (H, h)
T (I, i)
T (J, j)
T (K, k)
T (L, l)
T (M, m)
as testcase and looking for "\tld\t%f0,".
While g++ 9 with -std=c++17 used to pass in fpr just
A, g++ 9 -std=c++14, as well as current trunk -std=c++14 & 17
and clang++ from today -std=c++14 & 17 all pass A, B, C
in fpr and nothing else.  The intent stated by Jason seems to be
that A, B, C, F, G, J, K should all be passed in fpr.

Attached are two (updated) versions of the patch on top of the
powerpc+middle-end patch just posted.

The first one emits two separate -Wpsabi warnings like powerpc, one for
the -std=c++14 vs. -std=c++17 ABI difference and one for GCC 9 vs. 10
[[no_unique_address]] passing changes, the other one is silent about the
second case.

2020-04-29  Jakub Jelinek  <jakub@redhat.com>

	PR target/94704
	* config/s390/s390.c (s390_function_arg_vector,
	s390_function_arg_float): Use DECL_FIELD_ABI_IGNORED instead of
	cxx17_empty_base_field_p.  In -Wpsabi diagnostics use the type
	passed to the function rather than the type of the single element.
	Rename cxx17_empty_base_seen variable to empty_base_seen, change
	type to int, and adjust diagnostics depending on if the field
	has [[no_unique_attribute]] or not.

	* g++.target/s390/s390.exp: New file.
	* g++.target/s390/pr94704-1.C: New test.
	* g++.target/s390/pr94704-2.C: New test.
	* g++.target/s390/pr94704-3.C: New test.
	* g++.target/s390/pr94704-4.C: New test.