combination of read/write and earlyclobber constraint modifier

[Tom de Vries <Tom_deVries at mentor dot com>]

Vladimir,

There are a few patterns which use both the read/write constraint modifier (+) 
and the earlyclobber constraint modifier (&):
...
$ grep -c 'match_operand.*+.*&' gcc/config/*/* | grep -v :0
gcc/config/aarch64/aarch64-simd.md:1
gcc/config/arc/arc.md:1
gcc/config/arm/ldmstm.md:30
gcc/config/rs6000/spe.md:8
...

F.i., this one in gcc/config/aarch64/aarch64-simd.md:
...
(define_insn "vec_pack_trunc_<mode>"
 [(set (match_operand:<VNARROWQ2> 0 "register_operand" "+&w")
       (vec_concat:<VNARROWQ2>
         (truncate:<VNARROWQ> (match_operand:VQN 1 "register_operand" "w"))
         (truncate:<VNARROWQ> (match_operand:VQN 2 "register_operand" "w"))))]
...

The documentation ( 
https://gcc.gnu.org/onlinedocs/gccint/Modifiers.html#Modifiers ) states:
...
'‘&’ does not obviate the need to write ‘=’.
...
which seems to state that '&' implies '='.

An earlyclobber operand is defined as 'modified before the instruction is 
finished using the input operands'. AFAIU that would indeed exclude the 
possibility that the earlyclobber operand is an input/output operand it self, 
but perhaps I misunderstand.

So my question is: is the combination of '&' and '+' supported ? If so, what is 
the exact semantics ? If not, should we warn or give an error ?

Thanks,
- Tom