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 ?