'TYPE_EMPTY_P' vs. code offloading [PR120308]

We've got 'gcc/stor-layout.cc:finalize_type_size':

    /* Handle empty records as per the x86-64 psABI.  */
    TYPE_EMPTY_P (type) = targetm.calls.empty_record_p (type);

(Indeed x86_64 is still the only target to define 'TARGET_EMPTY_RECORD_P',
calling 'gcc/tree.cc-default_is_empty_record'.)

And so it happens that for an empty struct used in code offloaded from x86_64
host (but not powerpc64le host, for example), we get to see 'TYPE_EMPTY_P' in
offloading compilation (where the offload targets (currently?) don't use it
themselves, and therefore aren't prepared to handle it).

For nvptx offloading compilation, this causes wrong code generation:
'ptxas [...] error : Call has wrong number of parameters', as nvptx code
generation for function definition doesn't pay attention to this flag (say, in
'gcc/config/nvptx/nvptx.cc:pass_in_memory', or whereever else would be
appropriate to handle that), but the generic code 'gcc/calls.cc:expand_call'
via 'gcc/function.cc:aggregate_value_p' does pay attention to it, and we thus
get mismatching function definition vs. function call.

This issue apparently isn't a problem for GCN offloading, but I don't know if
that's by design or by accident.

Richard Biener:
> It looks like TYPE_EMPTY_P is only used during RTL expansion for ABI
> purposes, so computing it during layout_type is premature as shown here.
>
> I would suggest to simply re-compute it at offload stream-in time.

(For avoidance of doubt, the additions to 'gcc.target/nvptx/abi-struct-arg.c',
'gcc.target/nvptx/abi-struct-ret.c' are not dependent on the offload streaming
code changes, but are just to mirror the changes to
'libgomp.oacc-c-c++-common/abi-struct-1.c'.)

	PR lto/120308
	gcc/
	* lto-streamer-out.cc (hash_tree): Don't handle 'TYPE_EMPTY_P' for
	'lto_stream_offload_p'.
	* tree-streamer-in.cc (unpack_ts_type_common_value_fields):
	Likewise.
	* tree-streamer-out.cc (pack_ts_type_common_value_fields):
	Likewise.
	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/abi-struct-1.c: Add empty
	structure testing.
	gcc/testsuite/
	* gcc.target/nvptx/abi-struct-arg.c: Add empty structure testing.
	* gcc.target/nvptx/abi-struct-ret.c: Likewise.

(cherry picked from commit 9063810c86beee6274d745b91d8fb43a81c9683e)

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