expand: Fix up empty class return optimization [PR101160]

expand: Fix up empty class return optimization [PR101160]

On Mon, Jun 14, 2021 at 11:24:22PM -0400, Jason Merrill via Gcc-patches wrote:
> The x86_64 psABI says that an empty class isn't passed or returned in memory or
> registers, so we shouldn't set %eax in this function.  Is this a reasonable
> place to implement that?  Another possibility would be to remove the hack to
> prevent i386.c:function_value_64 from returning NULL in this case and fix the
> callers to deal, but that seems like more work.
>
> The df-scan hunk catches the case where we look at a 0-length reg and build
> a range the length of unsigned int, which happened before I changed
> assign_parms to match expand_function_end.

The assign_params change unfortunately breaks e.g. the following testcase.
The problem is that some passes (e.g. subreg lowering but assign_parms
comments also talk about delayed slot scheduling) rely on crtl->return_rtx
not to contain pseudo registers, and the assign_parms change results
in the pseudo in there not being replaced with a hard register.

The following patch instead clears the crtl->return_rtx if a function
returns TYPE_EMPTY_P structure, that way (use (pseudo)) is not emitted
into the IL and it is treated like more like functions returning void.

I've also changed the effective target on the empty-class1.C testcase, so
that it doesn't fail on x86_64-linux with -m32 testing.

2021-06-22  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101160
	* function.c (assign_parms): For decl_result with TYPE_EMPTY_P type
	clear crtl->return_rtx instead of keeping it referencing a pseudo.

	* g++.target/i386/empty-class1.C: Require lp64 effective target
	instead of x86_64-*-*.
	* g++.target/i386/empty-class2.C: New test.