https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96895
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #5)
> (In reply to Michael Matz from comment #2)
> > The psABI doesn't say anything about such types, no. Maybe it could in some
> > additional info pages, but it's always a problem to codify behaviour
> > retroactively
> > in it, when conflicting implementations already exist. It is about
> > extension types, though, so we might be fine.
> >
> > FWIW, even ignoring the obvious relation of v1Xf to Xf, GCC behaviour for
> > float and clang behaviour for double is the most logical one: this extended
> > type is most
> > similar to a struct containing one float/double, and such are passed in XMM
> > registers per psABI. As this is also consistent with how a single
> > top-level float is passed, this choice is the most consistent one. This is
> > also
> > what the psABI _would_ say, if we had written it into it, so at least both
> > compilers would need a change to implement it.
>
> So vector types with element type T and N, a power-of-two, not otherwise
> specified are passes the same as
>
> struct S { T a[N] };
>
> ? I guess there's mismatch then for AVX types with -mno-avx then at least
> (AVX512 are probably too big to be passed in registers).
Ugh. We pass struct S { double a[4]; } in %rdi _and_ on the stack?! And
return by invisible reference _and_ in %eax?
typedef struct S { double a[4]; } v4df;
v4df foo (v4df x)
{
return x;
}
foo:
.LFB0:
.cfi_startproc
movq %rdi, %rax
movdqu 8(%rsp), %xmm0
movups %xmm0, (%rdi)
movdqu 24(%rsp), %xmm1
movups %xmm1, 16(%rdi)
ret
but a two-element array is passed/returned in two %xmm regs
