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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/96895] ABI of returning V1DF differs between GCC and clang
Date: Wed, 02 Sep 2020 14:16:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96895

--- 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.
> >=20
> > FWIW, even ignoring the obvious relation of v1Xf to Xf, GCC behaviour f=
or
> > float and clang behaviour for double is the most logical one: this exte=
nded
> > 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.  Thi=
s is
> > also
> > what the psABI _would_ say, if we had written it into it, so at least b=
oth
> > compilers would need a change to implement it.
>=20
> So vector types with element type T and N, a power-of-two, not otherwise
> specified are passes the same as
>=20
> struct S { T a[N] };
>=20
> ?  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=