From: Xi Ruoyao <xry111@xry111.site>
To: Lulu Cheng <chenglulu@loongson.cn>,
gcc-patches@gcc.gnu.org, Jonathan Wakely <jwakely@redhat.com>
Cc: i@xen0n.name, xuchenghua@loongson.cn
Subject: Re: [PATCH] LoongArch: Fix the problem of structure parameter passing in C++. This structure has empty structure members and less than three floating point members.
Date: Wed, 24 May 2023 16:41:25 +0800 [thread overview]
Message-ID: <356c3dfd86b82689483fc96f97790909a2e57c47.camel@xry111.site> (raw)
In-Reply-To: <2d33bf204b0d59f16df8714123ee812be5754617.camel@xry111.site>
Wang Lei raised some concerns about Itanium C++ ABI, so let's ask a C++
expert here...
Jonathan: AFAIK the standard and the Itanium ABI treats an empty class
as size 1 in order to guarantee unique address, so for the following:
class Empty {};
class Test { Empty empty; double a, b; };
When we pass "Test" via registers, we may only allocate the registers
for Test::a and Test::b, and complete ignore Test::empty because there
is no addresses of registers. Is this correct or not?
On Wed, 2023-05-24 at 14:45 +0800, Xi Ruoyao via Gcc-patches wrote:
> On Wed, 2023-05-24 at 14:04 +0800, Lulu Cheng wrote:
> > An empty struct type that is not non-trivial for the purposes of
> > calls
> > will be treated as though it were the following C type:
> >
> > struct {
> > char c;
> > };
> >
> > Before this patch was added, a structure parameter containing an
> > empty structure and
> > less than three floating-point members was passed through one or two
> > floating-point
> > registers, while nested empty structures are ignored. Which did not
> > conform to the
> > calling convention.
>
> No, it's a deliberate decision I've made in
> https://gcc.gnu.org/r12-8294. And we already agreed "the ABI needs to
> be updated" when we applied r12-8294, but I've never improved my
> English
> skill to revise the ABI myself :(.
>
> We are also using the same "de-facto" ABI throwing away the empty
> struct
> for Clang++ (https://reviews.llvm.org/D132285). So we should update
> the
> spec here, instead of changing every implementation.
>
> The C++ standard treats the empty struct as size 1 for ensuring the
> semantics of pointer comparison operations. When we pass it through
> the
> registers, there is no need to really consider the empty field because
> there is no pointers to registers.
>
--
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University
next prev parent reply other threads:[~2023-05-24 8:41 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-24 6:04 Lulu Cheng
2023-05-24 6:45 ` Xi Ruoyao
2023-05-24 8:41 ` Xi Ruoyao [this message]
2023-05-24 8:59 ` Jonathan Wakely
2023-05-24 20:15 ` Jason Merrill
2023-05-25 2:46 ` Lulu Cheng
2023-05-25 2:52 ` WANG Xuerui
2023-05-25 3:41 ` Lulu Cheng
2023-05-25 8:23 ` Jonathan Wakely
2023-05-24 8:47 ` Lulu Cheng
2023-05-24 9:25 ` Xi Ruoyao
2023-05-24 10:07 ` Lulu Cheng
2023-05-24 14:55 ` Xi Ruoyao
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