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Ulrich Weigand via llvm-dev

2020-Jul-07 12:25 UTC

[llvm-dev] [RFC] C++20 ABI issue on several platforms

Hello,

as discussed here in more detail: https://reviews.llvm.org/D81583

the introduction of the C++20 [[no_unique_address]] attribute exposes an
ABI issue on platforms that require special handling for structs/classes
that are "equivalent" to a single floating-point member (or in some
cases,
a "homogeneous" set of floating-point members).  This is because we
can now
for the first time have "empty" members in a C++ class, which affects
that
determination.

The Itanium C++ ABI document was updated to include these new cases, and
GCC 10 was changed accordingly.  However, current clang/LLVM mainline does
not comply with this new ABI.   The Phabricator review I posted above fixes
that for the SystemZ target, but because I'm touching some amount of common
code, and because -to the best of my understanding- other platforms would
actually likewise be affected, I'm asking for comments here as well.

Given that we're nearing the time frame to create the LLVM 11 branch, it
would really be good if this issue could be fixed before the branch.


Bye,
Ulrich
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Hubert Tong via llvm-dev

2020-Jul-07 14:24 UTC

head link

[llvm-dev] [RFC] C++20 ABI issue on several platforms

As information, it seems no_unique_address handling already works for
PPC64LE:
struct A {
  int : 0;
};

struct B {
  float f;
  struct A a [[no_unique_address]];
};

void f(B);

void g(B *bp) {
  f(*bp);
}

Uses:
lfsx 1, 0, 3

Compiler Explorer link: https://godbolt.org/z/tHkSRQ

On Tue, Jul 7, 2020 at 8:25 AM Ulrich Weigand via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello,
>
> as discussed here in more detail: https://reviews.llvm.org/D81583
>
> the introduction of the C++20 [[no_unique_address]] attribute exposes an
> ABI issue on platforms that require special handling for structs/classes
> that are "equivalent" to a single floating-point member (or in
some cases,
> a "homogeneous" set of floating-point members). This is because
we can now
> for the first time have "empty" members in a C++ class, which
affects that
> determination.
>
> The Itanium C++ ABI document was updated to include these new cases, and
> GCC 10 was changed accordingly. However, current clang/LLVM mainline does
> not comply with this new ABI. The Phabricator review I posted above fixes
> that for the SystemZ target, but because I'm touching some amount of
common
> code, and because -to the best of my understanding- other platforms would
> actually likewise be affected, I'm asking for comments here as well.
>
> Given that we're nearing the time frame to create the LLVM 11 branch,
it
> would really be good if this issue could be fixed before the branch.
>
>
> Bye,
> Ulrich
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Jinsong Ji via llvm-dev

2020-Jul-07 15:03 UTC

head link

[llvm-dev] [RFC] C++20 ABI issue on several platforms

Thanks Hubert.

This simple example may work for ppc64le, but I think we still need to fix
some other issues.
eg: We do use isSingleElementStruct in getParamTypeAlignment as well.


Best,

Jinsong Ji (纪金松), PhD.

XL/LLVM on Power Compiler Development
E-mail: jji at us.ibm.com



From:	Hubert Tong via llvm-dev <llvm-dev at lists.llvm.org>
To:	Ulrich Weigand <Ulrich.Weigand at de.ibm.com>
Cc:	LLVM Developers Mailing List <llvm-dev at lists.llvm.org>, Clang
            Dev <cfe-dev at lists.llvm.org>
Date:	07/07/2020 10:25 AM
Subject:	[EXTERNAL] Re: [llvm-dev] [RFC] C++20 ABI issue on several
            platforms
Sent by:	"llvm-dev" <llvm-dev-bounces at lists.llvm.org>



As information, it seems no_unique_address handling already works for
PPC64LE:
struct A {
  int : 0;
};

struct B {
  float f;
  struct A a [[no_unique_address]];
};

void f(B);

void g(B *bp) {
  f(*bp);
}

Uses:
lfsx 1, 0, 3

Compiler Explorer link: https://godbolt.org/z/tHkSRQ

On Tue, Jul 7, 2020 at 8:25 AM Ulrich Weigand via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
  Hello,

  as discussed here in more detail: https://reviews.llvm.org/D81583

  the introduction of the C++20 [[no_unique_address]] attribute exposes an
  ABI issue on platforms that require special handling for structs/classes
  that are "equivalent" to a single floating-point member (or in some
  cases, a "homogeneous" set of floating-point members). This is
because we
  can now for the first time have "empty" members in a C++ class,
which
  affects that determination.

  The Itanium C++ ABI document was updated to include these new cases, and
  GCC 10 was changed accordingly. However, current clang/LLVM mainline does
  not comply with this new ABI. The Phabricator review I posted above fixes
  that for the SystemZ target, but because I'm touching some amount of
  common code, and because -to the best of my understanding- other
  platforms would actually likewise be affected, I'm asking for comments
  here as well.

  Given that we're nearing the time frame to create the LLVM 11 branch, it
  would really be good if this issue could be fixed before the branch.


  Bye,
  Ulrich

  _______________________________________________
  LLVM Developers mailing list
  llvm-dev at lists.llvm.org
  https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
  _______________________________________________
  LLVM Developers mailing list
  llvm-dev at lists.llvm.org
 
https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.llvm.org_cgi-2Dbin_mailman_listinfo_llvm-2Ddev&d=DwIGaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=DvnnfavFQBGT2CDyHzTr_Q&m=ZxkpwoIX2r0-rd7A8lfvGW_KeOUZq0vUxnwEMX3aVSM&s=JfHfgBqIoPVo_ovA83BQviEkBuMrdXoMESaKoeH61XA&e


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Ulrich Weigand via llvm-dev

2020-Jul-07 15:27 UTC

head link

[llvm-dev] [RFC] C++20 ABI issue on several platforms

Hi Jinsong,

yes, exactly.  I was just trying a more complicated case:

struct A {
  int : 0;
};

struct B {
  struct A a [[no_unique_address]];
};

struct C : B {
  float f;
};

void f(C);

void g(C *bp) {
  f(*bp);
}

and this is not recognized as homogeneous aggregate right now.   (It is on
Z with my patch.  It is also recognized by current GCC on both platforms.)


Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

--
  Dr. Ulrich Weigand | Phone: +49-7031/16-3727
  Distinguished Engineer, Open source compilers and toolchain
  IBM Deutschland Research & Development GmbH
  Vors. des Aufsichtsrats: Gregor Pillen | Geschäftsführung: Dirk Wittkopp
  Sitz d. Ges.: Böblingen | Registergericht: Amtsgericht Stuttgart, HRB
243294
  Data Privacy Statement der IBM: https://www.ibm.com/privacy/us/en/



From:	Jinsong Ji/Jacksonville/IBM
To:	Hubert Tong <hubert.reinterpretcast at gmail.com>
Cc:	Clang Dev <cfe-dev at lists.llvm.org>, LLVM Developers Mailing
            List <llvm-dev at lists.llvm.org>, "llvm-dev"
            <llvm-dev-bounces at lists.llvm.org>, Ulrich Weigand
            <Ulrich.Weigand at de.ibm.com>
Date:	07.07.2020 17:03
Subject:	Re: [EXTERNAL] Re: [llvm-dev] [RFC] C++20 ABI issue on several
            platforms


Thanks Hubert.

This simple example may work for ppc64le, but I think we still need to fix
some other issues.
eg: We do use isSingleElementStruct in getParamTypeAlignment as well.


Best,

Jinsong Ji (纪金松), PhD.

XL/LLVM on Power Compiler Development
E-mail: jji at us.ibm.com




From:	Hubert Tong via llvm-dev <llvm-dev at lists.llvm.org>
To:	Ulrich Weigand <Ulrich.Weigand at de.ibm.com>
Cc:	LLVM Developers Mailing List <llvm-dev at lists.llvm.org>, Clang
            Dev <cfe-dev at lists.llvm.org>
Date:	07/07/2020 10:25 AM
Subject:	[EXTERNAL] Re: [llvm-dev] [RFC] C++20 ABI issue on several
            platforms
Sent by:	"llvm-dev" <llvm-dev-bounces at lists.llvm.org>



As information, it seems no_unique_address handling already works for
PPC64LE:
struct A {
  int : 0;
};

struct B {
  float f;
  struct A a [[no_unique_address]];
};

void f(B);

void g(B *bp) {
  f(*bp);
}

Uses:
lfsx 1, 0, 3

Compiler Explorer link: https://godbolt.org/z/tHkSRQ

On Tue, Jul 7, 2020 at 8:25 AM Ulrich Weigand via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
  Hello,

  as discussed here in more detail: https://reviews.llvm.org/D81583

  the introduction of the C++20 [[no_unique_address]] attribute exposes an
  ABI issue on platforms that require special handling for structs/classes
  that are "equivalent" to a single floating-point member (or in some
  cases, a "homogeneous" set of floating-point members). This is
because we
  can now for the first time have "empty" members in a C++ class,
which
  affects that determination.

  The Itanium C++ ABI document was updated to include these new cases, and
  GCC 10 was changed accordingly. However, current clang/LLVM mainline does
  not comply with this new ABI. The Phabricator review I posted above fixes
  that for the SystemZ target, but because I'm touching some amount of
  common code, and because -to the best of my understanding- other
  platforms would actually likewise be affected, I'm asking for comments
  here as well.

  Given that we're nearing the time frame to create the LLVM 11 branch, it
  would really be good if this issue could be fixed before the branch.


  Bye,
  Ulrich

  _______________________________________________
  LLVM Developers mailing list
  llvm-dev at lists.llvm.org
  https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
  _______________________________________________
  LLVM Developers mailing list
  llvm-dev at lists.llvm.org
 
https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.llvm.org_cgi-2Dbin_mailman_listinfo_llvm-2Ddev&d=DwIGaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=DvnnfavFQBGT2CDyHzTr_Q&m=ZxkpwoIX2r0-rd7A8lfvGW_KeOUZq0vUxnwEMX3aVSM&s=JfHfgBqIoPVo_ovA83BQviEkBuMrdXoMESaKoeH61XA&e



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Jinsong Ji via llvm-dev

2020-Jul-07 15:48 UTC

head link

[llvm-dev] [RFC] C++20 ABI issue on several platforms

Thanks Ulrich! We will have a look for ppc64le.

Best,

Jinsong Ji (纪金松), PhD.

XL/LLVM on Power Compiler Development
E-mail: jji at us.ibm.com



From:	Ulrich Weigand/Germany/IBM
To:	Jinsong Ji/Jacksonville/IBM at IBMUS
Cc:	Hubert Tong <hubert.reinterpretcast at gmail.com>, Clang Dev
            <cfe-dev at lists.llvm.org>, LLVM Developers Mailing List
            <llvm-dev at lists.llvm.org>
Date:	07/07/2020 11:27 AM
Subject:	Re: [llvm-dev] [RFC] C++20 ABI issue on several platforms


Hi Jinsong,

yes, exactly.  I was just trying a more complicated case:

struct A {
  int : 0;
};

struct B {
  struct A a [[no_unique_address]];
};

struct C : B {
  float f;
};

void f(C);

void g(C *bp) {
  f(*bp);
}

and this is not recognized as homogeneous aggregate right now.   (It is on
Z with my patch.  It is also recognized by current GCC on both platforms.)


Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

--
  Dr. Ulrich Weigand | Phone: +49-7031/16-3727
  Distinguished Engineer, Open source compilers and toolchain
  IBM Deutschland Research & Development GmbH
  Vors. des Aufsichtsrats: Gregor Pillen | Geschäftsführung: Dirk Wittkopp
  Sitz d. Ges.: Böblingen | Registergericht: Amtsgericht Stuttgart, HRB
243294
  Data Privacy Statement der IBM: https://www.ibm.com/privacy/us/en/




From:	Jinsong Ji/Jacksonville/IBM
To:	Hubert Tong <hubert.reinterpretcast at gmail.com>
Cc:	Clang Dev <cfe-dev at lists.llvm.org>, LLVM Developers Mailing
            List <llvm-dev at lists.llvm.org>, "llvm-dev"
            <llvm-dev-bounces at lists.llvm.org>, Ulrich Weigand
            <Ulrich.Weigand at de.ibm.com>
Date:	07.07.2020 17:03
Subject:	Re: [EXTERNAL] Re: [llvm-dev] [RFC] C++20 ABI issue on several
            platforms


Thanks Hubert.

This simple example may work for ppc64le, but I think we still need to fix
some other issues.
eg: We do use isSingleElementStruct in getParamTypeAlignment as well.


Best,

Jinsong Ji (纪金松), PhD.

XL/LLVM on Power Compiler Development
E-mail: jji at us.ibm.com




From:	Hubert Tong via llvm-dev <llvm-dev at lists.llvm.org>
To:	Ulrich Weigand <Ulrich.Weigand at de.ibm.com>
Cc:	LLVM Developers Mailing List <llvm-dev at lists.llvm.org>, Clang
            Dev <cfe-dev at lists.llvm.org>
Date:	07/07/2020 10:25 AM
Subject:	[EXTERNAL] Re: [llvm-dev] [RFC] C++20 ABI issue on several
            platforms
Sent by:	"llvm-dev" <llvm-dev-bounces at lists.llvm.org>



As information, it seems no_unique_address handling already works for
PPC64LE:
struct A {
  int : 0;
};

struct B {
  float f;
  struct A a [[no_unique_address]];
};

void f(B);

void g(B *bp) {
  f(*bp);
}

Uses:
lfsx 1, 0, 3

Compiler Explorer link: https://godbolt.org/z/tHkSRQ

On Tue, Jul 7, 2020 at 8:25 AM Ulrich Weigand via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
  Hello,

  as discussed here in more detail: https://reviews.llvm.org/D81583

  the introduction of the C++20 [[no_unique_address]] attribute exposes an
  ABI issue on platforms that require special handling for structs/classes
  that are "equivalent" to a single floating-point member (or in some
  cases, a "homogeneous" set of floating-point members). This is
because we
  can now for the first time have "empty" members in a C++ class,
which
  affects that determination.

  The Itanium C++ ABI document was updated to include these new cases, and
  GCC 10 was changed accordingly. However, current clang/LLVM mainline does
  not comply with this new ABI. The Phabricator review I posted above fixes
  that for the SystemZ target, but because I'm touching some amount of
  common code, and because -to the best of my understanding- other
  platforms would actually likewise be affected, I'm asking for comments
  here as well.

  Given that we're nearing the time frame to create the LLVM 11 branch, it
  would really be good if this issue could be fixed before the branch.


  Bye,
  Ulrich

  _______________________________________________
  LLVM Developers mailing list
  llvm-dev at lists.llvm.org
  https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
  _______________________________________________
  LLVM Developers mailing list
  llvm-dev at lists.llvm.org
 
https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.llvm.org_cgi-2Dbin_mailman_listinfo_llvm-2Ddev&d=DwIGaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=DvnnfavFQBGT2CDyHzTr_Q&m=ZxkpwoIX2r0-rd7A8lfvGW_KeOUZq0vUxnwEMX3aVSM&s=JfHfgBqIoPVo_ovA83BQviEkBuMrdXoMESaKoeH61XA&e




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Hans Wennborg via llvm-dev

2020-Jul-27 14:58 UTC

head link

[llvm-dev] [RFC] C++20 ABI issue on several platforms

On Tue, Jul 7, 2020 at 2:25 PM Ulrich Weigand via llvm-dev
<llvm-dev at lists.llvm.org> wrote:>
> Hello,
>
> as discussed here in more detail: https://reviews.llvm.org/D81583
>
> the introduction of the C++20 [[no_unique_address]] attribute exposes an
ABI issue on platforms that require special handling for structs/classes that
are "equivalent" to a single floating-point member (or in some cases,
a "homogeneous" set of floating-point members). This is because we can
now for the first time have "empty" members in a C++ class, which
affects that determination.
>
> The Itanium C++ ABI document was updated to include these new cases, and
GCC 10 was changed accordingly. However, current clang/LLVM mainline does not
comply with this new ABI. The Phabricator review I posted above fixes that for
the SystemZ target, but because I'm touching some amount of common code, and
because -to the best of my understanding- other platforms would actually
likewise be affected, I'm asking for comments here as well.
>
> Given that we're nearing the time frame to create the LLVM 11 branch,
it would really be good if this issue could be fixed before the branch.
Hi Ulrich,

I see that the patch linked above landed before the branch date. Do I
understand that there's nothing more that needs to be done here for
LLVM 11?

Thanks,
Hans

Ulrich Weigand via llvm-dev

2020-Jul-27 15:40 UTC

head link

[llvm-dev] [RFC] C++20 ABI issue on several platforms

Hans Wennborg <hans at chromium.org> wrote on 27.07.2020 16:58:57:
> Hi Ulrich,
>
> I see that the patch linked above landed before the branch date. Do I
> understand that there's nothing more that needs to be done here for
> LLVM 11?
Hi Hans,

yes, this has all been merged, so there's nothing more to do be done.

Thanks,
Ulrich
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llvm dev - Jul 2020 - [RFC] C++20 ABI issue on several platforms

[llvm-dev] [RFC] C++20 ABI issue on several platforms

[llvm-dev] [RFC] C++20 ABI issue on several platforms

[llvm-dev] [RFC] C++20 ABI issue on several platforms

[llvm-dev] [RFC] C++20 ABI issue on several platforms

[llvm-dev] [RFC] C++20 ABI issue on several platforms

[llvm-dev] [RFC] C++20 ABI issue on several platforms

[llvm-dev] [RFC] C++20 ABI issue on several platforms