On Thu, Jul 16, 2020 at 3:05 PM Markus Böck via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> COFF targets currently do not support COMDAT groups. On MinGW targets
> GCC instead puts symbols part of a COMDAT group inside of sections
> annotated with the .linkonce GAS directive. This leads to GAS
> generating a section so that the COMDAT name is the same as the name
> of the actual symbol.
>
> When using LTO however we never go through any of those mechanisms and
> instead output the COMDAT into the LTO IR.
I think the intent is that the very same mechanism is then triggered during
LTRANS - why does that not happen?
Richard.
> This patch fixes this by
> basically replicating the above chain by instead writing the name into
> the IR file.
>
> This case only occurs in cases where multiple inheritance is used and
> non virtual thunks are created. This problem was found while trying to
> compile Qt using LTO on a MinGW target. In the patch a minimal
> reproducible testcase is added which fails to link without the patch
> and links successfully with the patch. No regressions were observed in
> the gcc and g++ testsuite after the patch has been added.
>
> gcc/ChangeLog:
>
> 2020-07-16 Markus Böck <markus.boec...@gmail.com>
>
> * lto-streamer-out.c (write_symol): Write symbol name instead of
> COMDAT group
> on PE/COFF Targets
>
> gcc/testsuite/ChangeLog:
>
> 2020-07-16 Markus Böck <markus.boec...@gmail.com>
>
> * g++.dg/lto/virtual-thunk-comdat_0.C: New test.
> * g++.dg/lto/virtual-thunk-comdat.h: New test.
> * g++.dg/lto/virtual-thunk-comdat_1.C: New test.
>
> ------
>
> Index: gcc/lto-streamer-out.c
> ===================================================================
> --- gcc/lto-streamer-out.c (revision 932e9140d3268cf2033c1c3e93219541c53fcd29)
> +++ gcc/lto-streamer-out.c (date 1594903384922)
> @@ -2779,7 +2779,12 @@
> size = 0;
>
> if (DECL_ONE_ONLY (t))
> + {
> + if (TARGET_PECOFF)
> + comdat = name;
> + else
> comdat = IDENTIFIER_POINTER (decl_comdat_group_id (t));
> + }
> else
> comdat = "";
>
> Index: gcc/testsuite/g++.dg/lto/virtual-thunk-comdat_0.C
> ===================================================================
> --- gcc/testsuite/g++.dg/lto/virtual-thunk-comdat_0.C (date 1594903164805)
> +++ gcc/testsuite/g++.dg/lto/virtual-thunk-comdat_0.C (date 1594903164805)
> @@ -0,0 +1,15 @@
> +// { dg-lto-do link }
> +#include "virtual-thunk-comdat.h"
> +
> +QAccessibleInterface::~QAccessibleInterface() {}
> +
> +QAccessibleActionInterface::~QAccessibleActionInterface() {}
> +
> +QAccessibleEditableTextInterface::~QAccessibleEditableTextInterface() {}
> +
> +bool QAccessibleObject::isValid() const
> +{
> + return false;
> +}
> +
> +void QAccessibleLineEdit::deleteText(const char* string) {}
> Index: gcc/testsuite/g++.dg/lto/virtual-thunk-comdat.h
> ===================================================================
> --- gcc/testsuite/g++.dg/lto/virtual-thunk-comdat.h (date 1594901724581)
> +++ gcc/testsuite/g++.dg/lto/virtual-thunk-comdat.h (date 1594901724581)
> @@ -0,0 +1,39 @@
> +
> +class QAccessibleInterface
> +{
> +protected:
> + virtual ~QAccessibleInterface();
> +
> +public:
> + virtual bool isValid() const = 0;
> +};
> +
> +class QAccessibleActionInterface
> +{
> +public:
> + virtual ~QAccessibleActionInterface();
> +};
> +
> +class QAccessibleEditableTextInterface
> +{
> +public:
> + virtual ~QAccessibleEditableTextInterface();
> +
> + virtual void deleteText(const char*) = 0;
> +};
> +
> +class QAccessibleObject : public QAccessibleInterface
> +{
> +public:
> + bool isValid() const override;
> +};
> +
> +class QAccessibleWidget : public QAccessibleObject, public
> QAccessibleActionInterface
> +{
> +};
> +
> +class QAccessibleLineEdit : public QAccessibleWidget, public
> QAccessibleEditableTextInterface
> +{
> +public:
> + void deleteText(const char* string) override;
> +};
> Index: gcc/testsuite/g++.dg/lto/virtual-thunk-comdat_1.C
> ===================================================================
> --- gcc/testsuite/g++.dg/lto/virtual-thunk-comdat_1.C (date 1594903161383)
> +++ gcc/testsuite/g++.dg/lto/virtual-thunk-comdat_1.C (date 1594903161383)
> @@ -0,0 +1,3 @@
> +#include "virtual-thunk-comdat.h"
> +
> +int main(int argc, char **argv) { QAccessibleLineEdit lineEdit; }
