> The COFF linker errors with a multiple reference error before the
> lto-wrapper process is started. If I understand correctly this is due
> to how COMDAT works in PE/COFF as the associated string to the COMDAT
> section is stored in the symbol table. When using the
> --allow-multiple-definition flag as a workaround linkage succeeds.
So the problem is caused by fact that we give linker different comdat
group names from LTO symtab then the ones from ltrans since in first
case we do ELF style comdat group that is keyed differently than the
linkonce extension?
Honza
>
> On Fri, Jul 17, 2020 at 9:31 AM Richard Biener
> <richard.guent...@gmail.com> wrote:
> >
> > 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; }
