[Bug lto/109369] New: LTO drops explicitly referenced symbol _pei386_runtime_relocator

[Bug lto/109369] New: LTO drops explicitly referenced symbol _pei386_runtime_relocator

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

            Bug ID: 109369
           Summary: LTO drops explicitly referenced symbol
                    _pei386_runtime_relocator
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pali at kernel dot org
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---
            Target: Mingw32

When PE runtime-pseudo-reloc is used (e.g. referencing member of global array
from DLL library without being marked as dllimport), LTO drops
_pei386_runtime_relocator symbol even when it is explicitly referenced from
used symbol and then it complains that _pei386_runtime_relocator symbol was
dropped.

This is a bug because LTO compiler 1) should not drop any symbol which is
explicitly referenced from some used symbol and 2) should not drop special
_pei386_runtime_relocator symbol when it detected that PE runtime-pseudo-reloc
is used.



Test case:

Create simple DLL library with global array arr[]:

$ cat arr.c
int arr[2] = { 1, 2 };

$ i686-w64-mingw32-gcc -shared arr.c -o arr.dll



Define simple startup file for mingw (so to compile full test case without
mingw). Function _pei386_runtime_relocator() is explicitly referenced from the
startup function mainCRTStartup():

$ cat startup.c
extern void _pei386_runtime_relocator(void);
extern int main();

int __main() { }

__attribute__((force_align_arg_pointer))
__attribute__((noinline))
static int _mainCRTStartup(void) {
  _pei386_runtime_relocator();
  return main();
}

__attribute__((used)) /* required due to bug 109368 */
int mainCRTStartup(void) {
  return _mainCRTStartup();
}



Implement PE runtime-pseudo-reloc. For compile-only purposes (without runtime
tests) it can be empty:

$ cat pseudo-reloc.c
void _pei386_runtime_relocator(void) { }



And finally simple test program which uses global array from DLL library which
is not explicitly marked with dllimport.

$ cat main.c
extern int arr[];

int main() {
  return arr[1];
}



Without LTO this example compiles fine:

$ i686-w64-mingw32-gcc -Os -nostartfiles -nodefaultlibs -nostdlib startup.c
pseudo-reloc.c main.c arr.dll -o test.exe


With LTO enabled this example does not compile due to dropping explicitly
referenced symbol:

$ i686-w64-mingw32-gcc -Os -nostartfiles -nodefaultlibs -nostdlib startup.c
pseudo-reloc.c main.c arr.dll -o test.exe -flto
`__pei386_runtime_relocator' referenced in section `.rdata' of
test_exe_ertr000004.o: defined in discarded section `.text' of /tmp/ccDpfRvt.o
(symbol from plugin)
collect2: error: ld returned 1 exit status

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |lto

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This could be a binutils issue rather than a GCC lto issue too.

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amonakov at gcc dot gnu.org

--- Comment #2 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
GCC inlines _pei386_runtime_relocator and doesn't emit it because it doesn't
know that it's also referenced implicitly.

Does _pei386_runtime_relocator need to participate in LTO at all? If not, I'd
suggest moving it to a separate file that is compiled with -fno-lto.

If the linker can decide if it's needed only after seeing all post-LTO objects,
that might be the only solution.

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
There is always __attribute__((used))

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

--- Comment #4 from Pali Rohár <pali at kernel dot org> ---
I wanted to point that marking _pei386_runtime_relocator() function with
__attribute__((used)) is working fine.

And whether _pei386_runtime_relocator() should participate in LTO at all? I
would rather ask, why not? Is there any specific reason why
_pei386_runtime_relocator() should not be compiled with LTO? I would expect
from gcc/ld that whole application can be compiled with LTO.

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

--- Comment #5 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
Indeed, sorry, __attribute__((used)) seems a much better solution for symbols
that might be referenced implicitly, in a manner that LTO plugin cannot see.

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #5)
> Indeed, sorry, __attribute__((used)) seems a much better solution for
> symbols that might be referenced implicitly, in a manner that LTO plugin
> cannot see.

OTOH if the linker(?) introduces such use then it should properly communicate
this via the resolution info.  That would hint at a binutils bug.

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

--- Comment #7 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
Yes, ld should claim _pei386_runtime_relocator (even if later it becomes
unneeded due to zero relocations left to fix up) to make this work properly.
That's for Binutils to fix on their side.

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

--- Comment #8 from Pali Rohár <pali at kernel dot org> ---
So from the discussion, do I understand correctly that this is rather LD linker
issue?

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

--- Comment #9 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Pali Rohár from comment #8)
> So from the discussion, do I understand correctly that this is rather LD
> linker issue?

Yes, ld changes will be needed to make this work automatically, without adding
the attribute manually. I would suggest to move the bug to the Binutils
Bugzilla.

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[pali at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

--- Comment #10 from Pali Rohár <pali at kernel dot org> ---
> I would suggest to move the bug to the Binutils Bugzilla.

Done: https://sourceware.org/bugzilla/show_bug.cgi?id=30343

[Bug lto/109369] LTO drops explicitly referenced symbol _pei386_runtime_relocator

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109369

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org
         Resolution|---                         |MOVED
                URL|                            |https://sourceware.org/bugz
                   |                            |illa/show_bug.cgi?id=30343
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #11 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
Moved.