[Bug lto/98971] New: LTO removes __patchable_function_entries

["gabriel at inconstante dot net.br" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98971

            Bug ID: 98971
           Summary: LTO removes __patchable_function_entries
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabriel at inconstante dot net.br
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

Building with -flto removes the effects of -fpatchable-function-entries, or at
least some of them.

For instance, building the following code:

$ cat libtesta.c
int
testa7(void)
{
  return 7;
}

with:

$ gcc libtesta.c -fPIC -fpatchable-function-entry=4,2 -flto -c -o libtesta.o
$ gcc libtesta.o -flto -shared -o libtesta.so

removes the nop padding usually generated with -fpatchable-function-entry, as
well as it removes the __patchable_function_entries section from the resulting
binaries. The intermediate libtesta.o already lacks both:

$ readelf --sections libtesta.o | grep __patchable
$ readelf --sections libtesta.so | grep __patchable
$ objdump -d libtesta.so | grep "<testa7>:" -C3
00000000000010f0 <frame_dummy>:
    10f0:       e9 7b ff ff ff          jmpq   1070 <register_tm_clones>

00000000000010f5 <testa7>:
    10f5:       55                      push   %rbp
    10f6:       48 89 e5                mov    %rsp,%rbp
    10f9:       b8 07 00 00 00          mov    $0x7,%eax

Without -flto, I get what I expected:

$ readelf --sections libtesta.o | grep __patchable
  [ 4] __patchable_[...] PROGBITS         0000000000000000  00000050
$ readelf --sections libtesta.so | grep __patchable
  [19] __patchable_[...] PROGBITS         0000000000004020  00003020
$ objdump -d libtesta.so | grep "<testa7>:" -C3
    10f5:       90                      nop
    10f6:       90                      nop

00000000000010f7 <testa7>:
    10f7:       90                      nop
    10f8:       90                      nop
    10f9:       55                      push   %rbp

Building with a single gcc command, such as:

$ gcc libtesta.c -fPIC -fpatchable-function-entry=4,2 -flto -shared -o
libtesta.so

also works as I expected, i.e.: the nops and the __patchable_function_entries
are kept.


Is this the intended behavior? Am I using it wrong? Or is this a bug?

Cheers

PS: Tested with the branch for gcc 10, with trunk, and with the compilers from
openSUSE and Debian.