[Bug lto/78087] New: gcc 6.2.0 fails to link temacs from emacs-25.1 with -O0 -flto

[howarth.at.gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78087

            Bug ID: 78087
           Summary: gcc 6.2.0 fails to link temacs from emacs-25.1 with
                    -O0 -flto
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: howarth.at.gcc at gmail dot com
  Target Milestone: ---

The gcc 6.2.0 compiler fails to link temacs in a build of emacs 25.1 when the
"-O0 -flto" flag is passed on CFLAGS and "-flto" on LDFLAGS. This issue occurs
on x86_64-apple-darwin15 where the failure appears as...

  CCLD     temacs
/var/folders/vh/xthx1f251nqfj804049zl1wm0000gn/T//cc5Viitn.s:288358:1: error:
assembler local symbol 'L1' not defined
...
/var/folders/vh/xthx1f251nqfj804049zl1wm0000gn/T//cc5Viitn.s:288358:1: error:
assembler local symbol 'L174' not defined

and on x86_64 linux using a build gcc 6.2.0 with configure edited to set
build_lto_plugin=no on ELF to use the same code LTO code generation path on
linux as darwin...

  CCLD     temacs
/tmp/cc7ZBmmQ.ltrans0.ltrans.o:(.rodata+0x135a0): undefined reference to `.L1'
...
/tmp/cc7ZBmmQ.ltrans0.ltrans.o:(.rodata+0x13bc8): more undefined references to
`.L174' follow
collect2: fatal error: ld returned 1 exit status

On x86_64 linux, the issue is suppressed when the -O1 optimization level or
higher is used.
On x86_64 darwin, the linker failure in PR78077 occurs when the -O1
optimization level or higher is used.