[Bug tree-optimization/92860] New: [8,9,10 regression] Global flags affected by -O settings are clobbered by optimize attribute

[hubicka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92860

            Bug ID: 92860
           Summary: [8,9,10 regression] Global flags affected by -O
                    settings are clobbered by optimize attribute
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hubicka at gcc dot gnu.org
  Target Milestone: ---

Hi,
the following testcase:
void linker_error();
__attribute__ ((optimize("-O0")))
int a ()
{
}
static int remove_me ()
{
  linker_error ();
}
void
main()
{
}

builds with GCC6 but not with GCC8, GCC9 and GCC10:
hubicka@lomikamen-jh:/aux/hubicka/trunk4/gcc$ gcc -O2 t.c
hubicka@lomikamen-jh:/aux/hubicka/trunk4/gcc$
/aux/hubicka/trunk-install/bin/gcc -O2 t.c
/usr/local/bin/ld: /tmp/cckSFE5R.o: in function `remove_me':
t.c:(.text+0x17): undefined reference to `linker_error'
collect2: error: ld returned 1 exit status

The problem is that while processing the optimize attribute for a we overwritte
flag_toplevel_reorder that is affected by optimization flag but not marked as
Optimization.  I suppose there are other cases like this.