[Bug other/101459] New: Mismatch in description of option "-falign-functions" between source code and documentation

[Bug other/101459] New: Mismatch in description of option "-falign-functions" between source code and documentation

[ashimida at linux dot alibaba.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101459

            Bug ID: 101459
           Summary: Mismatch in description of option "-falign-functions"
                    between source code and documentation
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ashimida at linux dot alibaba.com
  Target Milestone: ---

As descripted in online doc [1], -falign-functions is enable at levels -O2/-O3. 
But from source code and test result, this options is worked for all options
except -Os(such as -O0).
Is this an error in the documentation, or am I misunderstanding something?

FYI:
// ./gcc/varasm.c
void assemble_start_function (tree decl, const char *fnname)
{
  ......
  if (! DECL_USER_ALIGN (decl) 
          && align_functions.levels[0].log > align
      && optimize_function_for_speed_p (cfun))
  {
      int align_log = align_functions.levels[0].log;
      ......
      ASM_OUTPUT_ALIGN (asm_out_file, align_functions.levels[0].log);
  }
  ......
}

[Bug other/101459] Mismatch in description of option "-falign-functions" between source code and documentation

[ashimida at linux dot alibaba.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101459

--- Comment #1 from ashimida <ashimida at linux dot alibaba.com> ---
(In reply to ashimida from comment #0)
> As descripted in online doc [1], -falign-functions is enable at levels
> -O2/-O3. 
> But from source code and test result, this options is worked for all options
> except -Os(such as -O0).
> Is this an error in the documentation, or am I misunderstanding something?
> 
> FYI:
> // ./gcc/varasm.c
> void assemble_start_function (tree decl, const char *fnname)
> {
>   ......
>   if (! DECL_USER_ALIGN (decl) 
> 	  && align_functions.levels[0].log > align
>       && optimize_function_for_speed_p (cfun))
>   {
>       int align_log = align_functions.levels[0].log;
>       ......
>       ASM_OUTPUT_ALIGN (asm_out_file, align_functions.levels[0].log);
>   }
>   ......
> }

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Optimize-Options

[Bug other/101459] Mismatch in description of option "-falign-functions" between source code and documentation

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101459

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WORKSFORME
             Status|UNCONFIRMED                 |RESOLVED
           Keywords|                            |documentation

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
The flag, -falign-functions is only enabled at -O2+ (but not -Os), but the
actual alignment is recorded in the 'align_functions' data which is only
populated when -falign-functions is specified so I think it works as
documented.

[Bug other/101459] Mismatch in description of option "-falign-functions" between source code and documentation

[ashimida at linux dot alibaba.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101459

--- Comment #3 from ashimida <ashimida at linux dot alibaba.com> ---
(In reply to Richard Biener from comment #2)
> The flag, -falign-functions is only enabled at -O2+ (but not -Os), but the
> actual alignment is recorded in the 'align_functions' data which is only
> populated when -falign-functions is specified so I think it works as
> documented.

I tested this option in gcc 7.5.0 and 9.2.0, with the source code:

int func1(void)                                                                 
{
    return 0;
}

int func2(void)
{
    return 0;
}

int main (void)
{
    return 1;
}

and the result is like:
ashimida@ubuntu:~/test1$ aarch64-linux-gnu-gcc -falign-functions=128 -O0 test.c
-o test
ashimida@ubuntu:~/test1$ readelf -s test|grep fun
    63: 0000000000000780     8 FUNC    GLOBAL DEFAULT   13 func1
    78: 0000000000000800     8 FUNC    GLOBAL DEFAULT   13 func2
ashimida@ubuntu:~/test1$ aarch64-linux-gnu-gcc -O0 test.c -o test
ashimida@ubuntu:~/test1$ readelf -s test|grep fun
    63: 00000000000006e4     8 FUNC    GLOBAL DEFAULT   13 func1
    78: 00000000000006ec     8 FUNC    GLOBAL DEFAULT   13 func2

so I think this flag is also worked at -O0

[Bug other/101459] Mismatch in description of option "-falign-functions" between source code and documentation

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101459

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to ashimida from comment #3)
> (In reply to Richard Biener from comment #2)
> > The flag, -falign-functions is only enabled at -O2+ (but not -Os), but the
> > actual alignment is recorded in the 'align_functions' data which is only
> > populated when -falign-functions is specified so I think it works as
> > documented.
> 
> so I think this flag is also worked at -O0

I think you are misunderstanding the documentation.  The documentation is
saying it is enabled by default at -O2 and above (but not -Os); not that it
will only work at -O2 and above.

[Bug other/101459] Mismatch in description of option "-falign-functions" between source code and documentation

[ashimida at linux dot alibaba.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101459

--- Comment #5 from ashimida <ashimida at linux dot alibaba.com> ---
(In reply to Andrew Pinski from comment #4)
> (In reply to ashimida from comment #3)
> > (In reply to Richard Biener from comment #2)
> > > The flag, -falign-functions is only enabled at -O2+ (but not -Os), but the
> > > actual alignment is recorded in the 'align_functions' data which is only
> > > populated when -falign-functions is specified so I think it works as
> > > documented.
> > 
> > so I think this flag is also worked at -O0
> 
> I think you are misunderstanding the documentation.  The documentation is
> saying it is enabled by default at -O2 and above (but not -Os); not that it
> will only work at -O2 and above.

okey, now I understand, I have misunderstood this option, thank you very much!