[Bug lto/55102] New: The options -flto and -On do not behave as described in GCC docs

[Bug lto/55102] New: The options -flto and -On do not behave as described in GCC docs

[d.g.gorbachev at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55102

             Bug #: 55102
           Summary: The options -flto and -On do not behave as described
                    in GCC docs
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: d.g.gorbachev@gmail.com


> Additionally, the optimization flags used to compile
> individual files are not necessarily related to those
> used at link time.  For instance,
>
>         gcc -c -O0 -flto foo.c
>         gcc -c -O0 -flto bar.c
>         gcc -o myprog -flto -O3 foo.o bar.o
>
> This produces individual object files with unoptimized
> assembler code, but the resulting binary myprog is
> optimized at -O3.  If, instead, the final binary is
> generated without -flto, then myprog is not optimized.

In fact, when you use -O3 when linking the .o files, it is already too late,
the resulting binary will not be fully optimized. You need to compile the .c
files with at least -O1. Thus, there is a bug either in GCC itself or in the
documentation.

====== 8< ======
int foo(void)
{
  return 0;
}

int main(void)
{
  return foo();
}
====== >8 ======

$ gcc -flto -O0 -c foo.c -o foo-O0.o
$ gcc -flto -O1 -c foo.c -o foo-O1.o
$ gcc -flto -O0 foo-O0.o -o prog-O0-O0
$ gcc -flto -O3 foo-O0.o -o prog-O0-O3
$ gcc -flto -O0 foo-O1.o -o prog-O1-O0
$ gcc -flto -O3 foo-O1.o -o prog-O1-O3
$ nm -A prog-O0-O0 prog-O0-O3 prog-O1-O0 prog-O1-O3 | grep foo
prog-O0-O0:080483f0 t foo.2337
prog-O0-O3:080483f0 t foo.2337
prog-O1-O0:080483f0 t foo.2337

GCC 4.6 gives a slight different result:

$ nm -A prog-O0-O0 prog-O0-O3 prog-O1-O0 prog-O1-O3 | grep foo
prog-O0-O0:08048381 t foo.1988
prog-O0-O3:08048380 t foo.1988

(GCC 4.5 crashes.)

[Bug lto/55102] The options -flto and -On do not behave as described in GCC docs

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55102

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-10-29
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> 2012-10-29 14:38:24 UTC ---
Yes, fact is that -O0 disables local optimizations before LTO streaming.
It also disables IPA pass local analysis phase which means that even if
enabling -O3 at WPA / LTRANS stages you will _not_ get IPA transforms
enabled.

Eventually we'd want to enable all IPA analysis phases at all -O levels...

[Bug lto/55102] The options -flto and -On do not behave as described in GCC docs

[ebotcazou at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55102

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |uran238 at web dot de

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2013-03-26 20:51:48 UTC ---
*** Bug 56700 has been marked as a duplicate of this bug. ***

[Bug lto/55102] The options -flto and -On do not behave as described in GCC docs

[hubicka at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55102

--- Comment #3 from Jan Hubicka <hubicka at gcc dot gnu.org> 2013-03-27 12:47:04 UTC ---
Doing IPA analysis at -O0 for LTO streaming won't really solve the fact that
functions are not early optimized.  I would vote for at least issuing a waning
when LTOing -O0 objects into -On, n>=1 LTO binary or simply declaring -O0 to be
non-LTO only.

But indeed, we probably should make analysis/summary streaming of all IPA
passes so -fno-ipa-cp and such works as expected all the time.  I have patch
fot that somewhere already.

We probably should lean the route of streaming the options used and honoring
them rather than taking whatever is passed to linker...

[Bug lto/55102] The options -flto and -On do not behave as described in GCC docs

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55102

--- Comment #4 from rguenther at suse dot de <rguenther at suse dot de> 2013-03-27 13:09:18 UTC ---
On Wed, 27 Mar 2013, hubicka at gcc dot gnu.org wrote:

> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55102
> 
> --- Comment #3 from Jan Hubicka <hubicka at gcc dot gnu.org> 2013-03-27 12:47:04 UTC ---
> Doing IPA analysis at -O0 for LTO streaming won't really solve the fact that
> functions are not early optimized.  I would vote for at least issuing a waning
> when LTOing -O0 objects into -On, n>=1 LTO binary or simply declaring -O0 to be
> non-LTO only.
> 
> But indeed, we probably should make analysis/summary streaming of all IPA
> passes so -fno-ipa-cp and such works as expected all the time.  I have patch
> fot that somewhere already.
> 
> We probably should lean the route of streaming the options used and honoring
> them rather than taking whatever is passed to linker...

Well, we _do_ stream them.  The issue is that we need to formally
define how to merge N sets of options from the N input files
at WPA stage to M sets of options for the M LTRANS units
(with eventually, but not necessarily, M == 1).

Oh, and implement it, of course.

At the moment the LTO driver (lto-wrapper.c) has a brief look at
options because it creates options for the WPA stage (which
shouldn't really care about the options passed ... in which
case it could do the option processing from the TUs and eventually
simply partition them into sets of TUs that have the same options).

So - Honza, what about first making WPA "ignore" all flags?
(all optimization and target flags)  IPA pass processing should
just unconditionally run and handle inputs which have the IPA
sections present.