Re: Enale -fno-fat-lto-objects by default

[Markus Trippelsdorf <markus at trippelsdorf dot de>]

On 2013.11.19 at 11:21 +0100, Jan Hubicka wrote:
> > On 2013.11.19 at 09:44 +0100, Paolo Bonzini wrote:
> > > Il 18/11/2013 20:09, Jan Hubicka ha scritto:
> > > >>> > > this patch switches the default for fat-lto-objects as was documented for a while.
> > > >>> > > -ffat-lto-objects doubles compilation time and often makes users to not notice that
> > > >>> > > LTO was not used at all (because they forgot to use gcc-ar/gcc-nm plugins).
> > > >>> > > 
> > > >>> > > Sadly I had to add -ffat-lto-objects to bootstrap. This is because I do not know
> > > >>> > > how to convince our build machinery to use gcc-ar/gcc-nm during the stage2+
> > > >> > 
> > > >> > I've posted a minimal patch set for slim-lto-bootstrap last year, see:
> > > >> > http://thread.gmane.org/gmane.comp.gcc.patches/270842
> > > >> > 
> > > >> > If there's interest I could repost it.
> > > > It would be really nice to have it in indeed.  I think we do not really need
> > > > lto-bootstrap.mk and slim-lto-bootstrap.mk, but otherwise the patch seems easy
> > > > enough and would save quite some of lto bootstrap testing time...
> > > 
> > > Patches 1 and 2 should go upstream first.
> > 
> > OK, but where is upstream?
> > Please note that a general libtool update would fix this issue, too.
> > So, maybe it is just time to upgrade libtool everywhere in gnu-land?
> > 
> > > Patch 3 in the series is wrong because Makefile.in is a generated file.
> > >  The message does not explain why it is necessary, and it is probably
> > > working around a bug elsewhere.
> > > For patch 4, I agree with Jan that we do not need a separate configuration.
> > 
> > The problem is that fixincl links with libiberty.a:
> > 
> > /var/tmp/gcc_build_dir/./gcc/xgcc -B/var/tmp/gcc_build_dir/./gcc/
> > -B/usr/x86_64-pc-linux-gnu/bin/ -B/usr/x86_64-pc-linux-gnu/lib/ -isystem
> > /usr/x86_64-pc-linux-gnu/include -isystem
> > /usr/x86_64-pc-linux-gnu/sys-include    -O2 -pipe -static-libstdc++
> > -static-libgcc  -o fixincl fixincl.o fixtests.o fixfixes.o server.o
> > procopen.o fixlib.o fixopts.o ../libiberty/libiberty.a
> > 
> > And this archive consists of object files with LTO sections only. So we
> > need to find a way to pass -fuse-linker-plugin to the invocation above.
> 
> -fuse-linker-plugin is now default at the same time as -fno-fat-object-files is,
> so there should be no need for using this switch explicitely.

Hmm, gcc/gcc.c still reads:

 690 /* Conditional to test whether the LTO plugin is used or not.
 691    FIXME: For slim LTO we will need to enable plugin unconditionally.  This
 692    still cause problems with PLUGIN_LD != LD and when plugin is built but
 693    not useable.  For GCC 4.6 we don't support slim LTO and thus we can enable
 694    plugin only when LTO is enabled.  We still honor explicit
 695    -fuse-linker-plugin if the linker used understands -plugin.  */
 696
 697 /* The linker has some plugin support.  */
 698 #if HAVE_LTO_PLUGIN > 0
 699 /* The linker used has full plugin support, use LTO plugin by default.  */
 700 #if HAVE_LTO_PLUGIN == 2
 701 #define PLUGIN_COND "!fno-use-linker-plugin:%{flto|flto=*|fuse-linker-plugin"
 702 #define PLUGIN_COND_CLOSE "}"
 703 #else
 704 /* The linker used has limited plugin support, use LTO plugin with explicit
 705    -fuse-linker-plugin.  */
 706 #define PLUGIN_COND "fuse-linker-plugin"
 707 #define PLUGIN_COND_CLOSE ""
 708 #endif
 709 #define LINK_PLUGIN_SPEC \
 710     "%{"PLUGIN_COND": \
 711     -plugin %(linker_plugin_file) \
 712     -plugin-opt=%(lto_wrapper) \
 713     -plugin-opt=-fresolution=%u.res \
 714     %{!nostdlib:%{!nodefaultlibs:%:pass-through-libs(%(link_gcc_c_sequence))}} \
 715     }"PLUGIN_COND_CLOSE
 716 #else
 717 /* The linker used doesn't support -plugin, reject -fuse-linker-plugin.  */
 718 #define LINK_PLUGIN_SPEC "%{fuse-linker-plugin:\
 719     %e-fuse-linker-plugin is not supported in this configuration}"
 720 #endif

-- 
Markus