https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65837
--- Comment #24 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 29 Apr 2015, ramana at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65837 > > --- Comment #23 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> --- > (In reply to rguent...@suse.de from comment #20) > > On Tue, 28 Apr 2015, prathamesh3492 at gcc dot gnu.org wrote: > > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65837 > > > > > > --- Comment #17 from prathamesh3492 at gcc dot gnu.org --- > > > (In reply to clyon from comment #16) > > > > (In reply to prathamesh3492 from comment #15) > > > > > > > > > I am not understanding why vfpv3-d16 appears in collect_gcc_options in > > > > > run_gcc(). > > > > Isn't this because you configured GCC --with-fpu=vfpv3-d16? > > > > > > COLLECT_GCC_OPTIONS is set by gcc.c:set_collect_gcc_options(): > > > /* Build COLLECT_GCC_OPTIONS to have all of the options specified to > > > the compiler. */ > > > obstack_grow (&collect_obstack, "COLLECT_GCC_OPTIONS=", > > > sizeof ("COLLECT_GCC_OPTIONS=") - 1); > > > > > > and at the end of set_collect_gcc_options(): > > > xputenv (XOBFINISH (&collect_obstack, char *)); > > > which makes it environment variable. > > > > > > set_collect_gcc_options() is called by do_spec, which is called by > > > driver::maybe_run_linker(), before executing linker. > > > So the driver has no knowledge of options passed at compile-time, > > > it sets the default -mfpu=vfpv3-d16. > > > > > > When lto-wrapper executes, > > > it gets linker command line options from environment variable > > > COLLECT_GCC_OPTIONS, > > > which contains -mfpu=vfpv3-d16. > > > and since that was being appended after compile-time options > > > (fdecoded_options), -mfpu=vfpv3-d16 overrides -mfpu=neon. > > > > > > This also explains why it works in one shot > > > arm-linux-gnueabihf -flto -mfpu=neon test.c > > > > > > COLLECT_GCC_OPTIONS will have "-mfpu=neon" since it's mentioned on command > > > line, and lto-wrapper has access to this COLLECT_GCC_OPTIONS. > > > > > > When compiler and linker are run separately, at link time, the driver has > > > no > > > knowledege of flags of compile-time run, > > > and hence sets default flags in COLLECT_GCC_OPTIONS. > > > > > > I think correct way to fix would be in run_gcc() to get values from > > > COLLECT_GCC_OPTIONS in decoded_options as is currently done. > > > run_gcc() walks through options in object file and saves it in > > > fdecoded_options. So override the value in > > > decoded_options for the same option found in fdecoded_options. > > > Would that be a right approach ? > > > > No, link-time options always override compile-time ones. > > > > I suspect the fix will be to somehow avoid setting defaults when linking? > > Well I'm not sure how easy that's going to be - You will need some amount of > defaults to get through especially if the user hasn't provided the options in > the first place :( . The other option is to special-case only those options that are slipping in that way (-march, -mtune, -mcpu, -mfpu?) and emit those always first, hoping that explicit ones will override that. Of course lto-wrapper cannot distinguish between implicitely and explicitely such given arguments at link-time. Which means the only solution would be to completely ignore these at link-time. But I suspect this might break otherwise working cases (due to the fact that which -march/-mtune/-mcpu/-mfpu options lto-wrapper chooses from the object files is essentially "random" if they don't agree for all objects). It's really unfortunate that these configure-time defaults appear as regular user command-line arguments :( I suppose this was done to make them visible to specs processing.
