Re: PATCH: Turn on -fomit-frame-pointer by default for 32bit Linux/x86

[Jack Howarth <howarth at bromo dot med dot uc dot edu>]

On Thu, Aug 12, 2010 at 10:03:27AM -0700, H.J. Lu wrote:
> On Thu, Aug 12, 2010 at 10:00 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> > On Thu, Aug 12, 2010 at 6:43 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> >>>> 2010-08-12 ?H.J. Lu ?<hongjiu.lu@intel.com>
> >>>> ? ? ? ? ? ?Uros Bizjak ?<ubizjak@gmail.com>
> >>>>
> >>>> ? ? ? ?* config.gcc: Handle --enable-frame-pointer.
> >>>>
> >>>> ? ? ? ?* configure.ac: Add --enable-frame-pointer.
> >>>> ? ? ? ?* configure: Regenerated.
> >>>>
> >>>> ? ? ? ?* config/i386/i386.c (USE_IX86_FRAME_POINTER): Default to 0.
> >>>> ? ? ? ?(override_options): Enable -fomit-frame-pointer for 32bit code
> >>>> ? ? ? ?if compiling for TARGET_MACHO and not optimizing for size
> >>>> ? ? ? ?unless configured with --enable-frame-pointer. ?Enable
> >>>> ? ? ? ?-fasynchronous-unwind-tables unless configured with
> >>>> ? ? ? ?--enable-frame-pointer. ?Enable -maccumulate-outgoing-args
> >>>> ? ? ? ?by default unless configured with --enable-frame-pointer.
> >>>>
> >>>
> >>> Please change the ChangeLog text to something like:
> >>>
> >>> If not configured with --enable-frame-pointer, enable
> >>> -fomit-frame-pointer (but not for TARGET_MACHO or when optimizing for
> >>> size), enable -fasynchronous-unwind-tables and
> >>> -maccumulate-outgoing-args by default.
> >>>
> >>
> >> I checked it in with updated ChangeLog.
> >>
> >> How should we document it? We currently have
> >>
> >> --
> >> @item -fomit-frame-pointer
> >> @opindex fomit-frame-pointer
> >> Don't keep the frame pointer in a register for functions that
> >> don't need one. ?This avoids the instructions to save, set up and
> >> restore frame pointers; it also makes an extra register available
> >> in many functions. ?@strong{It also makes debugging impossible on
> >> some machines.}
> >>
> >> On some machines, such as the VAX, this flag has no effect, because
> >> the standard calling sequence automatically handles the frame pointer
> >> and nothing is saved by pretending it doesn't exist. ?The
> >> machine-description macro @code{FRAME_POINTER_REQUIRED} controls
> >> whether a target machine supports this flag. ?@xref{Registers,,Register
> >> Usage, gccint, GNU Compiler Collection (GCC) Internals}.
> >>
> >> Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
> >> --
> >>
> >> It was never correct for x86 and is wrong today.
> >
> > Perhaps something like:
> >
> > Index: invoke.texi
> > ===================================================================
> > --- invoke.texi (revision 163191)
> > +++ invoke.texi (working copy)
> > @@ -5993,6 +5993,11 @@
> > ?whether a target machine supports this flag. ?@xref{Registers,,Register
> > ?Usage, gccint, GNU Compiler Collection (GCC) Internals}.
> >
> > +Starting from GCC version 4.6, the default setting for 32-bit x86 targets
> > +has been changed to @option{-fomit-frame-pointer}. New behavior can be
> > +reverted back to @option{-fno-omit-frame-pointer} by configuring GCC with
> > +the @option{--enable-frame-pointer} configure option.
> > +
> > ?Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
> >
> > ?@item -foptimize-sibling-calls
> >
> 
> We default to --disable-frame-pointer only for 32bit x86 Linux and
> MACHO target always has -fno-omit-frame-pointer as default.

HJ,
  I thought that, in i386.c, we only had omit-frame-pointer disabled
for 64-bit, no?

  /* Set the default values for switches whose default depends on TARGET_64BIT
     in case they weren't overwritten by command line options.  */
  if (TARGET_64BIT)
    {
      if (flag_zee == 2)
        flag_zee = 1;
      /* Mach-O doesn't support omitting the frame pointer for now.  */
      if (flag_omit_frame_pointer == 2)
	flag_omit_frame_pointer = (TARGET_MACHO ? 0 : 1);
      if (flag_asynchronous_unwind_tables == 2)
	flag_asynchronous_unwind_tables = 1;
      if (flag_pcc_struct_return == 2)
	flag_pcc_struct_return = 0;
    }
  else
    {
      if (flag_zee == 2)
        flag_zee = 0;
      if (flag_omit_frame_pointer == 2)
	flag_omit_frame_pointer = 0;
      if (flag_asynchronous_unwind_tables == 2)
	flag_asynchronous_unwind_tables = 0;
      if (flag_pcc_struct_return == 2)
	flag_pcc_struct_return = DEFAULT_PCC_STRUCT_RETURN;
    }

  Jack
> 
> 
> 
> -- 
> H.J.