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

["H.J. Lu" <hjl dot tools at gmail dot com>]

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.



-- 
H.J.