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Re: PATCH: Turn on -fomit-frame-pointer by default for 32bit Linux/x86
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Uros Bizjak <ubizjak at gmail dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Richard Guenther <rguenther at suse dot de>, Jakub Jelinek <jakub at redhat dot com>, Mark Mitchell <mark at codesourcery dot com>, Richard Henderson <rth at redhat dot com>, jh at suse dot cz, Andi Kleen <andi at firstfloor dot org>, Andrew Haley <aph at redhat dot com>, David Daney <ddaney at caviumnetworks dot com>, Xinliang David Li <davidxl at google dot com>, Chris Lattner <clattner at apple dot com>, Gerald Pfeifer <gerald at pfeifer dot com>
- Date: Thu, 12 Aug 2010 10:06:56 -0700
- Subject: Re: PATCH: Turn on -fomit-frame-pointer by default for 32bit Linux/x86
- References: <AANLkTinbviPWO-hDfgL=31x=st_LA2frCPPYXtscEAhX@mail.gmail.com> <AANLkTim1L=eJGVO92S8_g+VP1ZUF4-O2O8KBUMB1UcwB@mail.gmail.com> <AANLkTi=7pqkdQjtDbqTSh-TAEa1L3TjBXgzXX2=znS4M@mail.gmail.com> <AANLkTi=FxbGb=sqr-+L0_tHwunFQS+0AqabarkEG10n7@mail.gmail.com> <AANLkTikUX0WLkiBFF+1eE8X44PsKyVNL6GNShPapoGo-@mail.gmail.com>
On Thu, Aug 12, 2010 at 10:03 AM, H.J. Lu <hjl.tools@gmail.com> 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.
>
>
How about this patch?
--
H.J.
2010-08-12 Uros Bizjak <ubizjak@gmail.com>
* doc/invoke.texi: Update -fomit-frame-pointer for 32-bit Linux
x86 targets.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index edce703..dbcc212 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -5993,6 +5993,11 @@ 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}.
+Starting from GCC version 4.6, the default setting for 32-bit Linux 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