From patchwork Thu Aug 12 17:06:56 2010
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Subject: PATCH: Turn on -fomit-frame-pointer by default for 32bit Linux/x86
Date: Thu, 12 Aug 2010 07:06:56 -0000
From: "H.J. Lu" <hjl.tools@gmail.com>
X-Patchwork-Id: 61628
Message-Id: <AANLkTi==ByATdy=JQrZJ05LwwcvoC0x+opVApWAEiMyr@mail.gmail.com>
To: Uros Bizjak <ubizjak@gmail.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>,
 Richard Guenther <rguenther@suse.de>, Jakub Jelinek <jakub@redhat.com>,
 Mark Mitchell <mark@codesourcery.com>,
 Richard Henderson <rth@redhat.com>, jh@suse.cz,
 Andi Kleen <andi@firstfloor.org>, Andrew Haley <aph@redhat.com>,
 David Daney <ddaney@caviumnetworks.com>,
 Xinliang David Li <davidxl@google.com>,
 Chris Lattner <clattner@apple.com>, Gerald Pfeifer <gerald@pfeifer.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?


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}.