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


On Sun, Aug 8, 2010 at 7:56 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> Hello!
>
> After recent discussions, I would like to propose a transition to
> -fomit-frame-pointer for x86_32.
>
> The transition should be smooth as much as possible, should have
> option to revert to old behaviour and still providing path for the
> improvement. And we have learned something from cld issues, too
> (cough, cough...).
>
> I support the idea to change x86_32 defaults w.r.t. frame pointer (and
> unwind tables) to the same defaults as x86_64 has.
>
> The patch should also introduce --enable-frame-pointer configure
> option (off by default) that would revert back to old x86_32
> behaviour. So, if there are codes that depend on FP, their users (or
> distributions) should either (re-)configure the compiler with
> --enable-frame-pointer or they should use older compiler - 4.5.x will
> still be supported for many years. OTOH, it looks that users don't
> care that much whether backtraces on x86_64 are totally accurate, so
> IMO the sky won't fall down if x86_32 misses some backtraces in the
> same way. And as I have learned from the discussion, the problem is
> fixable with some effort on the user's side, thus fixing both targets
> in one shot.
>
> Of course, this change and the option to revert to the previous
> behaviour should be announced and documented in GCC release notes for
> 4.6.0.
>
> IMO, we have to bite the bullet from time to time in order to improve
> the generated code. We should not claim that gcc is
> "no-code-left-behind compiler" - from my experience, introducing new
> compiler always means that some parts of the code have to be fixed (as
> in case of the change to -fno-strict-aliasing).
>
> Uros.
>

I tested this patch on Linux/ia32 and Linux/x86-64. There are no regressions.

I don't have good wording for document:

--
For 32-bit x86 targets, it is not enabled at @option{-Os} by default.
This option also can be disabled by default on 32-bit x86 targets by
configuring GCC with the @option{--enable-frame-pointer} configure
option.
--

isn't very accurate.  Any suggestions?

Thanks.


-- 
H.J.
---
2010-08-09  H.J. Lu  <hongjiu.lu@intel.com>

	* config.gcc: Handle --enable-frame-pointer.

	* configure.ac: Add --enable-frame-pointer.
	* configure: Regenerated.

	* config/i386/i386.c (override_options): If not optimize for
	size, use -fomit-frame-pointer and -fasynchronous-unwind-tables
	by default for 32-bit code unless configured with
	--enable-frame-pointer.

Attachment: gcc-i386-fp-5.patch
Description: Text document


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