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Re: [PATCH] i386: Don't use frame pointer without stack access

From: Michael Matz <matz at suse dot de>
To: Arjan van de Ven <arjan at linux dot intel dot com>
Cc: Jakub Jelinek <jakub at redhat dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>, Alexander Monakov <amonakov at ispras dot ru>, Uros Bizjak <ubizjak at gmail dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
Date: Mon, 7 Aug 2017 18:28:27 +0200 (CEST)
Subject: Re: [PATCH] i386: Don't use frame pointer without stack access
Authentication-results: sourceware.org; auth=none
References: <20170806194037.GA25845@gmail.com> <CAFULd4Z-OicMFZTRS-3PiOQRSFxfNrkUP+O5pb7R-Uw-tSzfcw@mail.gmail.com> <alpine.LSU.2.21.1708071510120.15613@wotan.suse.de> <CAFULd4awp98Kw__PP0kL=U4zWgH3aZJ2ow6AuKNaKvGR0O9n7Q@mail.gmail.com> <CAMe9rOqUP01UYJTcqNWktVCeQJYnjH7fYhwDj7hkYotG_p=HcA@mail.gmail.com> <alpine.LSU.2.21.1708071531210.15613@wotan.suse.de> <CAMe9rOqf8NYeeEEsmbc2g+d-X4SpyA2gTpubL-RiJXj13op3kA@mail.gmail.com> <alpine.LSU.2.21.1708071543400.15613@wotan.suse.de> <alpine.LNX.2.20.13.1708071653450.29517@monopod.intra.ispras.ru> <CAMe9rOpRJFr4KvftoEWyU0OaC4CfWz9VBwx7=2S6z2C+=dguEg@mail.gmail.com> <20170807154311.GN2123@tucnak> <9c734d2b-7ba2-057b-7505-a7b300cde472@linux.intel.com> <alpine.LSU.2.21.1708071813160.15613@wotan.suse.de> <2c0c60fc-2bdc-9246-0165-cf6ef48e3e7e@linux.intel.com>

Hi,

On Mon, 7 Aug 2017, Arjan van de Ven wrote:

> I'm not surprised to see one.
> I'm surprised to see a useless one.
> 
> The "perf" benefit is real, and that's why I asked for one... but the reorder
> made it an expensive 3 instruction nop for all intents and purposes.
> If the pop was just before the ret, sure. It's not.

Okay, that seems a reasonable request.  But IMHO independend from the 
issue of simply ignoring -fno-omit-frame-pointer to which I object.

> Maybe a different angle would be for a peephole phase to just eliminate 
> the useless (even for those who do want a frame pointer) push/mov/pop

For instance.

Ciao,
Michael.

References:
- [PATCH] i386: Don't use frame pointer without stack access
  - From: H.J. Lu
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Uros Bizjak
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Michael Matz
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Uros Bizjak
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: H.J. Lu
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Michael Matz
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: H.J. Lu
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Michael Matz
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Alexander Monakov
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: H.J. Lu
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Jakub Jelinek
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Arjan van de Ven
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Michael Matz
- Re: [PATCH] i386: Don't use frame pointer without stack access
  - From: Arjan van de Ven

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