From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 90098 invoked by alias); 25 Jun 2019 08:14:14 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 90066 invoked by uid 89); 25 Jun 2019 08:14:14 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-6.8 required=5.0 tests=AWL,BAYES_00,GIT_PATCH_1,SPF_PASS autolearn=ham version=3.3.1 spammy=H*M:8138 X-HELO: mx1.suse.de Received: from mx2.suse.de (HELO mx1.suse.de) (195.135.220.15) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Tue, 25 Jun 2019 08:14:11 +0000 Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id EFE8CAE51; Tue, 25 Jun 2019 08:14:08 +0000 (UTC) Subject: Re: [PATCH] Add .gnu.lto_.meta section. To: Richard Biener Cc: Jan Hubicka , Thomas Koenig , Andrew Pinski , Jeff Law , GCC Development , GCC Patches References: <20190619192954.edwdfxns3gx2gt5m@kam.mff.cuni.cz> <122f53d2-5ae0-b801-81ab-36ce69f9efda@netcologne.de> <9417a329-c71c-4682-872c-60ff3524c47e@suse.cz> <20190620120225.s7ouywid2mntmhlm@kam.mff.cuni.cz> <20190621125718.qv2m5iygqj2b43fe@kam.mff.cuni.cz> <498ec44b-60ad-6f70-3c7a-b521f9fb6c56@suse.cz> <48330aa3-d678-89a1-aa86-09e948059733@suse.cz> From: =?UTF-8?Q?Martin_Li=c5=a1ka?= Message-ID: <06d3a9f1-2d2a-8138-99d1-c6ad4fc4c020@suse.cz> Date: Tue, 25 Jun 2019 08:14:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-IsSubscribed: yes X-SW-Source: 2019-06/txt/msg01521.txt.bz2 On 6/24/19 8:05 PM, Richard Biener wrote: > On Mon, Jun 24, 2019 at 3:31 PM Martin Liška wrote: >> >> On 6/24/19 2:44 PM, Richard Biener wrote: >>> On Mon, Jun 24, 2019 at 2:12 PM Martin Liška wrote: >>>> >>>> On 6/24/19 2:02 PM, Richard Biener wrote: >>>>> On Fri, Jun 21, 2019 at 4:01 PM Martin Liška wrote: >>>>>> >>>>>> On 6/21/19 2:57 PM, Jan Hubicka wrote: >>>>>>> This looks like good step (and please stream it in host independent >>>>>>> way). I suppose all these issues can be done one-by-one. >>>>>> >>>>>> So there's a working patch for that. However one will see following errors >>>>>> when using an older compiler or older LTO bytecode: >>>>>> >>>>>> $ gcc main9.o -flto >>>>>> lto1: fatal error: bytecode stream in file ‘main9.o’ generated with LTO version -25480.4493 instead of the expected 9.0 >>>>>> >>>>>> $ gcc main.o >>>>>> lto1: internal compiler error: compressed stream: data error >>>>> >>>>> This is because of your change to bitfields or because with the old >>>>> scheme the header with the >>>>> version is compressed (is it?). >>>> >>>> Because currently also the header is compressed. >>> >>> That was it, yeah :/ Stupid decisions in the past. >>> >>> I guess we have to bite the bullet and do this kind of incompatible >>> change, accepting >>> the odd error message above. >>> >>>>> I'd simply avoid any layout changes >>>>> in the version check range. >>>> >>>> Well, then we have to find out how to distinguish between compression algorithms. >>>> >>>>> >>>>>> To be honest, I would prefer the new .gnu.lto_.meta section. >>>>>> Richi why is that so ugly? >>>>> >>>>> Because it's a change in the wrong direction and doesn't solve the >>>>> issue we already >>>>> have (cannot determine if a section is compressed or not). >>>> >>>> That's not true, the .gnu.lto_.meta section will be always uncompressed and we can >>>> also backport changes to older compiler that can read it and print a proper error >>>> message about LTO bytecode version mismatch. >>> >>> We can always backport changes, yes, but I don't see why we have to. >> >> I'm fine with the backward compatibility break. But we should also consider lto-plugin.c >> that is parsing following 2 sections: >> >> 91 #define LTO_SECTION_PREFIX ".gnu.lto_.symtab" >> 92 #define LTO_SECTION_PREFIX_LEN (sizeof (LTO_SECTION_PREFIX) - 1) >> 93 #define OFFLOAD_SECTION ".gnu.offload_lto_.opts" >> 94 #define OFFLOAD_SECTION_LEN (sizeof (OFFLOAD_SECTION) - 1) > > Yeah, I know. And BFD and gold hard-coded those __gnu_lto_{v1,slim} symbols... Yep, they do, 'nm' is also using that. > >>> >>>>> ELF section overhead >>>>> is quite big if you have lots of small functions. >>>> >>>> My patch is actually shrinking space as I'm suggesting to add _one_ extra ELF section >>>> and remove the section header from all other LTO sections. That will save space >>>> for all function sections. >>> >>> But we want the header there to at least say if the section is >>> compressed or not. >>> The fact that we have so many ELF section means we have the redundant version >>> info everywhere. >>> >>> We should have a single .gnu.lto_ section (and also get rid of those >>> __gnu_lto_v1 and __gnu_lto_slim COMMON symbols - checking for >>> existence of a symbol is more expensive compared to existence >>> of a section). >> >> I like removal of the 2 aforementioned sections. To be honest I would recommend to >> add a new .gnu.lto_.meta section. > > Why .meta? Why not just .gnu.lto_? Works for me. > >> We can use it instead of __gnu_lto_v1 and we can >> have a flag there instead of __gnu_lto_slim. As a second step, I'm willing to concatenate all >> >> LTO_section_function_body, >> LTO_section_static_initializer >> >> sections into a single one. That will require an index that will have to be created. I can discuss >> that with Honza as he suggested using something smarter than function names. > > I think the index belongs to symtab? > > Let's properly do it if we want to change it. Removing of > __gnu_lto_v1/slim is going to be > the most intrusive change btw. and orthogonal to the section changes. I'm fine with a proper change. So do I understand that correctly that: - we'll come up with .gnu.lto_ section that will be used by bfd, gold and nm to detect LTO objects - for some time, we'll keep __gnu_lto_v1 and __gnu_lto_slim for backward compatibility with older binutils tool - in couple of year, the legacy support will be removed ? Martin > > Richard. > >> >> Thoughts? >> Martin >> >>> >>> Richard. >>> >>>> Martin >>>> >>>>> >>>>> Richard. >>>>> >>>>>> >>>>>> Martin >>>> >>