From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/7559 Path: news.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: Re: building musl libc.so with gcc -flto Date: Thu, 30 Apr 2015 19:44:51 -0400 Message-ID: <20150430234451.GY17573@brightrain.aerifal.cx> References: <1429742932-6026-1-git-send-email-armccurdy@gmail.com> <20150423022309.GH6817@brightrain.aerifal.cx> <5542949D.8000509@kernel.org> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1430437510 11419 80.91.229.3 (30 Apr 2015 23:45:10 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 30 Apr 2015 23:45:10 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-7572-gllmg-musl=m.gmane.org@lists.openwall.com Fri May 01 01:45:10 2015 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1Yny8h-0003L5-5V for gllmg-musl@m.gmane.org; Fri, 01 May 2015 01:45:07 +0200 Original-Received: (qmail 11898 invoked by uid 550); 30 Apr 2015 23:45:04 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 11880 invoked from network); 30 Apr 2015 23:45:04 -0000 Content-Disposition: inline In-Reply-To: <5542949D.8000509@kernel.org> User-Agent: Mutt/1.5.21 (2010-09-15) Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:7559 Archived-At: On Thu, Apr 30, 2015 at 01:46:21PM -0700, Andy Lutomirski wrote: > On 04/22/2015 07:23 PM, Rich Felker wrote: > >On Wed, Apr 22, 2015 at 03:48:52PM -0700, Andre McCurdy wrote: > >>Hi all, > >> > >>Below are some observations from building musl libc.so with gcc's -flto > >>(link time optimization) option. > > > >Interesting! > > > >>1) With today's master (afbcac68), adding -flto to CFLAGS causes the > >>build to fail: > >> > >> | `_dlstart_c' referenced in section `.text' of /tmp/cc8ceNIy.ltrans0.ltrans.o: defined in discarded section `.text' of src/ldso/dlstart.lo (symbol from plugin) > >> | collect2: error: ld returned 1 exit status > >> | make: *** [lib/libc.so] Error 1 > >> > >>Reverting f1faa0e1 (make _dlstart_c function use hidden visibility) > >>seems to be a workaround. > > > >I think the problem is that LTO is garbage collecting "unused" symbols > >before it gets to the step of linking with asm for which there is no > >IR code, thereby losing anything that's only referenced from asm. A > >better workaround might be to define _dlstart_c with a different name > >as a non-hidden function (e.g. call it __dls1) and then make > >_dlstart_c a hidden alias for it via: > > > >__attribute__((__visibility__("hidden"))) > >void _dlstart_c(size_t *, size_t *); > > > >weak_alias(__dls1, _dlstart_c); > > > >If you get a chance to try that, let me know if it works. Another > >option might be adding -Wl,-u,_dlstart_c to LDFLAGS. > > Wouldn't adding __attribute__((externally_visible)) to the relevant > symbols be more appropriate? It's intended to solve exactly this > problem. I'm not clear whether it would be reliable to use this or not. Semantically externally_visible and visibility=hidden are contradictory. Even if we weren't trying to avoid relying on additional GNU C features, I think it would be a bad idea to rely on this working since the behavior under such contradictory annotations could potentially vary widely between compilers. Rich