From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/7646 Path: news.gmane.org!not-for-mail From: Isaac Dunham Newsgroups: gmane.linux.lib.musl.general Subject: Re: Eliminating preference for avoiding thread pointer? Cost on MIPS? Date: Sat, 16 May 2015 09:33:20 -0700 Message-ID: <20150516163319.GA1530@localhost> References: <20150516035544.GA4274@brightrain.aerifal.cx> 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 1431794009 11974 80.91.229.3 (16 May 2015 16:33:29 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 16 May 2015 16:33:29 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-7658-gllmg-musl=m.gmane.org@lists.openwall.com Sat May 16 18:33:29 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 1Ytf1k-00025T-SL for gllmg-musl@m.gmane.org; Sat, 16 May 2015 18:33:28 +0200 Original-Received: (qmail 5735 invoked by uid 550); 16 May 2015 16:33:26 -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 5676 invoked from network); 16 May 2015 16:33:24 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; bh=EXffYiJI83v0DorNplducFvYjc01blyYyK2x3C2+0EM=; b=PhEpApqvkZbBVodD/K7TzBl3546eXVoGtsEfTSq3CpEvLi4ktcXaOL3PWuT0yn84/x re8HCj3QB1M27bPNQV8UB9O3njWvDdSn/fwzB1u/kg5Mel5+3q0i+dG1kW4yINC8kx+v eZyjGSqI6XuHAOUGCZConzfzyNu9rD7KAsU8t0plBR/zxYYzDpm1BEuZJfN3N1uKXNgT uoA7nnG+P8MZDcH+9XrTnBm9oZwau/JKd1+fizNn5Nso6tM/fDKWHa2KzovtbJO17F1r LLRHFmwSIN4LM8rkXSQ4n25gu/2zdRZT60UbgdLAY5qtO513w+hex447pUjMYZKsLIq0 OPqA== X-Received: by 10.70.131.193 with SMTP id oo1mr29097992pdb.63.1431793992937; Sat, 16 May 2015 09:33:12 -0700 (PDT) Content-Disposition: inline In-Reply-To: <20150516035544.GA4274@brightrain.aerifal.cx> User-Agent: Mutt/1.5.23 (2014-03-12) Xref: news.gmane.org gmane.linux.lib.musl.general:7646 Archived-At: On Fri, May 15, 2015 at 11:55:44PM -0400, Rich Felker wrote: > Traditionally, musl has gone to pretty great lengths to avoid > depending on the thread pointer. The original reason was that it was > not always initialized, and when it was, the init was lazy. This > resulted in a lot of cruft, where we would have lots of constructs of > the form: > > bar = some_predicate ? __pthread_self()->foo : global_foo > > or similar. Being that these predicates depend(ed) on globals, they > were/are rather expensive in position-independent code on most archs. > Now that the thread pointer is always initialized at startup (since > 1.1.0) and assumed to have succeeded (since 1.1.9; musl now performs > HCF if it fails), this seems to be an unnecessary cost. Not only does > it cost cycles; it also has a complexity cost in terms of code to > maintain the state of the predicates (e.g. the atomics for locale > state) and in terms of libc-internal assumptions. So I'd like to just > use the thread pointer directly wherever it makes sense, and take > advantage of the fact that we have it. > > Unfortunately, there's one arch where thread-pointer access may be > prohibitively costly: old MIPS. On the MIPS o32 ABI, the thread > pointer is accessed via the "rdhwr $3,$29" instruction, which was only > introduced in MIPS32rev2. MIPS-I, MIPS-II, and possibly the original > MIPS32 lack it, and while Linux has a "fast path" trap to emulate it, > I'm not clear on how "fast" it is. > > First, I'd like to find out how slow this trap is. If it's something > like 150 cycles, that's ugly but probably acceptable. If it's more > like 1000 cycles, that's a big problem. If anyone can run the attached > test program on real MIPS-I or MIPS-II hardware and give me the > results, please do! Compile it once with -O3 -DDO_RDHWR and once with > just -O3 and send the (one-line) output of both to the list. It > doesn't matter what libc your MIPS system is using -- any should be > fine, but you might need to link with -lrt on glibc or uclibc. dd-wrt micro on a WRT54Gv8.0: \u@\h:\w\$ cat /proc/version Linux version 2.4.37 (root@dd-wrt) (gcc version 3.4.6 (OpenWrt-2.0)) #13303 Thu Aug 12 04:47:54 CEST 2010 \u@\h:\w\$ wget http://192.168.2.114:8080/def-bin Connecting to 192.168.2.114:8080 (192.168.2.114:8080) \u@\h:\w\$ echo * def-bin \u@\h:\w\$ chmod +x def-bin \u@\h:\w\$ ./def-bin 0 0.016751000 \u@\h:\w\$ wget http://192.168.2.114:8080/rd-bin Connecting to 192.168.2.114:8080 (192.168.2.114:8080) \u@\h:\w\$ chmod +x rd-bin \u@\h:\w\$ ./rd-bin Illegal instruction def-bin is withou -DDO_RDHWR, rd-bin is with. Both compiled static with musl 1.1.6 (because that's the latest musl-cross toolchain) and stripped. free reports 448 kb of 5736 kb free. (In other words, there's a reason it's that stripped down.) Thanks, Isaac Dunham