lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Thu, 26 Nov 2015 00:50:08 +0000 From: Måns Rullgård <mans@...sr.com> To: Nicolas Pitre <nico@...xnic.net> Cc: Stephen Boyd <sboyd@...eaurora.org>, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, Michal Marek <mmarek@...e.com>, linux-kbuild@...r.kernel.org, Russell King - ARM Linux <linux@....linux.org.uk>, Arnd Bergmann <arnd@...db.de>, Steven Rostedt <rostedt@...dmis.org>, Thomas Petazzoni <thomas.petazzoni@...e-electrons.com> Subject: Re: [PATCH v2 2/2] ARM: Replace calls to __aeabi_{u}idiv with udiv/sdiv instructions Nicolas Pitre <nico@...xnic.net> writes: > On Thu, 26 Nov 2015, Måns Rullgård wrote: > >> Nicolas Pitre <nico@...xnic.net> writes: >> >> > 3) In fact I was wondering if the overhead of the branch and back is >> > really significant compared to the non trivial cost of a idiv >> > instruction and all the complex infrastructure required to patch >> > those branches directly, and consequently if the performance >> > difference is actually worth it versus simply doing (2) alone. >> >> Depending on the operands, the div instruction can take as few as 3 >> cycles on a Cortex-A7. > > Even the current software based implementation can produce a result with > about 5 simple ALU instructions depending on the operands. > > The average cycle count is more important than the easy-way-out case. > And then how significant the two branches around it are compared to idiv > alone from direct patching of every call to it. If not calling the function saves an I-cache miss, the benefit can be substantial. No, I have no proof of this being a problem, but it's something that could happen. Of course, none of this is going to be as good as letting the compiler generate div instructions directly. -- Måns Rullgård mans@...sr.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists