Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751042AbbF3Vc7 (ORCPT ); Tue, 30 Jun 2015 17:32:59 -0400 Received: from terminus.zytor.com ([198.137.202.10]:48188 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750940AbbF3Vcx (ORCPT ); Tue, 30 Jun 2015 17:32:53 -0400 Message-ID: <55930AE7.5090309@zytor.com> Date: Tue, 30 Jun 2015 14:32:23 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Andy Lutomirski , gcc@gcc.gnu.org, "linux-kernel@vger.kernel.org" , Linus Torvalds , Ingo Molnar , Thomas Gleixner Subject: Re: gcc feature request / RFC: extra clobbered regs References: In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1896 Lines: 47 On 06/30/2015 02:22 PM, Andy Lutomirski wrote: > Hi all- > > I'm working on a massive set of cleanups to Linux's syscall handling. > We currently have a nasty optimization in which we don't save rbx, > rbp, r12, r13, r14, and r15 on x86_64 before calling C functions. > This works, but it makes the code a huge mess. I'd rather save all > regs in asm and then call C code. > > Unfortunately, this will add five cycles (on SNB) to one of the > hottest paths in the kernel. To counteract it, I have a gcc feature > request that might not be all that crazy. When writing C functions > intended to be called from asm, what if we could do: > > __attribute__((extra_clobber("rbx", "rbp", "r12", "r13", "r14", > "r15"))) void func(void); > > This will save enough pushes and pops that it could easily give us our > five cycles back and then some. It's also easy to be compatible with > old GCC versions -- we could just omit the attribute, since preserving > a register is always safe. > > Thoughts? Is this totally crazy? Is it easy to implement? > > (I'm not necessarily suggesting that we do this for the syscall bodies > themselves. I want to do it for the entry and exit helpers, so we'd > still lose the five cycles in the full fast-path case, but we'd do > better in the slower paths, and the slower paths are becoming > increasingly important in real workloads.) > Some gcc targets have done this in the past. There are command-line options to do that, but using attributes you have to handle cross-ABI compilation. However, I don't see this being done in the upstream gcc. Keep in mind the runway that we'll need, though. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/