Date: Wed, 4 Mar 2020 10:48:59 -0800 From: Ryan Libby <rlibby@freebsd.org> To: Brooks Davis <brooks@freebsd.org> Cc: Guido Falsi <madpilot@freebsd.org>, "Alexander V. Chernikov" <melifaro@ipfw.ru>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>, vbox@freebsd.org Subject: Re: svn commit: r358439 - head/sys/amd64/include Message-ID: <CAHgpiFwaCpfUGijFsHiGadTg9QvnFw5WF38P0PntJM8xC8AKBg@mail.gmail.com> In-Reply-To: <20200304182801.GA95422@spindle.one-eyed-alien.net> References: <202002281832.01SIWaEL071685@repo.freebsd.org> <5767791583138727@sas1-c7aad230fe87.qloud-c.yandex.net> <CAHgpiFzg1Co6KY2twE3Pa7-ozQti4kzMr432R_7w9hukMQU00A@mail.gmail.com> <3d54ebc3-a511-a239-136d-c0f638a69351@FreeBSD.org> <20200304182801.GA95422@spindle.one-eyed-alien.net>
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On Wed, Mar 4, 2020 at 10:28 AM Brooks Davis <brooks@freebsd.org> wrote: > > On Wed, Mar 04, 2020 at 12:27:08PM +0100, Guido Falsi wrote: > > On 02/03/20 18:13, Ryan Libby wrote: > > > On Mon, Mar 2, 2020 at 12:45 AM Alexander V. Chernikov <melifaro@ipfw.ru> wrote: > > >> > > >> 28.02.2020, 18:32, "Ryan Libby" <rlibby@freebsd.org>: > > >>> Author: rlibby > > >>> Date: Fri Feb 28 18:32:36 2020 > > >>> New Revision: 358439 > > >>> URL: https://svnweb.freebsd.org/changeset/base/358439 > > >>> > > >>> Log: > > >>> amd64 atomic.h: minor codegen optimization in flag access > > >>> > > >>> Previously the pattern to extract status flags from inline assembly > > >>> blocks was to use setcc in the block to write the flag to a register. > > >>> This was suboptimal in a few ways: > > >>> - It would lead to code like: sete %cl; test %cl; jne, i.e. a flag > > >>> would just be loaded into a register and then reloaded to a flag. > > >>> - The setcc would force the block to use an additional register. > > >>> - If the client code didn't care for the flag value then the setcc > > >>> would be entirely pointless but could not be eliminated by the > > >>> optimizer. > > >>> > > >>> A more modern inline asm construct (since gcc 6 and clang 9) allows for > > >> This effectively restricts kernel builds by all older compilers. > > >> Is there any chance of making it conditional depending on the compiler version/features? > > > > > > Yes, it is possible to test for __GCC_ASM_FLAG_OUTPUTS__. It is more > > > maintenance effort going forward. If building current with an old cross > > > compiler is an important scenario, we can either revert this and the > > > following revision or work up a patch to make it conditional. I'll see > > > what that might look like. > > > > > > > Actually this causes emulators/virtualbox-ose port to fail to build: > > > > In file included from /usr/src/sys/sys/systm.h:44: > > /usr/include/machine/atomic.h:230:1: error: invalid output constraint > > '=@cce' in asm > > ATOMIC_CMPSET(char); > > ^ > > /usr/include/machine/atomic.h:205:4: note: expanded from macro > > 'ATOMIC_CMPSET' > > : "=@cce" (res), /* 0 */ \ > > ^ > > /usr/include/machine/atomic.h:230:1: error: invalid output constraint > > '=@cce' in asm > > > > (and so on) > > > > > > the virtualbox-ose port is forced to use an older clang version due to > > crashes when compiled with newer ones. > > > > Not sure whose responsibility is to fix this. > > I suspect that now that we don't care about gcc 4.2.1, we should > restructure machine/atomic.h to use __atomic compiler builtins in nearly > all cases. We could then conditionalize small sets of mircooptimized > assembly versions based on the availability of compiler features if they > add any value. > > On CheriBSD we've switched the RISC-V to use the C versions and are > overdue to do the same to MIPS. Reworking things to make this the > default would decrease our maintenance burden and it seems unlikely that > most of our platforms would benefit from handcode assembly (given the > general level of optimization in our lower-tier platforms). > > -- Brooks There's further discussion on that topic in the original review (D23869) and in D23661.
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