Date: Wed, 4 Mar 2020 12:27:08 +0100 From: Guido Falsi <madpilot@FreeBSD.org> To: Ryan Libby <rlibby@freebsd.org>, "Alexander V. Chernikov" <melifaro@ipfw.ru> Cc: "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: <3d54ebc3-a511-a239-136d-c0f638a69351@FreeBSD.org> In-Reply-To: <CAHgpiFzg1Co6KY2twE3Pa7-ozQti4kzMr432R_7w9hukMQU00A@mail.gmail.com> References: <202002281832.01SIWaEL071685@repo.freebsd.org> <5767791583138727@sas1-c7aad230fe87.qloud-c.yandex.net> <CAHgpiFzg1Co6KY2twE3Pa7-ozQti4kzMr432R_7w9hukMQU00A@mail.gmail.com>
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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. Should I file a bug report on bugzilla? -- Guido Falsi <madpilot@freebsd.org>
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