Date: Mon, 2 Mar 2020 20:26:53 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: "Alexander V. Chernikov" <melifaro@ipfw.ru> Cc: Ryan Libby <rlibby@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r358439 - head/sys/amd64/include Message-ID: <20200302182653.GQ29554@kib.kiev.ua> 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 Mon, Mar 02, 2020 at 09:13:53AM -0800, 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? What is the older compiler you need to use for base system ? And why ? > > 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.
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