Date: Tue, 11 Sep 2012 15:42:13 +0100 From: Attilio Rao <attilio@freebsd.org> To: Brooks Davis <brooks@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, toolchain@freebsd.org, current@freebsd.org Subject: Re: Clang as default compiler November 4th Message-ID: <CAJ-FndDnNzvV3YYXkCg6iCHWDb%2B1vhEOwq7m4gakAPwQL9jpKQ@mail.gmail.com> In-Reply-To: <20120911140825.GA73518@lor.one-eyed-alien.net> References: <20120910211207.GC64920@lor.one-eyed-alien.net> <20120911104518.GF37286@deviant.kiev.zoral.com.ua> <20120911140825.GA73518@lor.one-eyed-alien.net>
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On 9/11/12, Brooks Davis <brooks@freebsd.org> wrote: > On Tue, Sep 11, 2012 at 01:45:18PM +0300, Konstantin Belousov wrote: >> On Mon, Sep 10, 2012 at 04:12:07PM -0500, Brooks Davis wrote: >> > For the past several years we've been working towards migrating from >> > GCC to Clang/LLVM as our default compiler. We intend to ship FreeBSD >> > 10.0 with Clang as the default compiler on i386 and amd64 platforms. >> > To >> > this end, we will make WITH_CLANG_IS_CC the default on i386 and amd64 >> > platforms on November 4th. >> > >> > What does the mean to you? >> > >> > * When you build world after the default is changed /usr/bin/cc, cpp, >> > and >> > c++ will be links to clang. >> > >> > * This means the initial phase of buildworld and "old style" kernel >> > compilation will use clang instead of gcc. This is known to work. >> > >> > * It also means that ports will build with clang by default. A major >> > of ports work, but a significant number are broken or blocked by >> > broken ports. For more information see: >> > http://wiki.freebsd.org/PortsAndClang >> > >> > What issues remain? >> > >> > * The gcc->clang transition currently requires setting CC, CXX, and >> > CPP >> > in addition to WITH_CLANG_IS_CC. I will post a patch to toolchain@ >> > to address this shortly. >> > >> > * Ports compiler selection infrastructure is still under development. >> > >> > * Some ports could build with clang with appropriate tweaks. >> > >> > What can you do to help? >> > >> > * Switch (some of) your systems. Early adoption can help us find >> > bugs. >> > >> > * Fix ports to build with clang. If you don't have a clang system, >> > you >> > can use the CLANG/amd64 or CLANG/i386 build environments on >> > redports.org. >> > >> > tl;dr: Clang will become the default compiler for x86 architectures on >> > 2012-11-04 >> >> There was a chorus of voices talking about ports already. My POV >> is that suggesting to 'fix remaining ports to work with clang' is >> just a nonsense. You are proposing to fork the development of all the >> programs which do not compile with clang. Often, upstream developers >> do not care about clang at all since it not being default compiler in >> Debian/Fedora/Whatever Linux. The project simply do not have resources >> to maintain the fork of 20K programs. > > I may have phrased the above poorly, but in most cases I'd be happy with > using USE_GCC as a solution, but to the extent that port maintainers > can fix their ports to build with clang, that's a good thing. Having a > deadline will help focus efforts towards finding the right fix for the > most important ports in a timely manner. > > If we near the deadline and find that we need a few more weeks, nothing > prevents us from slipping the date a bit. > >> Another issue with the switch, which seems to be not only not addressed, >> but even not talked about, is the performance impact of the change. I >> do not remember any measurements, whatever silly they could be, of the >> performance change by the compiler switch. We often have serious and >> argumented push-back for kernel changes that give as low as 2-3% of >> the speed hit. What are the numbers for clang change, any numbers ? > > Florian Smeets (flo) did one round of benchmarks back in June with > sysbench/mysql. There is a small but measurable slowdown both with > world compiled with clang and with mysql compiled with clang. You can > see the results on the last page of this document: > > http://people.freebsd.org/~flo/perf.pdf > > The total impacts are on the order of 1-2%. That's more than I'd like > and I expect some pushback, but I feel it is in the range of acceptable > code debt to take on to accomplish a multi-year project goal. 1-2% on SMP workload can just be part of the variance due to memory layout changes. What I would like to see is benchmarks in UP configurations, like machine booting with only one process and doing make buildworld (no -j at all). This could give a good measurement if the compiler changed anything or not. Attilio -- Peace can only be achieved by understanding - A. Einstein
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