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. -- Brooks
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