Re: Clang as default compiler November 4th

On 9/11/12, Brooks Davis <brooks_at_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_at_
>> >    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


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