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Re: Simplest compiler for C

From David Brown <david.brown@hesbynett.no>
Newsgroups comp.lang.c
Subject Re: Simplest compiler for C
Date 2016-11-15 10:05 +0100
Organization A noiseless patient Spider
Message-ID <o0ej3s$5np$1@dont-email.me> (permalink)
References (1 earlier) <4232d4c1-5461-40b8-8eac-94d0e9e055b6@googlegroups.com> <o0cs8t$58a$2@dont-email.me> <o0cvdo$ij4$1@dont-email.me> <o0d24h$tbb$1@dont-email.me> <o0d42c$53u$1@dont-email.me>

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On 14/11/16 20:42, BartC wrote:
> On 14/11/2016 19:09, jacobnavia wrote:
>>
>> So, you are running a simulation of linux under windows. In that
>> emulation you run the windows emulator, that eventually runs  your
>> program.
>>
>> No wonder!
> 
> AFAIK native code code ought to run at full speed, as it is not
> emulating the x86 processor. And some programs do.

Yes.

> 
> But others (like some of mine!) run slower than they ought, even leaving
> wine out of it, but perhaps 30-50% slower. And one seemed to be
> sensitive to some cache issues. So something about it (Virtualbox) is
> causing problems.

Make sure you isolate any startup delays from the timing - Wine has to
create a bit of Windows-like environment at startup.

Cache issues /can/ have an effect, though usually the speed difference
of the main x86 code between native and VirtualBox is very minor.  One
thing to note is that when an OS is running natively, it has more
information about the processors and can make more intelligent
scheduling decisions.  For example, if you have 4 cores with SMT, then
the Linux kernel knows which cores are shared between two threads, and
takes that into account when scheduling up to 8 running processes.  But
if you create a virtual machine and tell it to use up to 8 processors,
the kernel in the virtual machine only sees 8 cpus but knows nothing
about the SMT pairs, or cache sizes, etc.  It also loses (almost) all
information about processor affinity and cache contents - normally the
Linux kernel will try to re-schedule processes on the same core in order
to minimise cache misses.  It can't do that when a Windows host OS has
played with the cpus and caches behind its back.  (I have no idea how
smart the cpu scheduler is on Windows these days, I only know about the
Linux one.)


> 
> At the moment however I can only generate executable code to run
> natively on Linux via gcc, which is yet another factor. (And I didn't
> forget the -O3!)
> 

Often -O2 is faster than -O3, as -O3 does a lot more loop unrolling and
other code transformations that increase the size of code.  L1 caches,
prefetches, BTBs, etc., usually mean that the smaller code in -O2 is
faster than the expanded -O3 code.  But it can depend on details of the
code - picking the best switches for the very fastest code generation is
more of an art than a science.  Unless you are interested in reading the
documentation and doing a lot of trial and error, then stick to these:

-O2	// Basic solid optimisation
-march=native	// Tell gcc to take full advantage of /your/ cpu
-ffast-math	// Makes floating point much faster, but not IEEE


And try with something a little more demanding than "Hello, world!",
even if it is little more than a big summation loop, to isolate the
actual code performance.

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Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-14 18:57 +0800
  Re: Simplest compiler for C Mark Storkamp <mstorkamp@yahoo.com> - 2016-11-14 08:44 -0600
    Re: Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-14 22:46 +0800
  Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-14 08:15 -0800
    Re: Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-15 01:29 +0800
      Re: Simplest compiler for C Dutch Ingraham <spikeheron@att.net> - 2016-11-14 11:43 -0600
        Re: Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-15 19:18 +0800
      Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-14 18:23 +0000
        Re: Simplest compiler for C jacobnavia <jacob@jacob.remcomp.fr> - 2016-11-14 20:09 +0100
          Re: Simplest compiler for C Gareth Owen <gwowen@gmail.com> - 2016-11-14 19:11 +0000
          Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-14 19:42 +0000
            Re: Simplest compiler for C fir <profesor.fir@gmail.com> - 2016-11-14 12:00 -0800
              Re: Simplest compiler for C fir <profesor.fir@gmail.com> - 2016-11-14 12:17 -0800
                Re: Simplest compiler for C fir <profesor.fir@gmail.com> - 2016-11-14 12:19 -0800
              Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-14 12:40 -0800
                Re: Simplest compiler for C fir <profesor.fir@gmail.com> - 2016-11-14 13:08 -0800
                Re: Simplest compiler for C fir <profesor.fir@gmail.com> - 2016-11-14 13:12 -0800
            Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-14 12:10 -0800
            Re: Simplest compiler for C David Brown <david.brown@hesbynett.no> - 2016-11-15 10:05 +0100
          Re: Simplest compiler for C David Brown <david.brown@hesbynett.no> - 2016-11-15 09:50 +0100
    Re: Simplest compiler for C luser droog <luser.droog@gmail.com> - 2016-11-14 09:39 -0800
    Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-14 09:53 -0800
      Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-14 10:31 -0800
        Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-14 18:57 +0000
          Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-14 11:07 -0800
        Re: optimization "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-15 19:20 +0800
        Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-15 12:56 +0000
          Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-15 05:34 -0800
            Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-15 16:07 +0000
              Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-15 08:59 -0800
                Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-15 21:35 +0000
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-15 15:33 -0800
                Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-16 14:58 +0000
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-16 08:44 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-16 08:54 -0800
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-16 09:11 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-16 09:36 -0800
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-16 11:41 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-16 11:45 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-16 18:54 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-17 04:44 -0800
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-17 08:42 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-17 09:17 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-17 09:42 -0800
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-17 09:57 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-17 10:07 -0800
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-17 11:35 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-17 12:02 -0800
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-17 13:23 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-17 13:55 -0800
                Re: Simplest compiler for C supercat@casperkitty.com - 2016-11-17 15:15 -0800
                Re: Simplest compiler for C David Brown <david.brown@hesbynett.no> - 2016-11-18 10:41 +0100
                Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-18 12:12 +0000
                Re: Simplest compiler for C David Brown <david.brown@hesbynett.no> - 2016-11-18 13:24 +0100
                Re: Simplest compiler for C Jerry Stuckle <jstucklex@attglobal.net> - 2016-11-18 09:45 -0500
                Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-17 20:08 +0000
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-17 12:21 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-18 03:22 -0800
                Re: Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-18 19:23 +0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-18 03:36 -0800
                Re: Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-18 19:52 +0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-19 17:55 -0800
                Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-16 17:19 +0000
                Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-16 19:09 +0000
                Re: Simplest compiler for C David Brown <david.brown@hesbynett.no> - 2016-11-16 23:12 +0100
                Re: Simplest compiler for C "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-16 14:21 -0800
                Re: Simplest compiler for C "James R. Kuyper" <jameskuyper@verizon.net> - 2016-11-16 18:02 -0500
                Re: Simplest compiler for C "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-17 17:01 -0800
                Re: Simplest compiler for C "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-18 01:05 -0800
                Re: Simplest compiler for C jameskuyper@verizon.net - 2016-11-18 10:38 -0800
                Re: Simplest compiler for C "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-20 15:38 -0800
                Re: Simplest compiler for C David Brown <david.brown@hesbynett.no> - 2016-11-17 09:57 +0100
                Re: Simplest compiler for C BartC <bc@freeuk.com> - 2016-11-17 11:25 +0000
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-16 08:56 -0800
                Re: Simplest compiler for C fir <profesor.fir@gmail.com> - 2016-11-16 09:10 -0800
                Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-16 09:34 -0800
          Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-15 06:32 -0800
            [OT] Output of C compilers and CPU architectures "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-15 22:50 +0800
              Re: [OT] Output of C compilers and CPU architectures "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-15 07:46 -0800
                Re: [OT] Output of C compilers and CPU architectures "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-16 00:12 +0800
                Re: [OT] Output of C compilers and CPU architectures "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-15 09:02 -0800
                Re: [OT] Output of C compilers and CPU architectures "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-16 01:14 +0800
                Re: [OT] Output of C compilers and CPU architectures "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-15 09:24 -0800
                Re: [OT] Output of C compilers and CPU architectures "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-16 01:27 +0800
    Re: Simplest compiler for C Anton Shepelev <anton.txt@gmail.com> - 2016-11-14 21:49 +0300
      Re: Simplest compiler for C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2016-11-14 11:02 -0800
  Re: Simplest compiler for C Anton Shepelev <anton.txt@gmail.com> - 2016-11-14 21:37 +0300
    Re: Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-15 18:07 +0800
    Re: Simplest compiler for C "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2016-11-15 20:15 +0800

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