Re: Type system protocols rest atop fundamental data

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: Type system protocols rest atop fundamental data
Date	2017-06-21 15:33 +0200
Organization	A noiseless patient Spider
Message-ID	<oidscf$92j$1@dont-email.me> (permalink)
References	(18 earlier) <316e0ae4-9177-4b08-beb1-a1c54d40fa77@googlegroups.com> <oiaihc$vdl$1@dont-email.me> <64822b00-b983-4b36-be6d-27167aa918eb@googlegroups.com> <oibvcq$p2c$1@dont-email.me> <84c41593-bee7-4081-8685-56ab68b587fd@googlegroups.com>

Show all headers | View raw

On 20/06/17 23:04, supercat@casperkitty.com wrote:
> On Tuesday, June 20, 2017 at 3:12:45 PM UTC-5, David Brown wrote:
>> On 20/06/17 17:53, supercat wrote:
>>> On Tuesday, June 20, 2017 at 2:27:23 AM UTC-5, David Brown wrote:
>>>> You would like some cases where the standards today say "undefined
>>>> behaviour" to be replaced by "unpredictable results"?  Such as saying
>>>> that if you add two integers that overflow, then instead of being
>>>> undefined behaviour it should result in an integer whose value is
>>>> unpredictable?  Basically, that would be the same as "unspecified".
>>>
>>> Yes, though I would allow implementations to also specify either that
>>> integer overflows will (pick one):
> 
>> They can do that today.  "Undefined behaviour" means that the C standard 
>> does not define the behaviour.  It does not mean that the implementation 
>> may /not/ define the behaviour.
> 
> So if I want to write a program that will only compile on implementations
> that can uphold promises #4 and #B, how would I do that?

Only use implementations that can uphold promises #4 and #8 (whatever
they were) to compile your code.

An implementation can add any extra guarantees it wants beyond the
standard - that is the choice of the implementer.  A programmer can
require additional guarantees for their code to be correct - that is the
choice of the programmer.

> 
>>> Microprocessor systems started including dual processors before processors
>>> started including write-back caches (or, for that matter, any data caches).
>>
>> Dual processor systems were very rare before caches.  But even if they 
>> existed historically, they are irrelevant to the point here.
> 
> Such things were rare, but microprocessors entered the multi-processing
> world with strong-memory-model systems.  High-end multi-processor systems
> may have used looser memory models from the get-go, but microprocessor-based
> systems didn't.

I think you are making up your own terms, your own distinctions about
what counts as a "processor" and what counts as a "microprocessor"
(there is no fixed definition), and your own history here.  It is
possible that you are coincidentally correct, but it is all totally
/irrelevant/.  It does not /matter/ if the first SMP system had strong
or weak memory models, or write-back or write-through caches or no
caches at all.  The facts are simple:

1. On all realistic SMP systems, write-through cache would be a
bottleneck and a significant hinder to performance.  Therefore it is
rarely used on SMP systems.

2. Strong memory models are expensive to implement on most systems, and
scale badly when sizes increase.  Therefore most processors have weaker
memory models.

3. Writing code that is correct for a weak memory model is not much
harder than writing code that is correct for a strong memory model, in
the way that most people write code.  It might be harder for the OS
developers and the folks writing the implementations of atomic access
libraries, but they are not "most people".

> 
>>> A write-through cache is no good for a processor that's running multiple
>>> instructions per clock cycle, hooked up to memory that takes dozens of cycles
>>> to access, but it's just the ticket for a processor that takes at least a
>>> cycle per instruction, hooked up to memory that takes 3-4 cycles to access.
>>
>> No, it is not.  The cost is not as bad, but it is still slower to have 
>> write-through except perhaps for parts of memory that are very likely to 
>> be shared.  Your memory quickly becomes a bottleneck even with one 
>> processor on such a system - with two, it is worse.
> 
> A write-through cache is considerably cheaper than a write-back cache, and
> can support things like DMA without requiring software changes.  

No it cannot.  I have worked on these things, and know the issues.
Having a write-through cache does /not/ mean that you get all the data
out to memory, as seen by the DMA, at the time you might expect.  You
still have to use compiler memory barriers and appropriate processor
data synchronisation instructions to ensure data has hit the memory
before you start the DMA.  You may need additional setup or instructions
to handle re-ordering in memory write buffers, DDR controllers, etc.
And you still need cache control for when you are reading back data that
is written by DMA.

Having write-back cache means that when sending data /to/ the DMA
memory, you also need cache flush instructions.  But that is just one
more out of the half-dozen things you need to remember and get write -
it does not make an easy task hard, it merely makes a hard task
marginally harder.

> On a system
> with a write-back cache, software compatibility with DMA would require that a
> processor be able to monitor DMA requests, recognize attempts by an external
> device to fetch memory which is dirty, and substitute cache data for data
> from main RAM.  Rather complicated.

Actually, it is quite simple.  It is just cache snooping, something that
is common on SMP processors.

Thread

Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 06:14 -0700
  Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 06:18 -0700
    Re: Type system protocols rest atop fundamental data mark.bluemel@gmail.com - 2017-06-14 08:00 -0700
      Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 08:06 -0700
    Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-14 20:39 +0100
      Re: Type system protocols rest atop fundamental data Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-14 21:51 +0100
        Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-14 22:25 +0100
          Re: Type system protocols rest atop fundamental data Keith Thompson <kst-u@mib.org> - 2017-06-14 14:37 -0700
            Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-15 00:32 +0100
              Re: Type system protocols rest atop fundamental data Keith Thompson <kst-u@mib.org> - 2017-06-14 17:02 -0700
                Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-15 01:34 +0100
                Re: Type system protocols rest atop fundamental data Robert Wessel <robertwessel2@yahoo.com> - 2017-06-14 20:28 -0500
                Re: Type system protocols rest atop fundamental data raltbos@xs4all.nl (Richard Bos) - 2017-06-18 10:49 +0000
                Re: Type system protocols rest atop fundamental data Keith Thompson <kst-u@mib.org> - 2017-06-14 20:02 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-15 10:29 +0200
                Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-15 10:54 +0100
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-15 13:15 +0200
                Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-15 13:51 +0100
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-15 16:13 +0200
                Re: Type system protocols rest atop fundamental data Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-15 16:01 +0100
                Re: Type system protocols rest atop fundamental data Keith Thompson <kst-u@mib.org> - 2017-06-15 09:27 -0700
                Re: Type system protocols rest atop fundamental data raltbos@xs4all.nl (Richard Bos) - 2017-06-18 10:49 +0000
                Re: Type system protocols rest atop fundamental data Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-16 23:35 -0700
                Re: Type system protocols rest atop fundamental data Keith Thompson <kst-u@mib.org> - 2017-06-17 16:10 -0700
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-17 19:23 -0700
              Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-16 13:13 +1200
                Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-15 18:44 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-16 13:42 +0200
                Re: Type system protocols rest atop fundamental data Gareth Owen <gwowen@gmail.com> - 2017-06-16 20:20 +0100
                Re: Type system protocols rest atop fundamental data Robert Wessel <robertwessel2@yahoo.com> - 2017-06-18 00:32 -0500
                Re: Type system protocols rest atop fundamental data raltbos@xs4all.nl (Richard Bos) - 2017-06-18 11:06 +0000
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-18 15:26 +0200
                Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-18 14:58 +0100
          Re: Type system protocols rest atop fundamental data Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-15 01:42 +0100
            Re: Type system protocols rest atop fundamental data bartc <bc@freeuk.com> - 2017-06-15 11:52 +0100
              Re: Type system protocols rest atop fundamental data Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-15 16:07 +0100
                Re: Type system protocols rest atop fundamental data Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-16 23:37 -0700
              Re: Type system protocols rest atop fundamental data Keith Thompson <kst-u@mib.org> - 2017-06-15 09:52 -0700
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-15 10:36 -0700
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-15 17:58 +0000
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-15 11:48 -0700
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-15 19:01 +0000
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-15 12:29 -0700
                Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-16 13:47 +1200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-15 19:33 -0700
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-15 13:02 -0700
                Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-15 13:09 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-15 22:25 +0200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-15 14:23 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-15 23:53 +0200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-15 15:39 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-16 13:50 +0200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-16 08:40 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-18 16:49 +0200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-19 10:33 -0700
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-19 17:50 +0000
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-19 12:12 -0700
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-19 19:27 +0000
                Re: Type system protocols rest atop fundamental data Ike Naar <ike@iceland.freeshell.org> - 2017-06-20 15:18 +0000
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-20 09:30 -0700
                Re: Type system protocols rest atop fundamental data David Kleinecke <dkleinecke@gmail.com> - 2017-06-16 12:05 -0700
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-16 12:30 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-18 16:54 +0200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-19 13:25 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-20 09:27 +0200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-20 08:53 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-20 22:12 +0200
                Re: Type system protocols rest atop fundamental data "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-06-20 13:14 -0700
                Re: Type system protocols rest atop fundamental data "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-06-20 13:16 -0700
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-20 14:04 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-21 15:33 +0200
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-20 13:25 +0000
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-16 19:42 +0000
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-18 16:58 +0200
                Re: Type system protocols rest atop fundamental data raltbos@xs4all.nl (Richard Bos) - 2017-06-17 19:18 +0000
                Re: Type system protocols rest atop fundamental data Robert Wessel <robertwessel2@yahoo.com> - 2017-06-15 23:40 -0500
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-16 13:51 +0200
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-16 12:55 +0000
      Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 14:23 -0700
        Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-16 13:48 +1200
          Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-15 19:03 -0700
            Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-16 15:12 +1200
              Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-15 20:20 -0700
              Re: Type system protocols rest atop fundamental data Richard Heathfield <rjh@cpax.org.uk> - 2017-06-16 07:03 +0100
                Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-16 20:01 +1200
                Re: Type system protocols rest atop fundamental data Richard Heathfield <rjh@cpax.org.uk> - 2017-06-16 19:21 +0100
                Re: Type system protocols rest atop fundamental data "Chris M. Thomasson" <invalid@invalid.invalid> - 2017-06-16 12:07 -0700
                Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-17 16:18 +1200
                Re: Type system protocols rest atop fundamental data Richard Heathfield <rjh@cpax.org.uk> - 2017-06-17 07:19 +0100
                Re: Type system protocols rest atop fundamental data raltbos@xs4all.nl (Richard Bos) - 2017-06-17 19:19 +0000
                Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-18 08:29 +1200
                Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-17 18:49 -0700
                Re: Type system protocols rest atop fundamental data Ian Collins <ian-news@hotmail.com> - 2017-06-18 16:18 +1200
                Re: Type system protocols rest atop fundamental data Öö Tiib <ootiib@hot.ee> - 2017-06-18 06:28 -0700
                Re: Type system protocols rest atop fundamental data Keith Thompson <kst-u@mib.org> - 2017-06-16 09:11 -0700
                Re: Type system protocols rest atop fundamental data Tim Rentsch <txr@alumni.caltech.edu> - 2017-06-16 23:37 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-18 17:04 +0200
            Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-16 12:56 +0000
              Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-16 07:10 -0700
              Re: Type system protocols rest atop fundamental data GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-06-16 16:38 +0200
                Re: Type system protocols rest atop fundamental data scott@slp53.sl.home (Scott Lurndal) - 2017-06-16 15:02 +0000
                Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-16 08:11 -0700
                Re: Type system protocols rest atop fundamental data David Brown <david.brown@hesbynett.no> - 2017-06-18 17:13 +0200
      Re: Type system protocols rest atop fundamental data supercat@casperkitty.com - 2017-06-14 14:42 -0700
  Re: Type system protocols rest atop fundamental data Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-14 21:32 +0100
    Re: Type system protocols rest atop fundamental data "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-06-14 14:28 -0700

csiph-web