Re: Portable 16 bit binary integer write

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: Portable 16 bit binary integer write
Date	2016-09-20 19:43 +0200
Organization	A noiseless patient Spider
Message-ID	<nrrsg1$bpm$1@dont-email.me> (permalink)
References	(18 earlier) <lnshsvwxte.fsf@kst-u.example.com> <nrpgro$d3u$1@dont-email.me> <5299a08d-ac1e-4a51-a672-216c9d6e3516@googlegroups.com> <nrqpdv$egf$1@dont-email.me> <734d2b06-c5e8-4b56-b381-a97a0d522657@googlegroups.com>

Show all headers | View raw

On 20/09/16 16:49, supercat@casperkitty.com wrote:
> On Tuesday, September 20, 2016 at 2:45:18 AM UTC-5, David Brown wrote:
>> Modern ARMs are quite happy with operations on smaller values.  The
>> reason they don't have "8-bit add" instructions is that they are not
>> needed - they only have 8-bit and 16-bit instructions when such
>> instructions are different from the 32-bit instructions.  (There would
>> be a difference in the way some flags are updated, but that is usually
>> irrelevant, and it is not worth the cost in instruction code space.)
>
> On a 32-bit two's-complement system, given "int16_t x,y,z;" the behavior
> of x=y+z; will be defined in cases where the result would overflow a 16-bit
> value, and the lack of 16-bit "add" instruction would require the addition
> of an extra instruction (or, on the ARM7-TDMI, two instructions) to yield
> the Standard-mandated result.

The standard does not mandate behaviour here - the conversion of an 
int32_t to an int16_t is implementation defined (if the value does not fit).

Sometimes this will mean an extra "truncate" instruction is needed.  But 
it is rare - or at least, it is rare that it will have a significant 
effect.  One does not often write code that does nothing but add a 
couple of numbers.  And if the result is going to be stored in memory, 
the truncation happens automatically as part of the store instruction.

>  Using "int32_t x,y,z;" would be faster.  If
> values will be stored in RAM rather than registers, however, using int16_t
> will be just as fast as int32_t, and if caching is involved may be faster.
>
> What would be optimal for efficiency would be a type that allowed a compiler
> to arbitrarily use 16 or 32 bits at its convenience based upon context, but
> the C Standard does not allow for such types.
>

Yes, the C standard allows the compiler the freedom to use whatever type 
suits it - as long as the observable behaviour matches the behaviour 
given in the code.  It is unlikely that a compiler will choose to store 
a value as 16-bit in memory when the code says 32-bit, but it is 
conceivable.  It is certainly possible for the compiler to work with 
32-bit values internally even though the code says 16-bit, as long as 
the results are correct.

Thread

Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-10 14:23 -0700
  Re: Portable 16 bit binary integer write Barry Schwarz <schwarzb@dqel.com> - 2016-09-10 17:11 -0700
  Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-11 07:00 -0700
  Re: Portable 16 bit binary integer write Richard Damon <Richard@Damon-Family.org> - 2016-09-13 08:41 -0400
    Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-13 06:24 -0700
      Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-13 16:19 +0200
        Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-13 10:20 -0700
          Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-13 11:07 -0700
        Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-13 20:47 -0700
          Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-14 08:24 -0700
            Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-15 12:17 -0700
          Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-15 11:27 +0200
            Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-15 09:03 -0700
              Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-15 11:21 -0700
              Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-15 18:39 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-15 12:26 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-15 23:12 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-15 16:17 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-15 18:35 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-16 11:10 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-16 08:43 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-16 19:59 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-16 13:34 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-16 16:23 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-16 15:31 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-16 19:06 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-16 23:27 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-17 15:04 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-17 20:35 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-18 00:09 -0700
                Re: Portable 16 bit binary integer write Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-18 19:36 +0100
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-18 15:29 -0700
                Re: Portable 16 bit binary integer write Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-19 01:20 +0100
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-18 09:48 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-18 19:38 +0200
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-18 15:33 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-18 13:58 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-18 14:15 -0700
                Re: Portable 16 bit binary integer write BartC <bc@freeuk.com> - 2016-09-18 23:15 +0100
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-18 15:46 -0700
                Re: Portable 16 bit binary integer write Ian Collins <ian-news@hotmail.com> - 2016-09-19 17:03 +1200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-19 07:40 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-19 10:54 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-19 10:51 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-19 22:03 +0200
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-19 15:44 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-20 09:02 +0200
                Re: Portable 16 bit binary integer write BartC <bc@freeuk.com> - 2016-09-19 21:12 +0100
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-19 15:58 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-19 09:49 +0200
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-19 02:37 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-19 13:20 +0200
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-19 06:22 -0700
                Re: Portable 16 bit binary integer write BartC <bc@freeuk.com> - 2016-09-19 12:46 +0100
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-19 16:20 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-19 07:32 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-19 11:26 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-19 09:10 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-19 09:33 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-19 11:54 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-19 22:14 +0200
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-19 22:12 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-19 14:32 -0700
                Re: Portable 16 bit binary integer write Robert Wessel <robertwessel2@yahoo.com> - 2016-09-19 22:58 -0500
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-20 09:45 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-20 07:49 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-20 19:43 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-20 11:15 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-19 22:36 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-22 21:30 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-22 23:58 -0700
                Re: Portable 16 bit binary integer write Ian Collins <ian-news@hotmail.com> - 2016-09-23 19:45 +1200
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-28 08:17 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-28 09:59 -0700
                Re: Portable 16 bit binary integer write Ian Collins <ian-news@hotmail.com> - 2016-09-29 07:59 +1300
                Re: Portable 16 bit binary integer write Ike Naar <ike@iceland.freeshell.org> - 2016-09-28 19:26 +0000
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-28 12:37 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-10-09 03:46 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-18 13:53 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-21 21:03 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-21 22:23 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-22 08:09 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-23 10:31 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-23 10:56 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-29 08:03 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-30 07:39 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-22 09:49 +0200
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-22 08:34 -0700
                Re: Portable 16 bit binary integer write BartC <bc@freeuk.com> - 2016-09-22 17:44 +0100
                Re: Portable 16 bit binary integer write Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-22 17:16 +0000
                Re: Portable 16 bit binary integer write BartC <bc@freeuk.com> - 2016-09-22 18:26 +0100
                Re: Portable 16 bit binary integer write Melzzzzz <Melzzzzz@zzzzz.com> - 2016-09-22 17:33 +0000
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-23 10:23 +0200
                Re: Portable 16 bit binary integer write Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-23 13:43 +0100
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-23 14:58 +0200
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-22 18:14 -0700
                Re: Portable 16 bit binary integer write luser droog <luser.droog@gmail.com> - 2016-09-22 20:28 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-23 00:12 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-22 10:57 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-23 10:18 +0200
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-23 11:48 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-23 15:58 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-29 08:21 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-30 07:42 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-23 23:38 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-24 11:42 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-24 14:13 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-24 23:03 -0700
                Re: Portable 16 bit binary integer write Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-25 11:29 +0100
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-25 14:02 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-25 15:17 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-25 16:52 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-25 13:59 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-25 15:13 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-25 15:50 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-28 08:13 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-24 16:06 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-24 11:53 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-24 14:23 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-25 14:16 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-25 15:18 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-25 16:39 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-25 23:33 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-26 02:08 -0700
                Re: Portable 16 bit binary integer write Richard Heathfield <rjh@cpax.org.uk> - 2016-09-26 10:25 +0100
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-26 03:05 -0700
                Re: Portable 16 bit binary integer write Richard Heathfield <rjh@cpax.org.uk> - 2016-09-26 11:09 +0100
                Re: Portable 16 bit binary integer write "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-09-26 09:29 -0700
                Re: Portable 16 bit binary integer write Richard Heathfield <rjh@cpax.org.uk> - 2016-09-26 17:37 +0100
                Re: Portable 16 bit binary integer write "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-09-26 12:29 -0700
                Re: Portable 16 bit binary integer write BartC <bc@freeuk.com> - 2016-09-26 11:12 +0100
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-26 13:19 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-26 18:34 -0700
                Re: Portable 16 bit binary integer write Ike Naar <ike@iceland.freeshell.org> - 2016-09-27 06:43 +0000
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-27 00:51 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-27 08:42 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-22 11:40 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-23 10:00 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-23 10:34 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-29 08:13 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-18 00:06 +0200
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-17 17:32 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-18 15:57 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-17 20:46 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-18 16:08 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-18 10:03 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-17 23:59 +0200
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-17 16:49 -0700
                Re: Portable 16 bit binary integer write Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-18 01:19 +0100
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-18 07:16 -0700
                Re: Portable 16 bit binary integer write "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-18 10:19 -0400
                Re: Portable 16 bit binary integer write luser droog <luser.droog@gmail.com> - 2016-09-18 13:26 -0700
                Re: Portable 16 bit binary integer write "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-18 17:04 -0400
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-22 01:21 -0700
            Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-15 12:07 -0700
              Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-16 00:32 +0200
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-15 16:30 -0700
                Re: Portable 16 bit binary integer write Keith Thompson <kst-u@mib.org> - 2016-09-15 18:35 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-15 22:00 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-15 22:17 -0700
                Re: Portable 16 bit binary integer write Ian Collins <ian-news@hotmail.com> - 2016-09-16 18:12 +1200
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-16 00:46 -0700
                Re: Portable 16 bit binary integer write Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-16 10:23 +0100
                Re: Portable 16 bit binary integer write BartC <bc@freeuk.com> - 2016-09-16 10:59 +0100
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-16 04:13 -0700
                Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-16 08:58 -0700
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-16 10:16 -0700
                Re: Portable 16 bit binary integer write Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-16 09:11 -0700
                Re: Portable 16 bit binary integer write David Brown <david.brown@hesbynett.no> - 2016-09-16 20:14 +0200
                Re: Portable 16 bit binary integer write Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-16 20:37 -0700
      Re: Portable 16 bit binary integer write supercat@casperkitty.com - 2016-09-13 07:47 -0700

csiph-web