Re: Is this code considered to transgress the signed overflow purism?

Newsgroups	comp.lang.c
Date	2017-04-27 08:37 -0700
References	<11ea6682-cdee-4513-97c2-6c90b585716a@googlegroups.com> <odt02g$k55$1@dont-email.me>
Message-ID	<307ebff5-b74f-46ab-beb7-7e2e4ff494fb@googlegroups.com> (permalink)
Subject	Re: Is this code considered to transgress the signed overflow purism?
From	supercat@casperkitty.com

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On Thursday, April 27, 2017 at 9:46:49 AM UTC-5, James Kuyper wrote:
> Well, you don't have to be an optimization sophomore to understand
> correctly that the behavior is undefined for signed types. All you have
> to do is be able to read English. Later in the same paragraph it says
> (about E1 << E2) that "If E1 has a signed type and nonnegative value,
> and E1 × 2^E2 is representable in the result type, then that is the
> resulting value; otherwise, the behavior is undefined."

The clear intention of the Standard is that on a sign-magnitude platform
whose hardware may randomly catch fire if the left-shift instruction is
fed a negative number, a compiler for such a platform would have no
obligation to prevent that from happening.  I see no evidence that the
authors of the Standard expected or intended that implementations
targeting two's-complement hardware would should not opt to behave "in a
documented manner characteristic of the environment".  The authors of C89
deliberately avoided defining behaviors that would be applicable to some
machines but not others, figuring that compiler writers and programmers
alike should be able to recognize the obvious useful cases.

Incidentally, I'm curious about which standard-conforming C compiler for
a two's-complement machine was the first one to perform an optimization
by treating a left shift of a negative number as something other than
repeated multiplication by two in cases where the latter would not
overflow.  I would guess it was this century, but it would be interesting
to know precisely.

Thread

Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 03:13 -0700
  Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 03:41 -0700
  Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-27 11:59 +0100
    Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 14:35 -0700
      Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 15:13 -0700
        Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 00:58 +0200
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 16:44 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 20:04 -0700
            Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 08:53 +0200
              Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 08:23 -0700
                Re: Is this code considered to transgress the signed overflow purism? Manfred <noname@invalid.add> - 2017-04-28 18:05 +0200
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 09:22 -0700
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 12:24 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 00:03 +0100
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 16:37 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-28 17:09 -0700
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 17:34 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 01:43 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-28 21:21 -0700
                Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-29 02:04 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 11:46 +0100
                Re: Is this code considered to transgress the signed overflow purism? Robert Wessel <robertwessel2@yahoo.com> - 2017-04-30 12:05 -0500
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 11:33 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-29 08:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 17:58 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-29 09:13 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-30 12:55 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-30 09:17 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-30 20:35 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Kleinecke <dkleinecke@gmail.com> - 2017-04-30 15:39 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-05-01 16:21 +0200
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-05-02 06:35 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-05-02 16:29 +0200
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 20:58 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 18:43 +0200
            Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-28 14:09 +0100
              Re: Is this code considered to transgress the signed overflow purism? scott@slp53.sl.home (Scott Lurndal) - 2017-04-28 14:23 +0000
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-28 16:17 +0100
                Re: Is this code considered to transgress the signed overflow purism? scott@slp53.sl.home (Scott Lurndal) - 2017-04-28 15:55 +0000
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-04-28 19:11 -0700
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 11:23 +0100
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-29 11:31 +0100
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 12:01 +0100
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-29 12:47 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 15:20 +0200
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-29 15:22 +0100
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 16:52 +0200
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-29 15:18 +0100
                Re: Is this code considered to transgress the signed overflow purism? Jean-Marc Bourguet <jm@bourguet.org> - 2017-04-29 14:25 +0200
                Re: Is this code considered to transgress the signed overflow purism? Jerry Stuckle <jstucklex@attglobal.net> - 2017-04-29 08:05 -0400
                Re: Is this code considered to transgress the signed overflow purism? Gareth Owen <gwowen@gmail.com> - 2017-04-29 13:34 +0100
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-04-29 05:55 -0700
                Re: Is this code considered to transgress the signed overflow purism? GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-04-29 15:11 +0200
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-29 15:24 +0200
                Re: Is this code considered to transgress the signed overflow purism? Gareth Owen <gwowen@gmail.com> - 2017-04-29 14:57 +0100
                Re: Is this code considered to transgress the signed overflow purism? asetofsymbols@gmail.com - 2017-05-03 12:17 -0700
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-05-03 16:36 -0700
                Re: Is this code considered to transgress the signed overflow purism? asetofsymbols@gmail.com - 2017-05-03 11:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-05-03 21:40 +0200
                Re: Is this code considered to transgress the signed overflow purism? asetofsymbols@gmail.com - 2017-05-03 23:46 -0700
              Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 08:55 -0700
        Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 20:20 -0700
          Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 20:26 -0700
          Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 10:19 +0200
            Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-28 09:57 -0400
              Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 17:06 +0200
            Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 08:38 -0700
              Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 18:51 +0200
            Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-29 12:43 -0700
              Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 14:02 -0700
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-30 14:25 -0700
                Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 15:01 -0700
          Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-28 09:48 -0400
          Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 08:07 -0700
            Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 18:56 +0200
              Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-28 11:30 -0700
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-28 08:35 -0700
      Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 00:54 +0200
        Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-27 21:05 -0700
          Re: Is this code considered to transgress the signed overflow purism? David Brown <david.brown@hesbynett.no> - 2017-04-28 10:58 +0200
            Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-28 10:50 +0100
          Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-28 10:01 -0400
            Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-02 05:06 -0700
      Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-04-28 23:57 +0100
      Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 12:28 -0700
        Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-30 12:41 -0700
          Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-04-30 21:46 +0100
            Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-01 05:41 -0700
              Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-01 13:57 +0100
            Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-02 05:30 -0700
              Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 16:13 +0100
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 09:12 -0700
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 18:15 +0100
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 11:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 21:39 +0100
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-05-02 13:53 -0700
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 14:10 -0700
                Re: Is this code considered to transgress the signed overflow purism? bartc <bc@freeuk.com> - 2017-05-02 22:36 +0100
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-02 15:27 -0700
                Re: Is this code considered to transgress the signed overflow purism? Robert Wessel <robertwessel2@yahoo.com> - 2017-05-03 00:45 -0500
                Re: Is this code considered to transgress the signed overflow purism? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-05-03 11:57 +0100
                Re: Is this code considered to transgress the signed overflow purism? Robert Wessel <robertwessel2@yahoo.com> - 2017-05-03 13:34 -0500
                Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-05-03 12:05 -0700
                Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-05 02:58 -0700
                Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-03 05:26 -0700
                Re: Is this code considered to transgress the signed overflow purism? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2017-05-03 05:54 -0700
                Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-03 06:49 -0700
                Re: Is this code considered to transgress the signed overflow purism? Thiago Adams <thiago.adams@gmail.com> - 2017-05-03 06:51 -0700
                Re: Is this code considered to transgress the signed overflow purism? jadill33@gmail.com - 2017-05-03 09:03 -0700
                Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-05-02 14:47 -0700
                Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-05-03 17:45 -0700
          Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 14:42 -0700
        Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-30 23:57 -0700
          Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-05-02 04:32 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-05-03 18:20 -0700
          Re: Is this code considered to transgress the signed overflow purism? luser droog <luser.droog@gmail.com> - 2017-05-08 23:12 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-05-09 19:54 -0700
              Re: Is this code considered to transgress the signed overflow purism? luser droog <luser.droog@gmail.com> - 2017-05-09 21:21 -0700
  Re: Is this code considered to transgress the signed overflow purism? James Kuyper <jameskuyper@verizon.net> - 2017-04-27 10:46 -0400
    Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-27 08:37 -0700
      Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 09:38 -0700
        Re: Is this code considered to transgress the signed overflow purism? supercat@casperkitty.com - 2017-04-27 10:17 -0700
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-27 12:10 -0700
        Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 13:50 -0700
          Re: Is this code considered to transgress the signed overflow purism? Keith Thompson <kst-u@mib.org> - 2017-04-30 14:11 -0700
            Re: Is this code considered to transgress the signed overflow purism? Tim Rentsch <txr@alumni.caltech.edu> - 2017-04-30 14:44 -0700
            Re: Is this code considered to transgress the signed overflow purism? joel.rees@gmail.com - 2017-04-30 22:05 -0700

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