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Re: Why is shifting too far undefined behvaior?

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On 2021-09-21, Philipp Klaus Krause <pkk@spth.de> wrote:
> Looking at the C23 draft N2596, section J.2 (but this AFAIK has been the
> same forever), we see that there is undefined behavior, when "An
> expression is shifted by a negative number or by an amount greater than
> or equal to the width of the promoted expression (6.5.7)." or "An
> expression having signed promoted type is left-shifted and either the
> value of the expression is negative or the result of shifting would not
> be representable in the promoted type (6.5.7)."
>
> Does anyone know why this was made undefined (as opposed to yielding an
> unspecified value)? The C99 rationale doesn't mention anything.
>
> The only idea that comes to my mind is that it would be to allow
> implementations to trap at runtime when shifting too far, which might
> help with debugging.

Another thing that comes to mind is that on a machine with N-bit
wordsize, the machine instruction for shifting might only use log2(N) bits
to specify the number of places to shift.
Shifting N places or more would not be possible on such hardware
(at least, not with a single shift instruction).

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Why is shifting too far undefined behvaior? Philipp Klaus Krause <pkk@spth.de> - 2021-09-21 13:09 +0200
  Re: Why is shifting too far undefined behvaior? Ike Naar <ike@faeroes.freeshell.org> - 2021-09-21 11:33 +0000
    Re: Why is shifting too far undefined behvaior? Richard Kettlewell <invalid@invalid.invalid> - 2021-09-21 12:47 +0100
  Re: Why is shifting too far undefined behvaior? Richard Damon <Richard@Damon-Family.org> - 2021-09-21 08:29 -0400
    Re: Why is shifting too far undefined behvaior? Vincent Lefevre <vincent-news@vinc17.net> - 2021-09-22 01:17 +0000
      Re: Why is shifting too far undefined behvaior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-21 19:05 -0700
        Re: Why is shifting too far undefined behvaior? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-22 00:02 -0400
          Re: Why is shifting too far undefined behvaior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-21 21:12 -0700
            Re: Why is shifting too far undefined behvaior? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-22 09:42 -0400
          Re: Why is shifting too far undefined behvaior? Richard Damon <Richard@Damon-Family.org> - 2021-09-22 07:08 -0400
            Re: Why is shifting too far undefined behvaior? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-22 09:42 -0400
              Re: Why is shifting too far undefined behvaior? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-23 12:18 -0400
          Re: Why is shifting too far undefined behvaior? Vincent Lefevre <vincent-news@vinc17.net> - 2021-09-23 11:10 +0000
            Re: Why is shifting too far undefined behvaior? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-09-23 12:22 -0400
        Re: Why is shifting too far undefined behvaior? Vincent Lefevre <vincent-news@vinc17.net> - 2021-09-23 11:08 +0000
  Re: Why is shifting too far undefined behvaior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-09-21 11:01 -0700
    Re: Why is shifting too far undefined behvaior? Philipp Klaus Krause <pkk@spth.de> - 2021-09-21 20:12 +0200
  Re: Why is shifting too far undefined behvaior? Vincent Lefevre <vincent-news@vinc17.net> - 2021-09-22 01:13 +0000
  Re: Why is shifting too far undefined behvaior? David Brown <david.brown@hesbynett.no> - 2021-09-22 14:25 +0200
    Re: Why is shifting too far undefined behvaior? Vincent Lefevre <vincent-news@vinc17.net> - 2021-09-23 11:17 +0000
  Re: Why is shifting too far undefined behvaior? Tim Rentsch <txr@shamko.com> - 2021-10-09 04:27 -0700

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