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Re: left shift count >= width of type

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James Kuyper <jameskuyper@verizon.net> writes:
[...]
> There's no guarantee that the shift count >= the width of the type,
> since 'int' could be, for instance, a 64-bit type.
>
> The relevant clause is phrased in terms of whether not the the result
> can be represented in the type, not the width of the type, For the
> expression E1 << E2, "If E1 has a signed type and nonnegative value, and
> E1 x 2^E2 is representable in the result type, then that is the
> resulting value; otherwise, the behavior is undefined" (6.5.7p5)
>
> Therefore, for a 32-bit int, even 1 << 31 is problematic. That's an
> example of why it's better to use unsigned types for most bit-masking
> purposes.
>
> However - and this is the key point - this is not a constraint, it's
> undefined behavior. Conforming implementations are NOT required to issue
> a diagnostic.

In addition (C99 6.5.7):

    If the value of the right operand is negative or is greater than or
    equal to the width of the promoted left operand, the behavior is
    undefined.

So for a 32-bit type, even 0<<32 or 0>>32 has undefined behavior, even
though the mathematical result is representable.

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

left shift count >= width of type Alessandro Basili <alessandro.basili@cern.ch> - 2011-11-02 10:55 +0100
  Re: left shift count >= width of type James Kuyper <jameskuyper@verizon.net> - 2011-11-02 07:34 -0400
    Re: left shift count >= width of type Keith Thompson <kst-u@mib.org> - 2011-11-02 13:47 -0700
    Re: left shift count >= width of type Alessandro Basili <alessandro.basili@cern.ch> - 2011-11-04 17:46 +0100
      Re: left shift count >= width of type James Kuyper <jameskuyper@verizon.net> - 2011-11-04 13:12 -0400
        Re: left shift count >= width of type Alessandro Basili <alessandro.basili@cern.ch> - 2011-11-05 19:17 +0100
  Re: left shift count >= width of type Thad Smith <ThadSmith@acm.org> - 2011-11-02 20:22 -0600

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