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Re: left shift count >= width of type

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On 11/2/2011 3:55 AM, Alessandro Basili wrote:
> Hi,
> here's a simple program snippet that I'm dealing with and worried about
> potential pitfalls:
>
> #define BITS_PER_UNIT 32
> [...]
> int foo, bar;
>
> bar = 0xAAAA5555;
> foo = bar&  ~ ((int) 1<<  BITS_PER_UNIT);
> [...]
>
> Strictly speaking the shift operations should report a warning (left
> shift count>= width of type), since 1 is of type int (correct me if I'm
> wrong).
> Since this snippet is part of a very old implementation of g21k compiler
> for ADSP21xxx, I'm wondering why do we need to shift at all, isn't the
> above foo assignment equal to the following:
>
> foo = bar;

Others have mentioned the undefined behavior if int is 32 bits.

Let's assume that ints contain more than 33 bits to eliminate the undefined 
behavior.  I'll assume that ints are 40 bits.

The assignment to foo is equivalent to
  foo = bar & (int)0xfeffffffff;

when the programmer probably intended

  foo = bar & 0x00ffffffff;

To keep the lower BITS_PER_UNIT bits (at least one) I would have written

  foo = bar & (((1u << (BITS_PER_UNIT-1))-1)*2 +1);

That should work, assuming the implementation has at least BITS_PER_UNIT bits in 
an unsigned int.  If bar is an unsigned int, then replace 1u with 1ul.

The embedded compiler I work with optimizes out anding with all one bits, so it 
would have no run-time penalty.


Thad

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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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