[Bug c/63420] New: GCC 4.8.2: Bitshift second operand range not as per manual.

[Bug c/63420] New: GCC 4.8.2: Bitshift second operand range not as per manual.

[aainscow at uk dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63420

            Bug ID: 63420
           Summary: GCC 4.8.2: Bitshift second operand range not as per
                    manual.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aainscow at uk dot ibm.com

Created attachment 33625
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33625&action=edit
Demo code

Most likely this is simply a problem with the GCC manual.

The GCC documentation states (30th Sept 2014):

-- 
You use the left-shift operator << to shift its first operand's bits to the
left. The second operand denotes the number of bit places to shift. Bits
shifted off the left side of the value are discarded; new bits added on the
right side will all be 0. 

...

For both << and >>, if the second operand is greater than the bit-width of the
first operand, or the second operand is negative, the behavior is undefined. 
--

NOTE: "greater than" not "greater than or equal to".

The following code should therefore not be undefined and return 0.

int main() {
    int a = sizeof(int)*8;
    return ((int)1) << a;
}

However:
aainscow@morwenna:~/cake$ gcc bitshift_bug.c 
aainscow@morwenna:~/cake$ ./a.out ; echo $?
1

Most likely this is a problem with documentation.

[Bug c/63420] GCC 4.8.2: Bitshift second operand range not as per manual.

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63420

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
What matters is what the C and C++ standards (differs between different
versions thereof) say, the documentation is certainly wrong, but even some
shifts where
shift count is smaller than bitsize can be invalid in some standards (for
signed types)).

[Bug c/63420] GCC 4.8.2: Bitshift second operand range not as per manual.

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63420

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Can you explain where in the documentation you find it though?
I can't find any wording like that.

[Bug c/63420] GCC 4.8.2: Bitshift second operand range not as per manual.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63420

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |mpolacek at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
That words come from http://www.gnu.org/software/gnu-c-manual/gnu-c-manual.html
That is not a part of GCC though, so closing.