[Bug libc/15512] New: __bswap_constant_16 not compiled when -Werror -Wsign-conversion is given

["edjee at google dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15512

             Bug #: 15512
           Summary: __bswap_constant_16 not compiled when -Werror
                    -Wsign-conversion is given
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: unassigned@sourceware.org
        ReportedBy: edjee@google.com
                CC: drepper.fsp@gmail.com
    Classification: Unclassified


(Filling a bug, following Andreas' suggestion)

I'm using gcc and glibc on 32bit LE ARM (But all the other platforms that use
the architecture-independent bit/byteswap.h may have similar issue).

I found that the following code is not compiled, if I give -Werror
-Wsign-conversion arguments to gcc.


#include <stdio.h>
#include <byteswap.h>

int main(int argc, char *argv[]) {
  unsigned short int x = 0xbeef;
  printf("bswap_16(0x%x)=0x%x\n", x, bswap_16(x));
  return 0;
}


I saw compilation error "conversion to 'unsigned int' from 'int' may change the
sign of the result".

It looks like that a similar issue has been solved for x86 by
http://sourceware.org/git/?p=glibc.git;a=commit;h=69da074d7adfab7b57004a0dea9403a928e310a5
.
But the architecture-independent bit/byteswap.h has not been modified. In that
file, __bswap_constant_16 macro looks like this:

#define __bswap_constant_16(x) \
        ((unsigned short int)((((x) >> 8) & 0xffu) | (((x) & 0xffu) << 8)))


It seems that ((x) >> 8) promotes the data to signed value, even if x is
unsigned. So (((x) >> 8) && 0xffu) seems to fail when -Werror -Wsign-conversion
is given.

All the architecture-specific versions of byteswap.h uses 0xff but this file
uses 0xffu. Is this intended?

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