[Bug c/40404] New: Comparison involving unsigned int:17 bitfield seems wrong

["foo at mailinator dot com" <gcc-bugzilla@gcc.gnu.org>]

#include <stdio.h>
struct S {
    unsigned int ui17 : 17;
} s;
int main()
{
    s.ui17 = 0x1ffff;
    printf("%x\n", (unsigned int)s.ui17);
    if (s.ui17 >= 0xfffffffeu)
        puts("FAIL");
    return 0;
}

Maybe I don't understand GCC's rules for the promotion of bitfields, but it
seems to me that s.ui17 should evaluate to ((unsigned int)0x1ffff), which is
less than 0xfffffffeu, so "FAIL" should not be printed. But "FAIL" is printed.
I can't explain why; it's almost as if GCC is truncating the rhs to 0x1fffe
before doing the comparison!

Changing s.ui17 from 17 bits to 15 bits makes the test pass (i.e., not print
"FAIL"). I believe in that case GCC promotes s.ui17 to "unsigned short"... but
that should undergo usual arithmetic conversion to "unsigned int" at which
point we ought to see the same FAIL behavior happening.

Reproduced with GCC 4.2.4 and 4.3.3.


-- 
           Summary: Comparison involving unsigned int:17 bitfield seems
                    wrong
           Product: gcc
           Version: 4.3.3
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: foo at mailinator dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40404