c/10040: Incorrect warning message for bit fields

[peterson at austin dot ibm dot com]

>Number:         10040
>Category:       c
>Synopsis:       Incorrect warning message for bit fields
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 12 16:26:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     James Peterson, IBM Austin Research Lab
>Release:        3.2.2
>Organization:
>Environment:
RedHat Linux 7.1 on AMD Athlon
>Description:
warning: comparison is always 0 due to width of bit-field

message is generated because a 1 bit field is tested against
one:

    int _v: 1;

    if ((*tlb)._hi._v == 1)

This message comes from around line 3132 in fold-const.c
where a comment says "the constant shifted right by the 
field length should be all zero", and then proceeds to 
create trees for (1 >> (lbitsize-1)) and see if the result
is zero (or all ones).  But lbitsize is 1, so lbitsize-1
is zero and this quantity is then (1 >> 0) which is still
1, and hence non-zero (but is all one's to the size of the
bit field, so maybe the problem is integer_all_onesp).

This code is followed by explicit code dealing with 
"single-bit compares".

In trying to explain the problem, I'm beginning to see
that this error message is a result of the bit field 
being an "int" (signed) rather than "unsigned int". 
In which case, maybe the problem is best addressed by
a different warning message -- "bit field too small for
value and sign bit; consider unsigned type for field"

>How-To-Repeat:

>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: