Re: [PATCH] gdb/microblaze-tdep.c: Check whether less than zero in conditional expression

[Chen Gang <gang.chen.5i5j@gmail.com>]

On 07/24/2014 06:17 AM, Michael Eager wrote:
> On 07/23/14 15:06, Chen Gang wrote:
>> On 07/24/2014 04:04 AM, Michael Eager wrote:
>>> On 07/20/14 07:03, Chen Gang wrote:
>>>> Use typecast 'size_t' on 'reg', not only avoid the related warning, but
>>>> also check whether less than zero -- for 'reg' is type 'int', and sizeof
>>>> (dwarf2_to_reg_map) is less than 0x7fff.
>>>>
>>>> It is quoted in gdb_assert(), so need check 'reg' whether less than zero.
>>>> And the related warning (with '-W'):
>>>>
>>>>     ../../binutils-gdb/gdb/microblaze-tdep.c:667:3: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
>>>>
>>>> ChangeLog:
>>>>
>>>>    * microblaze-tdep.c (microblaze_dwarf2_reg_to_regnum): Check whether
>>>>    less tha zero in conditional expression.
>>>>
>>>>
>>>> Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
>>>> ---
>>>>    gdb/microblaze-tdep.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/gdb/microblaze-tdep.c b/gdb/microblaze-tdep.c
>>>> index 7e89241..9bec260 100644
>>>> --- a/gdb/microblaze-tdep.c
>>>> +++ b/gdb/microblaze-tdep.c
>>>> @@ -664,7 +664,7 @@ static int dwarf2_to_reg_map[78] =
>>>>    static int
>>>>    microblaze_dwarf2_reg_to_regnum (struct gdbarch *gdbarch, int reg)
>>>>    {
>>>> -  gdb_assert (reg < sizeof (dwarf2_to_reg_map));
>>>> +  gdb_assert ((size_t) reg < sizeof (dwarf2_to_reg_map));
>>>>      return dwarf2_to_reg_map[reg];
>>>>    }
>>>
>>> I don't see anything in the patch which does what you describe,
>>> checking whether reg is less than zero.  Converting a signed
>>> integer to an unsigned integer is not a way to check whether
>>> it is less than zero.  This is better:
>>>
>>> + gdb_assert (reg >= 0 && (size_t) reg < sizeof (dwarf2_to_reg_map));
>>>
>>
>> Yeah, it is common statement. It is also OK to me, although after type
>> cast, 'reg >=0' can be omited (it can let code simpler, but let code
>> not quit easy understanding).
> 
> No, if you want to verify that the value is greater than zero,
> this cannot be omitted.  A negative value would converted to
> a positive value by the cast.  There no reason to believe that
> this would cause the other half of the test to fail.
> 

When an 'int' negative value converted to a positive value, it will be
larger than 0x7fff which must be larget than 'sizeof (dwarf2_to_reg_map)'.

Thanks.
-- 
Chen Gang

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