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 06:20 AM, Chen Gang wrote:
> 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)'.
> 

If what I said is correct, your idea/suggestions is still OK to me: easy
understanding has higher priority than keeping source code simple.


Thanks.
-- 
Chen Gang

Open, share, and attitude like air, water, and life which God blessed