bug#24630: guile-2.0.12: Comparison is always true warning - may cause problems.

[Jens Bauer]

Hi Andy.

Thanks for looking into this.
Your reply is correct and I can see that I made an error when manually reducing 
the expression.

FIY: The warning shows up in GCC-4.2.1, not clang (I cannot run clang on any of 
my Macs, they're PowerPC based).
-So the warning does show up when buiding using GCC-4.2.1.

Love
Jens

On Wed, 01 Mar 2017 10:18:28 +0100, Andy Wingo wrote:
> Hi,
> 
> On Thu 06 Oct 2016 22:49, Jens Bauer <address@hidden> writes:
> 
>> I get the following warnings, when building on Mac OS X.
>> (It should show up for all platforms, though):
>> 
>> In file included from 
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/numbers.c:9731:
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/conv-integer.i.c: 
>> In function 'scm_to_int8':
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/conv-integer.i.c:94: 
>> warning: comparison is always true due to limited range of data type
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/conv-integer.i.c:94: 
>> warning: comparison is always true due to limited range of data type
> 
> These are not really bugs.  I mean, we shouldn't produce warnings, but
> GCC doesn't warn on these, so clearly there is a heuristic which clang
> has set differently; but the actual code is fine.
> 
> In your investigations below there are some errors.  I include a couple
> of inline comments for your enjoyment.
> 
>> In file included from 
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/numbers.c:9747:
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/conv-integer.i.c: 
>> In function 'scm_to_int16':
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/conv-integer.i.c:94: 
>> warning: comparison is always true due to limited range of data type
>> /Users/jens/open-source/Source/guile-2.0.12/libguile/conv-integer.i.c:94: 
>> warning: comparison is always true due to limited range of data type
>> 
>> Notice that it's only from line 94, which reads...
>>        if (n >= TYPE_MIN && n <= TYPE_MAX)
>> 
>> ... looking at the top of the file, it says: "It is only for signed 
>> types", so I look in ...
>> 
>>   numbers.c:9731
>>   numbers.c:9747
>> ... which is int8 and int16 (signed integers); this should be as intended.
>> 
>> The variable 'n' is declared as scm_t_signed_bits, which is a 
>> scm_t_intptr, which is an intptr_t, which is just a 'long'.
>> 
>> So my guess is that the problem must be with TYPE_MIN and TYPE_MAX.
>> 
>> In numbers.c, line 9742, they're defined as follows:
>> #define TYPE                     scm_t_int16
>> #define TYPE_MIN                 SCM_T_INT16_MIN
>> #define TYPE_MAX                 SCM_T_INT16_MAX
>> 
>> ... looks good to me, but where's the definition of SCM_T_INT16_MIN 
>> and SCM_T_INT16_MAX ?
>> -It seems to be in __scm.h:
>> 
>> #define SCM_I_UTYPE_MAX(type)      ((type)-1)
>> #define SCM_I_TYPE_MAX(type,umax)  ((type)((umax)/2))
>> #define SCM_I_TYPE_MIN(type,umax)  (-((type)((umax)/2))-1)
>> 
>> #define SCM_T_UINT8_MAX   SCM_I_UTYPE_MAX(scm_t_uint8)
>> #define SCM_T_INT8_MIN    SCM_I_TYPE_MIN(scm_t_int8,SCM_T_UINT8_MAX)
>> #define SCM_T_INT8_MAX    SCM_I_TYPE_MAX(scm_t_int8,SCM_T_UINT8_MAX)
>> 
>> #define SCM_T_UINT16_MAX  SCM_I_UTYPE_MAX(scm_t_uint16)
>> #define SCM_T_INT16_MIN   SCM_I_TYPE_MIN(scm_t_int16,SCM_T_UINT16_MAX)
>> #define SCM_T_INT16_MAX   SCM_I_TYPE_MAX(scm_t_int16,SCM_T_UINT16_MAX)
>> 
>> Now, this is where things get interesting. The macros are cool, but 
>> I think the use seems to be incorrect.
>> 
>> Let's try an example (SCM_T_INT16_MIN):
>> SCM_T_INT16_MIN = SCM_I_TYPE_MIN(scm_t_int16,SCM_T_UINT16_MAX)
>> Expands to ...
>> SCM_T_INT16_MIN = (-((scm_t_int16)((-1)/2))-1)
> 
> SCM_T_UINT16_MAX expands to ((scm_t_uint16)-1) which expands to the
> uint16_t value 0xffff.  (These intermediate expansions have type in
> addition to value.)  SCM_T_INT16_MIN is -(0xffff/2)-1, which is
> (int16_t)-0x8000.
> 
>> ... which can be cleaned up ...
>> 
>> SCM_T_INT16_MIN = (-(((-1)/2))-1)
>> 
>> A signed integer of value -1 divided by 2, is the same as 
>> bitshifting to the right by using ASR; the result will be -1.
>> 
>> SCM_T_INT16_MIN = (-(((-1)))-1)
>> SCM_T_INT16_MIN = (-((-1))-1)
>> SCM_T_INT16_MIN = (-(-1)-1)
>> SCM_T_INT16_MIN = (+1-1)
>> SCM_T_INT16_MIN = (0)
>> 
>> ... Ehm ... Did I do something wrong ?
>> I expected the value -32768, but got 0.
>> 
>> Wouldn't it be correct to typecast as scm_t_uint16 instead of 
>> scm_t_int16 (and thus scm_t_uint8 instead of scm_t_int8) ?
>> 
> 
> Happy hacking,
> 
> Andy