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

[Jens Bauer]

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
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)
... 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) ?


Love
Jens