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 <jens-guile-...@plustv.dk> 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
