modbus-data.c: fix for bswap_16 fallback

[dreamflow]

when compiling the newest libmodbus master branch with
GCC 6.3.0 (MinGW-w64 MSYS) on Win7 32bit via the steps:

autogen.sh
./configure
make

make warns:

modbus-data.c:54:4: warning: #warning "Fallback on C functions for bswap_16" [-Wcpp]
 #  warning "Fallback on C functions for bswap_16"

it can be resolved by adding in src/modbus-data.c directly under the line 42

// Since GCC >= 4.30, GCC provides __builtin_bswapXX() alternatives so we switch to them

these 2 new lines

#    undef bswap_16
#    define bswap_16 __builtin_bswap16

[stephane]

Thank you

[stephane]

I should have tested to avoid a blind push...

[dreamflow]

the TravisCI report in the revert commit bf30047 was very helpful to find the cause:
__builtin_bswap16 was in a later gcc version available than __builtin_bswap32 .
__builtin_bswap16 is available in gcc since 4.8.0 .

therefore a possible solution would be in src/modbus-data.c to change also line 41 to

# if GCC_VERSION > 480

all in all, if in src/modbus-data.c the lines 39-46 would change to

#if defined(__GNUC__)
#  define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__ * 10)
#  if GCC_VERSION > 480
// GCC > 4.8.0 provides __builtin_bswapXX() alternatives so we switch to them
#    undef bswap_16
#    define bswap_16 __builtin_bswap16
#    undef bswap_32
#    define bswap_32 __builtin_bswap32
#  endif
#endif

then the __builtin_bswap16 could compile without the "fallback" warning in
MinGW-w64 windows builds with a gcc compiler > 4.8.0 .

today i've already made a live test on real modbus-RS485-hardware,
where i've used the new libmodbus function modbus_get_float_badc
with the above suggested patch. this modbus_get_float_badc function
uses internaly gcc's __builtin_bswap16 via bswap_16 .
the result of the hardware-test is: it works fine.

[stephane]

What do you think of my patch (still use bswap32 for older releases and >=)?

[dreamflow]

your patch is very good. i've test-compiled the bswap_16-#377-branch
with the result: no compile-warnings/errors on Win7 32bit with
GCC 6.3.0 (MinGW-w64 MSYS).

[stephane]

Merged. Thank you.