[erlang-questions] ei patch to handle NEW_FLOAT_EXT

[Serge Aleynikov]

Ever since the emulator support of the compact IEEE 754 double encoding 
in external binary format was introduced I've been patiently waiting for 
ei to support this feature.  Finally I gave up and implemented this 
myself as I recently needed to marshal lots of doubles between Erlang 
and C++ and size of the binary stream did matter.

Attached is a patch for R12B-2 that implements this feature.  The only 
thing I wasn't sure about was whether backward compatibility of 
ei_{encode,decode}_double() is truly required.  It exists in the 
emulator, where the new functionality is controlled by 
term_to_binary(Term, [{minor_version, 1}]), and is essential indeed, but 
is it really needed for ei?  In order to be properly backward 
compatible, it would likely require to introduce yet another pair of 
functions, such as:

     int ei_encode_double_new(char *buf, int *index, double p)
     int ei_x_encode_double_new(ei_x_buff* x, const void *p, long len)

which gets a little messy as there are already a fair amount of ei 
functions.

Since the internals of the external binary format are hidden behind the 
facade of:

     int ei_encode_double(char *buf, int *index, double p)
     int ei_decode_double(const char *buf, int *index, double *p)

Perhaps retaining the compatibility of lower-level implementation is not 
that crucial?  In the patch the old functionality is controlled by 
setting USE_OLD_FLOAT_ENCODER macro, but this is left discretionary to 
the OTP team.

Regards,

Serge
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