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A lot of code is duplicated in unicodeobject.c to manipulate ("encode/decode") surrogates. Each function has from one to three different implementations. The new decode_ucs4() function adds a new implementation. Attached patch replaces this code by macros.
I think that only the implementations of IS_HIGH_SURROGATE and IS_LOW_SURROGATE are important for speed. ((ch & 0xFFFFFC00UL) == 0xD800) (from decode_ucs4) is *a little bit* faster than (0xD800 <= ch && ch <= 0xDBFF) on my CPU (Atom Z520 @ 1.3 GHz): running test_unicode 4 times takes ~54 sec instead of ~57 sec (-3%).
These 3 macros have to be checked, I wrote the first one:
#define IS_SURROGATE(ch) (((ch) & 0xFFFFF800UL) == 0xD800)
#define IS_HIGH_SURROGATE(ch) (((ch) & 0xFFFFFC00UL) == 0xD800)
#define IS_LOW_SURROGATE(ch) (((ch) & 0xFFFFFC00UL) == 0xDC00)
I added cast to Py_UCS4 in COMBINE_SURROGATES to avoid integer overflow if Py_UNICODE is 16 bits (narrow build). It's maybe useless.
#define COMBINE_SURROGATES(ch1, ch2) \
 (((((Py_UCS4)(ch1) & 0x3FF) << 10) | ((Py_UCS4)(ch2) & 0x3FF)) + 0x10000)
HIGH_SURROGATE and LOW_SURROGATE require that their ordinal argument has been preproceed to fit in [0; 0xFFFF]. I added this requirement in the comment of these macros. It would be better to have only one macro to do the two operations, but because "*p++" (dereference and increment) is usually used, I prefer to avoid one unique macro (I don't like passing *p++ in a macro using its argument more than once).
Or we may add a third macro using HIGH_SURROGATE and LOW_SURROGATE.
I rewrote the main loop of PyUnicode_EncodeUTF16() to avoid an useless test on ch2 on narrow build.
I also added a IS_NONBMP macro just because I prefer macro over hardcoded constants.