问题
I've been trying for days to install GMP library under MINGW. I have been using for weeks __uint128_t with gcc under a linux64 bit environment, then ported the same program under GMP and mingw (32 bit version). I used mpz_class integers instead of __uint128_t. Then I started my new program and...! With __uint128_t and 64 bit it takes 16 minutes to complete, with GMP and MINGW it takes 91 HOURS!!!
What should I do to speed up things a bit? Is there any faster way to do 128 bit integer math under a 32 bit environment? I don't need more than 128 bit, so is there any way to tell GMP "ok, I just need 128 bits, keep presicion fixed but please GO FASTER"?
回答1:
No, when you are using an mpz_t, you can't limit GMP to fixed-len integers. mpz_t is a struct with lenght of limbs array (allocated; used) and a pointer to actual value which is stored as array of ints (limbs; array of int32 or int64). GMP is ready to expand length of any value when it will grow to big.
You can allocate 128 bits for every mpz_t at init, using mpz_init2:
mpz_init2(mpz_t*, bit_number);
But the speedup from this is small, there is still data-indirection and length-handling.
You can use limbs directly with switching to mpn_ low-level functions:
http://www.gnu.org/software/gmp/manual/html_node/Low-level-Functions.html#Low-level%20Functions
There will be no pointer to limbs (this is good for cache), no easy input/output code; and no automatic limbs size handling (nor auto-expand; nor allocation). You should do all storage by yourself; may be even some carry must be handled manually, but there will be GMP's fast *, / and % operations; you can reconstruct mpz_t for easy input/output with mpz_t t;t._mp_size = t._mp_alloc=limb_number;t._mp_d=pointer_to_limb_array.
Also, you just can use uint128_t if you will switch to 64-bit mingw.
回答2:
You can use MinGW-w64 instead, if the Windows machines you're targeting are new enough to ship with 64-bit Windows (e.g. Vista or 7).
来源：https://stackoverflow.com/questions/7264009/fixed-size-integers-with-gmp