c++ - How to let GCC compiler turn variable-division into mul(if faster) -

int a, b; scanf("%d %d", &a, &b); printf("%d\n", (unsigned int)a/(unsigned char)b); 

when compiling, got ...

    ::00401c1e::  c70424 24304000          mov dword ptr [esp],403024  %d %d     ::00401c25::  e8 36ffffff              call 00401b60               scanf     ::00401c2a::  0fb64c24 1c              movzx ecx,byte ptr [esp+1c]     ::00401c2f::  8b4424 18                mov eax,[esp+18]                             ::00401c33::  31d2                     xor edx,edx                                  ::00401c35::  f7f1                     div ecx                                      ::00401c37::  894424 04                mov [esp+4],eax                              ::00401c3b::  c70424 2a304000          mov dword ptr [esp],40302a  %d\x0a     ::00401c42::  e8 21ffffff              call 00401b68               printf 

will faster if div turn mul , use array store mulvalue? if so, how let compiler optimization?

int main() {     uint a, s=0, i, t;     scanf("%d", &a);     diviuint aa = a;     t = clock();     (i=0; i<1000000000; i++)         s += i/a;     printf("result:%10u\n", s);     printf("time:%12u\n", clock()-t);     return 0; } 

where diviuint(a) make memory of 1/a , use multiple instead using s+=i/aa makes speed 2 times of s+=i/a

replacing div mul may make sense (but doesn't have in cases) when 1 of values known @ compile time. when both user inputs, don't know what's range, usual tricks not work.

basically need handle both a , b between int_max , int_min. there's no space left scaling them up/down. if wanted extend them larger types, take longer time invert b , check result consistent.