c++ - Why is (a % 256) different than (a & 0xFF)? -


i assumed when doing (a % 256) optimizer naturally use efficient bitwise operation, if wrote (a & 0xff).

when testing on compiler explorer gcc-6.2 (-o3):

// type code here, or load example. int mod(int num) {     return num % 256; }  mod(int):     mov     edx, edi     sar     edx, 31     shr     edx, 24     lea     eax, [rdi+rdx]     movzx   eax, al     sub     eax, edx     ret

and when trying other code:

// type code here, or load example. int mod(int num) {     return num & 0xff; }  mod(int):     movzx   eax, dil     ret

seems i'm missing out. ideas?

it's not same. try num = -79, , different results both operations. (-79) % 256 = -79, while (-79) & 0xff positive number.

using unsigned int, operations same, , code same.

ps- commented

they shouldn't same, a % b defined a - b * floor (a / b).

that's not how defined in c, c++, objective-c (ie languages code in question compile).


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