Revers order of bytes unsigned char array :?

[newcslover]

hi,

i need fastest method to revers order of bytes in my char array.

for example i have:

unsigned char buf[8];
// consider data stored in buf is 88 77 66 55 44 33 22 11
// how to reverse it to: 11 22 33 44 55 66 77 88 

// currently i can do it by equal assignment , i make another buf like:

unsigned char buf_ok[8];
 
        buf_ok[0]=buf[7];
	buf_ok[1]=buf[6];
	buf_ok[2]=buf[5];
	buf_ok[3]=buf[4];
	buf_ok[4]=buf[3];
	buf_ok[5]=buf[2];
	buf_ok[6]=buf[1];
	buf_ok[7]=buf[0];

// This does revers the bytes as i want but its very slow , i am looking for fast method ..

[newcslover]

hi,

anyone please know better and fast method ?

[Peter87]

Slow? You compiled with optimizations enabled when you came to this conclusion?

[newcslover]

hi,

sorry i am new to c++ dont know how to compile with optimization i use ubuntu ssh cli to compile .. g++ myprogram.c -o program

[Peter87]

pass -O2 (or -O3) to the compiler and see if there is a difference.

g++ -O2 myprogram.c -o program

[newcslover]

thanks i test and find execution time ... i am running a for loop that generate bytes , using this above assignment i am able to generate 30,000 8 bytes of keys / second i want to increase speed ...

[newcslover]

WOOOW compiling with -O3 result now : KEYS/SECOND: 93657

without -O3 30,000 and as program runs more time the speed droping ..


-O3 did the job thanks Peter87 :)

[newcslover]

removing printf from for loop speed rised upto : KEYS/SECOND: 131120

Thank you brother for suggesting -O option :)

i will try using threads and multiple process to speed up more :) thanks again ..

[newcslover]

-Ofast more faster then -O3 :D

[Peter87]

I don't know how you are testing this but note that if the you don't use the result of the code you are testing the compiler might optimize away too much. For example, in the code you posted, if the buf_ok array is never used later in the code the compiler could simply ignore lines 7-16.

[newcslover]

The results are by using buf_ok array and assignments as mentioned in 1st post.

i wanted alternate way to reverse the bytes order , in program i think i have big endian and littel endian formate problem with revers the input array i am researching on it , finding different ways to speed up my program ...

currently the program read my array all functions works correctly , it dont ignor any line ..

Thank you peter :)