I have a piece of code that runs at ~1.2 million runs per second after performing some tasks, the bulkiest is setting a uint8_t array with bitshifted data from two uint32_t pieces of data. The excerpt code is as follows:
static inline uint32_t RotateRight(uint32_t val, int n)
{
return (val >> n) + (val << (32 - n));
}
static inline uint32_t CSUInt32BE(const uint8_t *b)
{
return ((uint32_t)b[0] << 24) | ((uint32_t)b[1] << 16) | ((uint32_t)b[2] << 8) | (uint32_t)b[3];
}
static uint32_t ReverseBits(uint32_t val) // Usually just static, tried inline/static inline
{
// uint32_t res = 0;
// for (int i = 0; i<32; i++)
// {
// res <<= 1;
// res |= val & 1;
// val >>= 1;
// }
// Original code above, benched ~220k l/s
//val = ((val & 0x55555555) << 1) | ((val >> 1) & 0x55555555);
//val = ((val & 0x33333333) << 2) | ((val >> 2) & 0x33333333);
//val = ((val & 0x0F0F0F0F) << 4) | ((val >> 4) & 0x0F0F0F0F);
//val = ((val & 0x00FF00FF) << 8) | ((val >> 8) & 0x00FF00FF);
//val = (val << 16) | (val >> 16);
// Option 0, benched ~770k on MBP
uint32_t c = 0;
c = (BitReverseTable256[val & 0xff] << 24) |
(BitReverseTable256[(val >> 8) & 0xff] << 16) |
(BitReverseTable256[(val >> 16) & 0xff] << 8) |
(BitReverseTable256[val >> 24]); // was (val >> 24) & 0xff
// Option 1, benched ~970k l/s on MBP, Current, minor tweak to 24
//unsigned char * p = (unsigned char *)&val;
//unsigned char * q = (unsigned char *)&c;
//q[3] = BitReverseTable256[p[0]];
//q[2] = BitReverseTable256[p[1]];
//q[1] = BitReverseTable256[p[2]];
//q[0] = BitReverseTable256[p[3]];
// Option 2 at ~970k l/s on MBP from http://stackoverflow.com/questions/746171/best-algorithm-for-bit-reversal-from-msb-lsb-to-lsb-msb-in-c
return c; // Current
// return val; // option 0
// return res; // original
//uint32_t m;
//val = (val >> 16) | (val << 16); // swap halfwords
//m = 0x00ff00ff; val = ((val >> 8) & m) | ((val << 8) & ~m); // swap bytes
//m = m^(m << 4); val = ((val >> 4) & m) | ((val << 4) & ~m); // swap nibbles
//m = m^(m << 2); val = ((val >> 2) & m) | ((val << 2) & ~m);
//m = m^(m << 1); val = ((val >> 1) & m) | ((val << 1) & ~m);
//return val;
// Benches at 850k l/s on MBP
//uint32_t t;
//val = (val << 15) | (val >> 17);
//t = (val ^ (val >> 10)) & 0x003f801f;
//val = (t + (t << 10)) ^ val;
//t = (val ^ (val >> 4)) & 0x0e038421;
//val = (t + (t << 4)) ^ val;
//t = (val ^ (val >> 2)) & 0x22488842;
//val = (t + (t << 2)) ^ val;
//return val;
// Benches at 820k l/s on MBP
}
static void StuffItDESCrypt(uint8_t data[8], StuffItDESKeySchedule *ks, BOOL enc)
{
uint32_t left = ReverseBits(CSUInt32BE(&data[0]));
uint32_t right = ReverseBits(CSUInt32BE(&data[4]));
right = RotateRight(right, 29);
left = RotateRight(left, 29);
//Encryption function runs here
left = RotateRight(left, 3);
right = RotateRight(right, 3);
uint32_t left1 = ReverseBits(left);
uint32_t right1 = ReverseBits(right);
data[0] = right1 >> 24;
data[1] = (right1 >> 16) & 0xff;
data[2] = (right1 >> 8) & 0xff;
data[3] = right1 & 0xff;
data[4] = left1 >> 24;
data[5] = (left1 >> 16) & 0xff;
data[6] = (left1 >> 8) & 0xff;
data[7] = left1 & 0xff;
Is this the most optimal way to accomplish this? I have a uint64_t version as well:
uint64_t both = ((uint64_t)ReverseBits(left) << 32) | (uint64_t)ReverseBits(right);
data[0] = (both >> 24 & 0xff);
data[1] = (both >> 16) & 0xff;
data[2] = (both >> 8) & 0xff;
data[3] = both & 0xff;
data[4] = (both >> 56);
data[5] = (both >> 48) & 0xff;
data[6] = (both >> 40) & 0xff;
data[7] = (both >> 32) & 0xff;
I tested what would happen if I completely skipped this assignment (the ReverseBits function is still done), and the code runs at ~6.5 million runs per second. In addition, this speed hit happens if I only do just one as well, leveling out at 1.2 million even without touching the other 7 assignments.
I'd hate to think that this operation takes a massive 80% speed hit due to this work and can't be made any faster.
This is on Windows Visual Studio 2015 (though I try to keep the source as portable to macOS and Linux as possible).
Edit: The full base code is at Github. I am not the original author of the code, however I have forked it and maintain a password recovery solution using a modified for speed version. You can see my speed up successes in ReverseBits with various solutions and benched speeds.
These files are 20+ years old, and has successfully recovered files albeit at a low speed for years. See blog post.
You're certainly doing more work than you need to do. Note how function ReverseBits() goes to some effort to put the bytes of the reversed word in the correct order, and how the next thing that happens -- the part to which you are attributing the slowdown -- is to reorder those same bytes.
You could write and use a modified version of ReverseBits() that puts the bytes of the reversed representation directly into the correct places in the array, instead of packing them into integers just to unpack them again. That ought to be at least a bit faster, as you would be strictly removing operations.