What's the quickest way to compute log2 of an integer in C#?
Solution 1
The Cleanest and Quickest... (works in .Net core 3.1 and up and takes the performance lead starting in .Net 5.0)
int val = BitOperations.Log2(x);
Runner up... (fastest in versions below .Net 5, 2nd place in .Net 5)
int val = (int)((BitConverter.DoubleToInt64Bits(x) >> 52) + 1) & 0xFF;
Notes:
- The idea of using the exponent in a floating-point was inspired by SPWorley 3/22/2009.
- This also supports more than 32 bits. I have not tested the max but did go to at least 2^38.
- Use with caution on production code since this can possibly fail on architectures that are not little-endianness.
Here are some benchmarks: (code here: https://github.com/SunsetQuest/Fast-Integer-Log2)
1-2^32 32-Bit Zero
Function Time1 Time2 Time3 Time4 Time5 Errors Support Support
Log2_SunsetQuest3 18 18 79167 19 18 255 N N
Log2_SunsetQuest4 18 18 86976 19 18 0 Y N
LeadingZeroCountSunsetq - - - 30 20 0 Y Y
Log2_SPWorley 18 18 90976 32 18 4096 N Y
MostSigBit_spender 20 19 86083 89 87 0 Y Y
Log2_HarrySvensson 26 29 93592 34 31 0 Y Y
Log2_WiegleyJ 27 23 95347 38 32 0 Y N
Leading0Count_phuclv - - - 33 20 10M N N
Log2_SunsetQuest1 31 28 78385 39 30 0 Y Y
HighestBitUnrolled_Kaz 33 33 284112 35 38 2.5M N Y
Log2_Flynn1179 58 52 96381 48 53 0 Y Y
BitOperationsLog2Sunsetq - - - 49 15 0 Y Y
GetMsb_user3177100 58 53 100932 60 59 0 Y Y
Log2_Papayaved 125 60 119161 90 82 0 Y Y
FloorLog2_SN17 102 43 121708 97 92 0 Y Y
Log2_DanielSig 28 24 960357 102 105 2M N Y
FloorLog2_Matthew_Watso 29 25 94222 104 102 0 Y Y
Log2_SunsetQuest2 118 140 163483 184 159 0 Y Y
Msb_version 136 118 1631797 212 207 0 Y Y
Log2_SunsetQuest0 206 202 128695 212 205 0 Y Y
BitScanReverse2 228 240 1132340 215 199 2M N Y
Log2floor_version 89 101 2 x 10^7 263 186 0 Y Y
UsingStrings_version 2346 1494 2 x 10^7 2079 2122 0 Y Y
Zero_Support = Supports Zero if the result is 0 or less
Full-32-Bit = Supports full 32-bit (some just support 31 bits)
Time1 = benchmark for sizes up to 32-bit (same number tried for each size)
Time2 = benchmark for sizes up to 16-bit (for measuring perf on small numbers)
Time3 = time to run entire 1-2^32 in sequence using Parallel.For. Most results range will on the larger end like 30/31 log2 results. (note: because random was not used some compiler optimization might have been applied so this result might not be accurate)
Time4 = .Net Core 3.1
Time5 = .Net 5
Benchmark notes: AMD Ryzen CPU, Release mode, no-debugger attached, .net core 3.1
I really like the one created by spender in another post. This one does not have the potential architecture issue and it also supports Zero while maintaining almost the same performance as the float method from SPWorley.
Update 3/13/2020: Steve noticed that there were some errors in Log2_SunsetQuest3 that were missed.
Update 4/26/2020: Added new .Net Core 3's BitOperations.LeadingZeroCount() as pointed out by phuclv.
Solution 2
Slight improvement to Guffa's answer... Since the amount you are adding to the result is always a power of two using bit operations can produce slight improvement on some architectures. Also since our context is bit patterns it is slightly more readable to use hexadecimal. In this case it is useful to shift the arithmetic by a power of 2.
int bits = 0;
if (n > 0xffff) {
n >>= 16;
bits = 0x10;
}
if (n > 0xff) {
n >>= 8;
bits |= 0x8;
}
if (n > 0xf) {
n >>= 4;
bits |= 0x4;
}
if (n > 0x3) {
n >>= 2;
bits |= 0x2;
}
if (n > 0x1) {
bits |= 0x1;
}
Further a check for n==0 should be added since the above will yield a result of 0 and Log(0) is undefined (regardless of base).
In ARM assembly this algorithm produces very compact code as the branch after comparison can be eliminated with conditional instructions which avoids pipeline flushing. For Example:
if (n > 0xff) {
n >>= 8;
bits |= 0x8;
}
becomes (let R0 = n, R1 = bits)
CMP R0, $0xff
MOVHI R0, R0, LSR $8
ORRHI R1, R1, $0x8
Solution 3
You can simply count how many times you have to remove bits until the value is zero:
int bits = 0;
while (n > 0) {
bits++;
n >>= 1;
}
More efficient for large numbers, you can count groups of bits first:
int bits = 0;
while (n > 255) {
bits += 8;
n >>= 8;
}
while (n > 0) {
bits++;
n >>= 1;
}
Edit:
The most efficient method would be to use the binary steps that Flynn1179 suggested (upvoted for the inspiration :), but expanding the loop into hard coded checks. This is at least twice as fast as the method above, but also more code:
int bits = 0;
if (n > 32767) {
n >>= 16;
bits += 16;
}
if (n > 127) {
n >>= 8;
bits += 8;
}
if (n > 7) {
n >>= 4;
bits += 4;
}
if (n > 1) {
n >>= 2;
bits += 2;
}
if (n > 0) {
bits++;
}
Solution 4
Efficiency in terms of lines of code, or runtime execution speed?
Code's easy: Math.log(n, 2)
.
Runtime speed's a little trickier, but you can do it with a kind of 'binary search':
int bits = 1;
for (int b = 16; b >=1; b/=2)
{
int s = 1 << b;
if (n >= s) { n>>=b; bits+=b; }
}
I'm not 100% certain I've got the logic right there, but hopefully the idea's clear. There might be some overheads in the .NET VM, but in principle it should be faster.
The 16
in the for loop initialializer is based on half the number of bits needed for an int. If you're working with longs, start it at 32, etc.
Solution 5
In .NET Core 3.0 there are BitOperations.LeadingZeroCount() and BitOperations.Log2. They're mapped to the underlying hardware instructino like x86's LZCNT/BSR so that should be the most efficient solution
int bits = BitOperations.Log2(x); // or
int bits = x == 0 ? 1 : 32 - BitOperations.LeadingZeroCount(x);
Comments
-
izb about 2 years
How can I most efficiently count the number of bits required by an integer (log base 2) in C#? For example:
int bits = 1 + log2(100); => bits == 7
-
Filip Navara over 12 yearsgraphics.stanford.edu/~seander/bithacks.html ... choose your poison :)
-
-
Emond over 12 yearsHow about negative numbers? They would appear to be pretty cheap :)
-
Flynn1179 over 12 yearsNegatives always need max; the top bit's set.
-
Guffa over 12 years@Emo: Good point. This only works for positive numbers. To handle negative numbers just add a
if (n < 0) { bits = 32; } else { ... }
around the loop. -
Guffa over 12 yearsThat logic doesn't work, as the number that you can represent with a specific number of bits can't be added. 2^24 isn't 2^16 + 2^8 but 2^16 * 2^8.
-
Flynn1179 over 12 yearsWell, that's not entirely true, it's not adding 2^16+2^8, it's adding 16+8, which SHOULD work, but there's an error somewhere else I can't quite put my finger on how to fix at the moment; I'll edit when I do. The principle of a 'binary search' should work though, if you can get the logic at each step right.
-
Flynn1179 over 12 yearsFixed it; I had got the logic slightly wrong, and you need to start with 1 bit, not 0.
-
Guffa over 12 yearsthe code is not directly adding 2^16+2^8, but subtracting each from the number, in the
n-=s;
statement. The bitmask should be formed fromb-1
rather thanb
, and instead of subtractings
from the number, you would need to discard the leastb
bits of the number:int s = 1 << (b - 1); if (n >= s) { n >>=b; bits+=b; }
. -
Guffa over 12 yearsIf you start from 1, then the number 0 still needs one bit.
-
Guffa over 12 yearsAlthough this method has fewer iterations, that doesn't make up for the added logic. I checked the performance, and this method is faster than the first method that I suggested (but only for large numbers), and slower than the second method that I suggested. Anyhow, the execution time ranges from 30 to 80 nanoseconds, so you rarely need anything faster than the simplest possible method. :)
-
Flynn1179 over 12 yearsFair enough; I think for very large numbers it would probably be faster in pure ASM/C, but yeah, if the performance is still lightning fast with simpler code, go with the more maintainable option, definitely.
-
harold over 12 yearsASM? might as well use BSR then. C? _BitScanReverse or equivalent.
-
Zéychin over 12 yearsSure they do;
log(-5) = log(5)+i pi
, for example. -
Zéychin over 12 yearsThat's why complex data types were created, such as in Fortran, and easily implementable in the C-derived languages.
-
Guffa over 12 years@Zéychin: You can use an imaginary data type to describe how many imaginary bits you need to store the number, but you can't use it to actually store the number. So, calculating the imaginary bits is totally useless.
-
Flynn1179 about 10 yearsHeh, how long have I gone without noticing that overload! Updated the answer, thanks.
-
Suraj Jain almost 6 yearsIt wont work when we use
32
in initializer, due to this. -
Kind Contributor over 5 yearsThis seems to be slightly faster out of 10M iterations than (int)ceiling(log(value)/log(2)). (int)ceiling(Log(value,2)) is actually slightly slower than both methods, but Log(value,2) is much easier to remember and reuse across projects, the slight slow-down isn't a concern.
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izb over 4 yearsIs the line "a.asInt = 0;" redundant? What does that part do?
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SunsetQuest over 4 yearsWithout the "a.asInt = 0;", the compiler throws an error of an uninitiated value for some reason when it runs across the "a.asInt >> 23". Maybe the compiler does not take into account the "a.asFloat = val;" that also initialized it. Sorry for replying 7 years late; hopefully, this will help someone else that falls upon this fastest-integer-log2 question.
-
phuclv about 4 years
BitOperations.LeadingZeroCount()
should be faster if you have .NET Core 3.0+ -
SunsetQuest about 4 years@phuclv - I was for sure thinking the LEadingZeroCount would be faster but it's actually 50% slower than the top one.
-
SunsetQuest about 4 years"int bits = x == 0 ? 1 : 64 - BitOperations.LeadingZeroCount(x);" I think you would want "int bits = 31 - BitOperations.LeadingZeroCount(x);" maybe.
-
phuclv about 4 yearsshould be
int bits = 32 - BitOperations.LeadingZeroCount(x);
for 32-bit ints if you want it to return 1 for 1 and 7 for 100 -
phuclv about 4 years@Sunsetquest I guess it has something to do with AMD. PDEP & PEXT are very slow on AMD and probably the same to LZCNT/TZCNT?
-
SunsetQuest about 4 yearsAre you sure? Log2(1) is 0. But if you use "int bits = 32 - BitOperations.LeadingZeroCount(x);" and put in a 1 for x it comes out to 1.
-
phuclv about 4 years@Sunsetquest yes it's 0, therefore
1 + log2(1)
as the OP expected will result in 1 -
phuclv about 4 yearswow I didn't even noticed there's
BitOperations.Log2
. But can you tell me how to build your project? I'm trying it on my PC but it always compiles to BenchmarkLeading0Count.dll instead of an exe file -
SunsetQuest about 4 years@phuclv It should just open with the VS 2017/2019 IDE. I just tested it. VS will compile the console app to a dll but it also places an exe next to it that it runs. It will be interesting what you get on an intel chip.