Answer by cipilo for What is the fastest/most efficient way to find the...
For a 4 bit group n, the value can be computed in 3 instructions using a 32bit unsigned value as a lookup table with 16 entries of two bits each:(0xffffaa50 >> (n << 1)) & 3This can be...
View ArticleAnswer by hochl for What is the fastest/most efficient way to find the...
I know I am massively late to the party, but I just came across this question and I wanted to share my template solution for getting the highest bit that is set in the argument as a value and not the...
View ArticleAnswer by ceorron for What is the fastest/most efficient way to find the...
Not too happy with the answers on here, I thought I'd give this one a go myself.Returns 0 if 0 is passed in or index of the bit + 1, for all other values. Handles signed and unsigned values. Is...
View ArticleAnswer by paperclip optimizer for What is the fastest/most efficient way to...
Since I seemingly have nothing else to do, I dedicated an inordinate amount of time to this problem during the weekend.Without direct hardware support, it SEEMED like it should be possible to do better...
View ArticleAnswer by Alex Guteniev for What is the fastest/most efficient way to find...
There's a proposal to add bit manipulation functions in C, specifically leading zeros is helpful to find highest bit set. See...
View ArticleAnswer by user15363887 for What is the fastest/most efficient way to find the...
This looks big but works really fast compared to loop thank from bluegsmithint Bit_Find_MSB_Fast(int x2){ long x = x2 & 0x0FFFFFFFFl; long num_even = x & 0xAAAAAAAA; long num_odds = x &...
View ArticleAnswer by Piotr Siupa for What is the fastest/most efficient way to find the...
Here is a fast solution for C that works in GCC and Clang; ready to be copied and pasted.#include <limits.h>unsigned int fls(const unsigned int value){ return (unsigned int)1 <<...
View ArticleAnswer by SpartanWar for What is the fastest/most efficient way to find the...
My humble method is very simple:MSB(x) = INT[Log(x) / Log(2)]Translation: The MSB of x is the integer value of (Log of Base x divided by the Log of Base 2).This can easily and quickly be adapted to any...
View ArticleAnswer by Antonin GAVREL for What is the fastest/most efficient way to find...
I assume your question is for an integer (called v below) and not an unsigned integer.int v = 612635685; // whatever value you wishunsigned int get_msb(int v){ int r = 31; // maximum number of...
View ArticleAnswer by Glenn Slayden for What is the fastest/most efficient way to find...
Another poster provided a lookup-table using a byte-wide lookup. In case you want to eke out a bit more performance (at the cost of 32K of memory instead of just 256 lookup entries) here is a solution...
View ArticleAnswer by Harry Svensson for What is the fastest/most efficient way to find...
Woaw, that was many answers. I am not sorry for answering on an old question.int result = 0;//could be a char or int8_t insteadif(value){//this assumes the value is 64bit...
View ArticleAnswer by Finnegan for What is the fastest/most efficient way to find the...
I know this question is very old, but just having implemented an msb() function myself,I found that most solutions presented here and on other websites are not necessarily the most efficient - at least...
View ArticleAnswer by VoidStar for What is the fastest/most efficient way to find the...
Some overly complex answers here. The Debruin technique should only be used when the input is already a power of two, otherwise there's a better way. For a power of 2 input, Debruin is the absolute...
View ArticleAnswer by ChuckCottrill for What is the fastest/most efficient way to find...
Note that what you are trying to do is calculate the integer log2 of an integer,#include <stdio.h>#include <stdlib.h>unsigned intLog2(unsigned long x){ unsigned long n = x; int bits =...
View ArticleAnswer by jemin for What is the fastest/most efficient way to find the...
The code: // x>=1; unsigned func(unsigned x) { double d = x ; int p= (*reinterpret_cast<long long*>(&d) >> 52) - 1023; printf( "The left-most non zero bit of %d is bit %d\n", x, p);...
View ArticleAnswer by user4924658 for What is the fastest/most efficient way to find the...
thats some kind of binary search, it works with all kinds of (unsigned!) integer types#include <climits>#define UINT (unsigned int)#define UINT_BIT (CHAR_BIT*sizeof(UINT))int msb(UINT x){ if(0 ==...
View ArticleAnswer by Jonathan Mee for What is the fastest/most efficient way to find the...
c99 has given us log2. This removes the need for all the special sauce log2 implementations you see on this page. You can use the standard's log2 implementation like this:const auto n = 13UL;const auto...
View ArticleAnswer by user3177100 for What is the fastest/most efficient way to find the...
A version in C using successive approximation:unsigned int getMsb(unsigned int n){ unsigned int msb = sizeof(n) * 4; unsigned int step = msb; while (step > 1) { step /=2; if (n>>msb) msb +=...
View ArticleAnswer by David C. Rankin for What is the fastest/most efficient way to find...
As the answers above point out, there are a number of ways to determine the most significant bit. However, as was also pointed out, the methods are likely to be unique to either 32bit or 64bit...
View ArticleAnswer by Marco Amagliani for What is the fastest/most efficient way to find...
What aboutint highest_bit(unsigned int a) { int count; std::frexp(a, &count); return count - 1;}?
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