Created
January 4, 2023 12:52
-
-
Save hnakamur/a65812da4e99257d187882438d3914ce to your computer and use it in GitHub Desktop.
Test ilog2
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| #include <math.h> | |
| #include <stdint.h> | |
| #include <stdio.h> | |
| #include <stdlib.h> | |
| #include <string.h> | |
| #include <gmp.h> | |
| #include <mpfr.h> | |
| int ilog2(uint64_t x) { return 63 - __builtin_clzll(x); } | |
| int ilog2b(uint64_t x) { | |
| double f = x; | |
| uint64_t v; | |
| memcpy(&v, &f, 8); | |
| return (v >> 52) - 1023; | |
| } | |
| int ilog2c(uint64_t x) { | |
| static const uint64_t debruijn_magic = 0x022fdd63cc95386dULL; | |
| static const uint64_t magic_table[] = { | |
| 0, 1, 2, 53, 3, 7, 54, 27, 4, 38, 41, 8, 34, 55, 48, 28, | |
| 62, 5, 39, 46, 44, 42, 22, 9, 24, 35, 59, 56, 49, 18, 29, 11, | |
| 63, 52, 6, 26, 37, 40, 33, 47, 61, 45, 43, 21, 23, 58, 17, 10, | |
| 51, 25, 36, 32, 60, 20, 57, 16, 50, 31, 19, 15, 30, 14, 13, 12, | |
| }; | |
| x |= (x >> 1); | |
| x |= (x >> 2); | |
| x |= (x >> 4); | |
| x |= (x >> 8); | |
| x |= (x >> 16); | |
| x |= (x >> 32); | |
| return (magic_table[((x & ~(x >> 1)) * debruijn_magic) >> 58]); | |
| } | |
| int ilog2d(uint64_t x) { | |
| union { | |
| double d; | |
| uint32_t u[2]; | |
| } ff; | |
| ff.d = x | 1; | |
| return (ff.u[1] >> 20) - 1023; | |
| } | |
| int ilog2_double(uint64_t x) { return (int)(log2(x)); } | |
| int ilog2_ref(uint64_t x) { | |
| mpfr_t hx, hy; | |
| int y; | |
| mpfr_init2(hx, 20); | |
| mpfr_init2(hy, 20); | |
| mpfr_set_ui(hx, (unsigned long int)x, MPFR_RNDD); | |
| mpfr_log2(hy, hx, MPFR_RNDD); | |
| y = (int)mpfr_get_si(hy, MPFR_RNDD); | |
| mpfr_clear(hx); | |
| mpfr_clear(hy); | |
| mpfr_free_cache(); | |
| return y; | |
| } | |
| void check(uint64_t x, int (*f)(uint64_t), int (*ref)(uint64_t)) { | |
| int got, want; | |
| got = f(x); | |
| want = ref(x); | |
| if (got != want) { | |
| printf("x=%lx, got=%d, want=%d, (uint64_t)(double)x=%lx\n", x, got, want, | |
| (uint64_t)(double)x); | |
| } | |
| } | |
| void check_fn(int (*f)(uint64_t)) { | |
| for (int x = 1; x < 100000; x++) { | |
| check(x, f, ilog2_ref); | |
| } | |
| for (int i = 1; i < 64; i++) { | |
| uint64_t x = (uint64_t)1 << i; | |
| check(x - 1, f, ilog2_ref); | |
| check(x, f, ilog2_ref); | |
| check(x + 1, f, ilog2_ref); | |
| } | |
| check(0xffffffffffffffff, f, ilog2_ref); | |
| } | |
| int main() { | |
| printf("ilog2 -----------\n"); | |
| check_fn(ilog2); | |
| printf("ilog2b -----------\n"); | |
| check_fn(ilog2b); | |
| printf("ilog2c -----------\n"); | |
| check_fn(ilog2c); | |
| printf("ilog2d -----------\n"); | |
| check_fn(ilog2d); | |
| printf("ilog2_double -----------\n"); | |
| check_fn(ilog2_double); | |
| } |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
$ gcc -o ilog2_test ilog2_test.c -lmpfr -lgmp -lm
$ ./ilog2_test
ilog2 -----------
ilog2b -----------
x=3fffffffffffff, got=54, want=53, (uint64_t)(double)x=40000000000000
x=7fffffffffffff, got=55, want=54, (uint64_t)(double)x=80000000000000
x=ffffffffffffff, got=56, want=55, (uint64_t)(double)x=100000000000000
x=1ffffffffffffff, got=57, want=56, (uint64_t)(double)x=200000000000000
x=3ffffffffffffff, got=58, want=57, (uint64_t)(double)x=400000000000000
x=7ffffffffffffff, got=59, want=58, (uint64_t)(double)x=800000000000000
x=fffffffffffffff, got=60, want=59, (uint64_t)(double)x=1000000000000000
x=1fffffffffffffff, got=61, want=60, (uint64_t)(double)x=2000000000000000
x=3fffffffffffffff, got=62, want=61, (uint64_t)(double)x=4000000000000000
x=7fffffffffffffff, got=63, want=62, (uint64_t)(double)x=8000000000000000
x=ffffffffffffffff, got=64, want=63, (uint64_t)(double)x=0
ilog2c -----------
ilog2d -----------
x=3fffffffffffff, got=54, want=53, (uint64_t)(double)x=40000000000000
x=7fffffffffffff, got=55, want=54, (uint64_t)(double)x=80000000000000
x=ffffffffffffff, got=56, want=55, (uint64_t)(double)x=100000000000000
x=1ffffffffffffff, got=57, want=56, (uint64_t)(double)x=200000000000000
x=3ffffffffffffff, got=58, want=57, (uint64_t)(double)x=400000000000000
x=7ffffffffffffff, got=59, want=58, (uint64_t)(double)x=800000000000000
x=fffffffffffffff, got=60, want=59, (uint64_t)(double)x=1000000000000000
x=1fffffffffffffff, got=61, want=60, (uint64_t)(double)x=2000000000000000
x=3fffffffffffffff, got=62, want=61, (uint64_t)(double)x=4000000000000000
x=7fffffffffffffff, got=63, want=62, (uint64_t)(double)x=8000000000000000
x=ffffffffffffffff, got=64, want=63, (uint64_t)(double)x=0
ilog2_double -----------
x=1ffffffffffff, got=49, want=48, (uint64_t)(double)x=1ffffffffffff
x=3ffffffffffff, got=50, want=49, (uint64_t)(double)x=3ffffffffffff
x=7ffffffffffff, got=51, want=50, (uint64_t)(double)x=7ffffffffffff
x=fffffffffffff, got=52, want=51, (uint64_t)(double)x=fffffffffffff
x=1fffffffffffff, got=53, want=52, (uint64_t)(double)x=1fffffffffffff
x=3fffffffffffff, got=54, want=53, (uint64_t)(double)x=40000000000000
x=7fffffffffffff, got=55, want=54, (uint64_t)(double)x=80000000000000
x=ffffffffffffff, got=56, want=55, (uint64_t)(double)x=100000000000000
x=1ffffffffffffff, got=57, want=56, (uint64_t)(double)x=200000000000000
x=3ffffffffffffff, got=58, want=57, (uint64_t)(double)x=400000000000000
x=7ffffffffffffff, got=59, want=58, (uint64_t)(double)x=800000000000000
x=fffffffffffffff, got=60, want=59, (uint64_t)(double)x=1000000000000000
x=1fffffffffffffff, got=61, want=60, (uint64_t)(double)x=2000000000000000
x=3fffffffffffffff, got=62, want=61, (uint64_t)(double)x=4000000000000000
x=7fffffffffffffff, got=63, want=62, (uint64_t)(double)x=8000000000000000
x=ffffffffffffffff, got=64, want=63, (uint64_t)(double)x=0