[Libguestfs] [PATCH nbdkit 3/5] common/include: Add next_power_of_2 function

It takes a 64 bit integer and finds the next power of 2, eg. next_power_of_2 (510) => 512 (2^9) Taken from https://jameshfisher.com/2018/03/30/round-up-power-2/ with some fixes. --- common/include/ispowerof2.h | 9 +++++++++ common/include/test-ispowerof2.c | 14 ++++++++++++++ 2 files changed, 23 insertions(+) diff --git a/common/include/ispowerof2.h b/common/include/ispowerof2.h index f067caf07..57ebcc4fd 100644 --- a/common/include/ispowerof2.h +++ b/common/include/ispowerof2.h @@ -59,4 +59,13 @@ log_2_bits (unsigned long v) return SIZEOF_LONG*8 - __builtin_clzl (v) - 1; } +/* Round up to next power of 2. + * https://jameshfisher.com/2018/03/30/round-up-power-2/ + */ +static inline uint64_t +next_power_of_2 (uint64_t x) +{ + return x == 1 ? 1 : UINT64_C(1) << (64 - __builtin_clzll (x-1)); +} + #endif /* NBDKIT_ISPOWEROF2_H */ diff --git a/common/include/test-ispowerof2.c b/common/include/test-ispowerof2.c index 9620192f0..fe37c4a32 100644 --- a/common/include/test-ispowerof2.c +++ b/common/include/test-ispowerof2.c @@ -68,5 +68,19 @@ main (void) assert (log_2_bits (0x8000000000000000) == 63); #endif + /* Test next power of 2. */ + assert (next_power_of_2 (1) == 1); + assert (next_power_of_2 (3) == 4); + assert (next_power_of_2 (8) == 8); + assert (next_power_of_2 (9) == 16); + assert (next_power_of_2 (0xffff) == 0x10000); + assert (next_power_of_2 (0x10000) == 0x10000); + assert (next_power_of_2 (UINT64_C ( 0xffffffff)) == 0x100000000); + assert (next_power_of_2 (UINT64_C (0x100000000)) == 0x100000000); + assert (next_power_of_2 (UINT64_C (0x200000001)) == 0x400000000); + assert (next_power_of_2 (UINT64_C (0x6ffffffff)) == 0x800000000); + assert (next_power_of_2 (UINT64_C (0x700000001)) == 0x800000000); + assert (next_power_of_2 (UINT64_C (0x800000000)) == 0x800000000); + exit (EXIT_SUCCESS); } -- 2.39.2