From 4cc67b048459bebb7a60b693044ec83fb853eba1 Mon Sep 17 00:00:00 2001 From: "Maciej W. Rozycki" Date: Sun, 11 Jan 2026 21:21:57 +0000 Subject: linux/log2.h: reduce instruction count for is_power_of_2() Follow an observation that (n ^ (n - 1)) will only ever retain the most significant bit set in the word operated on if that is the only bit set in the first place, and use it to determine whether a number is a whole power of 2, avoiding the need for an explicit check for nonzero. This reduces the sequence produced to 3 instructions only across Alpha, MIPS, and RISC-V targets, down from 4, 5, and 4 respectively, removing a branch in the two latter cases. And it's 5 instructions on POWER and x86-64 vs 8 and 9 respectively. There are no branches now emitted here for targets that have a suitable conditional set operation, although an inline expansion will often end with one, depending on what code a call to this function is used in. Credit goes to GCC authors for coming up with this optimisation used as the fallback for (__builtin_popcountl(n) == 1), equivalent to this code, for targets where the hardware population count operation is considered expensive. Link: https://lkml.kernel.org/r/alpine.DEB.2.21.2601111836250.30566@angie.orcam.me.uk Signed-off-by: Maciej W. Rozycki Cc: Jens Axboe Cc: John Garry Cc: "Martin K. Petersen" Cc: Su Hui Signed-off-by: Andrew Morton --- include/linux/log2.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/log2.h b/include/linux/log2.h index 2eac3fc9303d..e17ceb32e0c9 100644 --- a/include/linux/log2.h +++ b/include/linux/log2.h @@ -44,7 +44,7 @@ int __ilog2_u64(u64 n) static __always_inline __attribute__((const)) bool is_power_of_2(unsigned long n) { - return (n != 0 && ((n & (n - 1)) == 0)); + return n - 1 < (n ^ (n - 1)); } /** -- cgit v1.2.3