On 09/09/2025 07:57, Ojaswin Mujoo wrote:
+
+/*
+ * Round down n to nearest power of 2.
+ * If n is already a power of 2, return n;
+ */
+static int rounddown_pow_of_2(int n) {
+ int i = 0;
+
+ if (is_power_of_2(n))
+ return n;
+
+ for (; (1 << i) < n; i++);
+
+ return 1 << (i - 1);
Is this the neatest way to do this?
Well it is a straigforward o(logn) way. Do you have something else in
mind?
check what the kernel does is always a good place to start...
Thanks
So kernel pretty much does same thing:
unsigned long __rounddown_pow_of_two(unsigned long n)
{
return 1UL << (fls_long(n) - 1);
}
where fls*() variants jump into asm for efficiency. asm is obviously not
needed here so we use the for loop to calculate the fls (find last bit
set):
for (; (1 << i) < n; i++);
Ideally the compiler should do the right thing and optimize it.
I think that some versions of fls use builtin clz also, but I am not
sure if that (clz) is always available for supported toolchains here.
Thanks,
John
