Re: [PATCH bpf-next v8 2/8] selftests/bpf: Add basic libarena scaffolding

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Thu Apr 23, 2026 at 1:24 AM PDT, Matt Bobrowski wrote:
>> +
>> +int arena_fls(__u64 word)
>> +{
>> +	unsigned int num = 0;
>> +
>> +	if (!word)
>> +		return 0;
>> +
>> +	if (word & 0xffffffff00000000ULL) {
>> +		num += 32;
>> +		word >>= 32;
>> +	}
>> +
>> +	if (word & 0xffff0000) {
>> +		num += 16;
>> +		word >>= 16;
>> +	}
>> +
>> +	if (word & 0xff00) {
>> +		num += 8;
>> +		word >>= 8;
>> +	}
>> +
>> +	if (word & 0xf0) {
>> +		num += 4;
>> +		word >>= 4;
>> +	}
>> +
>> +	if (word & 0xc) {
>> +		num += 2;
>> +		word >>= 2;
>> +	}
>> +
>> +	if (word & 0x2)
>> +		num += 1;
>> +
>> +	return num + 1;
>> +}
>
> What's your reason for not using something like __builtin_clzll()
> instead here, for example:
>
> int arena_fls(u64 word)
> {
> 	if (!word)
> 		return 0;
>
> 	return 64 - __builtin_clzll(word);
> }

Great suggestion. If it works, let's adopt __builtin_*