Re: [PATCH v3 2/2] btrfs: use ilog2() to replace if () branches for btrfs_bg_flags_to_raid_index()

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2022/4/20 16:25, Johannes Thumshirn wrote:
> On 20/04/2022 10:09, Qu Wenruo wrote:
>> In function btrfs_bg_flags_to_raid_index(), we use quite some if () to
>> convert the BTRFS_BLOCK_GROUP_* bits to a index number.
>>
>> But the truth is, there is really no such need for so many branches at
>> all.
>> Since all BTRFS_BLOCK_GROUP_* flags are just one single bit set inside
>> BTRFS_BLOCK_GROUP_PROFILES_MASK, we can easily use ilog2() to calculate
>> their values.
>>
>> This calculation has an anchor point, the lowest PROFILE bit, which is
>> RAID0.
>>
>> Even it's fixed on-disk format and should never change, here I added
>> extra compile time checks to make it super safe:
>>
>> 1. Make sure RAID0 is always the lowest bit in PROFILE_MASK
>>     This is done by finding the first (least significant) bit set of
>>     RAID0 and PROFILE_MASK & ~RAID0.
>>
>> 2. Make sure RAID0 bit set beyond the highest bit of TYPE_MASK
>
> TBH I think this change obscures the code more than it improves it.
>
Right, that kinda makes sense.

Will update the patchset to remove that line if needed.

Thanks,
Qu