On 27.10.21 г. 8:28, 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.

Only one fixed offset is needed to make the index sequential (the
lowest profile bit starts at ilog2(1 << 3) while we have 0 reserved for
SINGLE).

Even with that calculation involved (one if(), one ilog2(), one minus),
it should still be way faster than the if () branches, and now it is
definitely small enough to be inlined.

Is this used in a performance critical path, are there any numbers which
prove that it's indeed faster?