[PATCH] iov_iter: optimise iter type checking

Pavel Begunkov
2020-11-21T14:41:18Z

The problem here is that iov_iter_is_*() helpers check types for
equality, but all iterate_* helpers do bitwise ands. This confuses
compilers, so even if some cases were handled separately with
iov_iter_is_*(), corresponding ifs in iterate*() right after are not
eliminated.

E.g. iov_iter_npages() first handles discards, but iterate_all_kinds()
still checks for discard iter type and generates unreachable code down
the line.

text data bss dec hex filename
before: 24409 805 0 25214 627e lib/iov_iter.o
after: 23977 805 0 24782 60ce lib/iov_iter.o

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
include/linux/uio.h | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 72d88566694e..c5970b2d3307 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -57,27 +57,27 @@ static inline enum iter_type iov_iter_type(const struct iov_iter *i)

static inline bool iter_is_iovec(const struct iov_iter *i)
{
- return iov_iter_type(i) == ITER_IOVEC;
+ return iov_iter_type(i) & ITER_IOVEC;
}

static inline bool iov_iter_is_kvec(const struct iov_iter *i)
{
- return iov_iter_type(i) == ITER_KVEC;
+ return iov_iter_type(i) & ITER_KVEC;
}

static inline bool iov_iter_is_bvec(const struct iov_iter *i)
{
- return iov_iter_type(i) == ITER_BVEC;
+ return iov_iter_type(i) & ITER_BVEC;
}

static inline bool iov_iter_is_pipe(const struct iov_iter *i)
{
- return iov_iter_type(i) == ITER_PIPE;
+ return iov_iter_type(i) & ITER_PIPE;
}

static inline bool iov_iter_is_discard(const struct iov_iter *i)
{
- return iov_iter_type(i) == ITER_DISCARD;
+ return iov_iter_type(i) & ITER_DISCARD;
}

static inline unsigned char iov_iter_rw(const struct iov_iter *i)
--
2.24.0