linux-kernel - [PATCH v2 2/2] iov_iter: optimise iter type checking

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Date:   Thu, 19 Nov 2020 23:24:39 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] iov_iter: optimise iter type checking

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 bvecs and discards, but
iterate_all_kinds() still checks those iter types and generates actually
unreachable code including iter init, for(), etc.

size lib/iov_iter.o
before:
   text    data     bss     dec     hex filename
  24409     805       0   25214    627e lib/iov_iter.o

after:
   text    data     bss     dec     hex filename
  23785     793       0   24578    6002 lib/iov_iter.o

Most of it is ifs that are executed but never true.

Reviewed-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@...il.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