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From:   David Howells <dhowells@redhat.com>
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To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     dhowells@redhat.com, Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Christian Brauner <christian@brauner.io>,
        Matthew Wilcox <willy@infradead.org>, jlayton@kernel.org,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] iov_iter: Convert iterate*() to inline funcs
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Date:   Fri, 11 Aug 2023 18:07:07 +0100
Message-ID: <3888331.1691773627@warthog.procyon.org.uk>
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Linus Torvalds <torvalds@linux-foundation.org> wrote:

> I like this generally, the code generation deprovement worries me a
> bit, but from a quick look on a test-branch it didn't really look all
> that bad (but the changes are too big to usefully show up as asm
> diffs)

Hmmm...  It seems that using if-if-if rather than switch() gets optimised
better in terms of .text space.  The attached change makes things a bit
smaller (by 69 bytes).

David
---
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 8943ac25e202..f61474ec1c89 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -190,22 +190,18 @@ size_t iterate_and_advance(struct iov_iter *iter, si=
ze_t len, void *priv,
 	if (unlikely(!len))
 		return 0;
 =

-	switch (iov_iter_type(iter)) {
-	case ITER_UBUF:
+	if (likely(iter_is_ubuf(iter)))
 		return iterate_ubuf(iter, len, priv, NULL, ustep);
-	case ITER_IOVEC:
+	if (likely(iter_is_iovec(iter)))
 		return iterate_iovec(iter, len, priv, NULL, ustep);
-	case ITER_KVEC:
+	if (iov_iter_is_kvec(iter))
 		return iterate_kvec(iter, len, priv, NULL, step);
-	case ITER_BVEC:
+	if (iov_iter_is_bvec(iter))
 		return iterate_bvec(iter, len, priv, NULL, step);
-	case ITER_XARRAY:
+	if (iov_iter_is_xarray(iter))
 		return iterate_xarray(iter, len, priv, NULL, step);
-	case ITER_DISCARD:
-		iter->count -=3D len;
-		return len;
-	}
-	return 0;
+	iter->count -=3D len;
+	return len;
 }
 =

 static __always_inline
@@ -217,22 +213,18 @@ size_t iterate_and_advance_csum(struct iov_iter *ite=
r, size_t len, void *priv,
 	if (unlikely(!len))
 		return 0;
 =

-	switch (iov_iter_type(iter)) {
-	case ITER_UBUF:
+	if (likely(iter_is_ubuf(iter)))
 		return iterate_ubuf(iter, len, priv, csum, ustep);
-	case ITER_IOVEC:
+	if (likely(iter_is_iovec(iter)))
 		return iterate_iovec(iter, len, priv, csum, ustep);
-	case ITER_KVEC:
+	if (iov_iter_is_kvec(iter))
 		return iterate_kvec(iter, len, priv, csum, step);
-	case ITER_BVEC:
+	if (iov_iter_is_bvec(iter))
 		return iterate_bvec(iter, len, priv, csum, step);
-	case ITER_XARRAY:
+	if (iov_iter_is_xarray(iter))
 		return iterate_xarray(iter, len, priv, csum, step);
-	case ITER_DISCARD:
-		iter->count -=3D len;
-		return len;
-	}
-	return 0;
+	iter->count -=3D len;
+	return len;
 }
 =

 static size_t copy_to_user_iter(void __user *iter_to, size_t progress,