Refactor the CRC-32 code to enhance its usability. Move the actual

CRC logic to a new function: crc32_raw() that obtains the initial
CRC value as well as leaves any post-processing to the caller. As
such, it can be used when the initial CRC value is not ~0U or when
the final CRC value does need to be inverted (bitwise). It also
means that crc32_raw() can be called repeatedly when the data is
not available as a single block, such as for scatter/gather lists
and the likes.

Avoid the additional call overhead incured by the refactoring by
moving the implementation off crc32() to sys/systm.h and making it
inlinable. Since crc32_raw() is itself trivial and since it may
be used in loops that iterate over fragments, having it available
for inlining can be beneficial. Hence, move its implementation
to sys/systm.h as well.

Keep the original implementation of crc32() in libkern/crc32.c for
documentation purposes (as a comment of course).

Triggered by: Jose M Rodriguez (josemi at freebsd dot jazztel dot es)
Discussed on: current@
Tested on: amd64, ia64 (BVO having GPT partitions)
Jargon file candidate: BVO = By Virtue Of :-)

1 files changed, 18 insertions, 15 deletions

diff --git a/sys/libkern/crc32.c b/sys/libkern/crc32.c
index a6a162b..4c937d2 100644
--- a/sys/libkern/crc32.c
+++ b/sys/libkern/crc32.c
@@ -94,18 +94,21 @@ uint32_t crc32_tab[] = {
 	0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
 };
 
-uint32_t
-crc32(const void *buf, size_t size)
-{
-	const uint8_t *p;
-	uint32_t crc;
-
-	p = buf;
-	crc = ~0U;
-
-	while (size--)
-		crc = crc32_tab[(crc ^ *p++) & 0xFF] ^ (crc >> 8);
-
-	return crc ^ ~0U;
-}
-
+/*
+ * A function that calculates the CRC-32 based on the table above is
+ * given below for documentation purposes. An equivalent implementation
+ * of this function that's actually used in the kernel can be found
+ * in sys/systm.h, where it can be inlined.
+ *
+ *	uint32_t
+ *	crc32(const void *buf, size_t size)
+ *	{
+ *		const uint8_t *p = buf;
+ *		uint32_t crc;
+ *
+ *		crc = ~0U;
+ *		while (size--)
+ *			crc = crc32_tab[(crc ^ *p++) & 0xFF] ^ (crc >> 8);
+ *		return crc ^ ~0U;
+ *	}
+ */