Re: [PATCH 02/17] bitops: Add generic parity calculation for u64

On February 24, 2025 5:34:31 AM PST, David Laight [off-list ref] wrote:

On Mon, 24 Feb 2025 08:09:43 +0100
Jiri Slaby [off-list ref] wrote:

quoted

On 23. 02. 25, 17:42, Kuan-Wei Chiu wrote:

quoted

Several parts of the kernel open-code parity calculations using
different methods. Add a generic parity64() helper implemented with the
same efficient approach as parity8().

Co-developed-by: Yu-Chun Lin <redacted>
Signed-off-by: Yu-Chun Lin <redacted>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
  include/linux/bitops.h | 22 ++++++++++++++++++++++
  1 file changed, 22 insertions(+)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index fb13dedad7aa..67677057f5e2 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h

@@ -281,6 +281,28 @@ static inline int parity32(u32 val)
  	return (0x6996 >> (val & 0xf)) & 1;
  }
  
+/**
+ * parity64 - get the parity of an u64 value
+ * @value: the value to be examined
+ *
+ * Determine the parity of the u64 argument.
+ *
+ * Returns:
+ * 0 for even parity, 1 for odd parity
+ */
+static inline int parity64(u64 val)
+{
+	/*
+	 * One explanation of this algorithm:
+	 * https://funloop.org/codex/problem/parity/README.html
+	 */
+	val ^= val >> 32;  

Do we need all these implementations? Can't we simply use parity64() for 
any 8, 16 and 32-bit values too? I.e. have one parity().

I'm not sure you can guarantee that the compiler will optimise away
the unnecessary operations.

But:
static inline int parity64(u64 val)
{
return parity32(val ^ (val >> 32))
}

should be ok.
It will also work on x86-32 where parity32() can just check the parity flag.
Although you are unlikely to manage to use the the PF the xor sets.

David

quoted

quoted

+	val ^= val >> 16;
+	val ^= val >> 8;
+	val ^= val >> 4;
+	return (0x6996 >> (val & 0xf)) & 1;
+}
+
  /**
   * __ffs64 - find first set bit in a 64 bit word
   * @word: The 64 bit word  

Sure you can; you do need an 8- and a 16-bit arch implementation though (the 16 bit one being xor %rh,%rl)