Le 23/10/2015 05:33, Scott Wood a écrit :

On Tue, 2015-09-22 at 16:34 +0200, Christophe Leroy wrote:

quoted

Simplify csum_add(a, b) in case a or b is constant 0

Signed-off-by: Christophe Leroy <redacted>
---
  arch/powerpc/include/asm/checksum.h | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/arch/powerpc/include/asm/checksum.h

b/arch/powerpc/include/asm/checksum.h
index 56deea8..f8a9704 100644

--- a/arch/powerpc/include/asm/checksum.h
+++ b/arch/powerpc/include/asm/checksum.h

@@ -119,7 +119,13 @@ static inline __wsum csum_add(__wsum csum, __wsum

addend)
  {
  #ifdef __powerpc64__
       u64 res = (__force u64)csum;
+#endif
+     if (__builtin_constant_p(csum) && csum == 0)
+             return addend;
+     if (__builtin_constant_p(addend) && addend == 0)
+             return csum;

+#ifdef __powerpc64__
       res += (__force u64)addend;
       return (__force __wsum)((u32)res + (res >> 32));
  #else

How often does this happen?

In the following patch (9/9), csum_add() is used to implement 
csum_partial() for small blocks.
In several places in the networking code, csum_partial() is called with 
0 as initial sum.

Christophe