Le 25/03/2015 02:30, Scott Wood a écrit :

On Tue, Feb 03, 2015 at 12:39:27PM +0100, LEROY Christophe wrote:

The C version of csum_add() as defined in include/net/checksum.h gives the
following assembly:
       0:       7c 04 1a 14     add     r0,r4,r3
       4:       7c 64 00 10     subfc   r3,r4,r0
       8:       7c 63 19 10     subfe   r3,r3,r3
       c:       7c 63 00 50     subf    r3,r3,r0

include/net/checksum.h also offers the possibility to define an arch specific
function.
This patch provides a ppc32 specific csum_add() inline function.

What makes it 32-bit specific?

As far as I understand, the 64-bit will do a 64 bit addition, so we will have to handle differently the carry, can't just be an addze like in 32-bit.

The generated code is most likely different on ppc64. I have no ppc64 compiler so I can't check what gcc generates for the following code:

__wsum csum_add(__wsum csum, __wsum addend)
{
	u32 res = (__force u32)csum;
	res += (__force u32)addend;
	return (__force __wsum)(res + (res < (__force u32)addend));
}

Can someone with a ppc64 compiler tell what we get ?

Christophe

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