RE: [PATCH 2/2] powerpc/32: Implement csum_sub

[David Laight <David.Laight@ACULAB.COM>]

From: Christophe Leroy
> Sent: 11 February 2022 10:25
> 
> When building kernel with CONFIG_CC_OPTIMISE_FOR_SIZE, several
> copies of csum_sub() are generated, with the following code:
> 
> 	00000170 <csum_sub>:
> 	     170:	7c 84 20 f8 	not     r4,r4
> 	     174:	7c 63 20 14 	addc    r3,r3,r4
> 	     178:	7c 63 01 94 	addze   r3,r3
> 	     17c:	4e 80 00 20 	blr
> 
> Let's define a PPC32 version with subc/addme, and for it's inlining.
> 
> It will return 0 instead of 0xffffffff when subtracting 0x80000000 to itself,
> this is not an issue as 0 and ~0 are equivalent, refer to RFC 1624.

They are not always equivalent.
In particular in the UDP checksum field one of them is (0?) 'checksum not calculated'.

I think all the Linux functions have to return a non-zero value (for non-zero input).

If the csum is going to be converted to 16 bit, inverted, and put into a packet
the code usually has to have a check that changes 0 to 0xffff.
However if the csum functions guarantee never to return zero they can feed
an extra 1 into the first csum_partial() then just invert and add 1 at the end.
Because (~csum_partion(buffer, 1) + 1) is the same as ~csum_partial(buffer, 0)
except when the buffer's csum is 0xffffffff.

I did do some experiments and the 64bit value can be reduced directly to
16bits using '% 0xffff'.
This is different because it returns 0 not 0xffff.
However gcc 'randomly' picks between the fast 'multiply by reciprocal'
and slow divide instruction paths.
The former is (probably) faster than reducing using shifts and adc.
The latter definitely slower.

	David

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