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SubjectRe: [PATCH] Revert "powerpc/64: Fix checksum folding in csum_add()"
On Tue, Apr 10, 2018 at 08:34:37AM +0200, Christophe Leroy wrote:
> This reverts commit 6ad966d7303b70165228dba1ee8da1a05c10eefe.
>
> That commit was pointless, because csum_add() sums two 32 bits
> values, so the sum is 0x1fffffffe at the maximum.
> And then when adding upper part (1) and lower part (0xfffffffe),
> the result is 0xffffffff which doesn't carry.
> Any lower value will not carry either.
>
> And behind the fact that this commit is useless, it also kills the
> whole purpose of having an arch specific inline csum_add()
> because the resulting code gets even worse than what is obtained
> with the generic implementation of csum_add()
>
> 0000000000000240 <.csum_add>:
> 240: 38 00 ff ff li r0,-1
> 244: 7c 84 1a 14 add r4,r4,r3
> 248: 78 00 00 20 clrldi r0,r0,32
> 24c: 78 89 00 22 rldicl r9,r4,32,32
> 250: 7c 80 00 38 and r0,r4,r0
> 254: 7c 09 02 14 add r0,r9,r0
> 258: 78 09 00 22 rldicl r9,r0,32,32
> 25c: 7c 00 4a 14 add r0,r0,r9
> 260: 78 03 00 20 clrldi r3,r0,32
> 264: 4e 80 00 20 blr
>
> In comparison, the generic implementation of csum_add() gives:
>
> 0000000000000290 <.csum_add>:
> 290: 7c 63 22 14 add r3,r3,r4
> 294: 7f 83 20 40 cmplw cr7,r3,r4
> 298: 7c 10 10 26 mfocrf r0,1
> 29c: 54 00 ef fe rlwinm r0,r0,29,31,31
> 2a0: 7c 60 1a 14 add r3,r0,r3
> 2a4: 78 63 00 20 clrldi r3,r3,32
> 2a8: 4e 80 00 20 blr
>
> And the reverted implementation for PPC64 gives:
>
> 0000000000000240 <.csum_add>:
> 240: 7c 84 1a 14 add r4,r4,r3
> 244: 78 80 00 22 rldicl r0,r4,32,32
> 248: 7c 80 22 14 add r4,r0,r4
> 24c: 78 83 00 20 clrldi r3,r4,32
> 250: 4e 80 00 20 blr
>
> Fixes: 6ad966d7303b7 ("powerpc/64: Fix checksum folding in csum_add()")
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

Seems I was right first time... :)

Acked-by: Paul Mackerras <paulus@ozlabs.org>

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