On Tue, 2018-04-10 at 06:34:35 UTC, Christophe Leroy wrote: > The current implementation of from64to32() gives a poor result: > > 0000000000000270 <.from64to32>: > 270: 38 00 ff ff li r0,-1 > 274: 78 69 00 22 rldicl r9,r3,32,32 > 278: 78 00 00 20 clrldi r0,r0,32 > 27c: 7c 60 00 38 and r0,r3,r0 > 280: 7c 09 02 14 add r0,r9,r0 > 284: 78 09 00 22 rldicl r9,r0,32,32 > 288: 7c 00 4a 14 add r0,r0,r9 > 28c: 78 03 00 20 clrldi r3,r0,32 > 290: 4e 80 00 20 blr > > This patch modifies from64to32() to operate in the same > spirit as csum_fold() > > It swaps the two 32-bit halves of sum then it adds it with the > unswapped sum. If there is a carry from adding the two 32-bit halves, > it will carry from the lower half into the upper half, giving us the > correct sum in the upper half. > > The resulting code is: > > 0000000000000260 <.from64to32>: > 260: 78 60 00 02 rotldi r0,r3,32 > 264: 7c 60 1a 14 add r3,r0,r3 > 268: 78 63 00 22 rldicl r3,r3,32,32 > 26c: 4e 80 00 20 blr > > Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr>
Applied to powerpc next, thanks. https://git.kernel.org/powerpc/c/55a0edf083022e402042255a0afb03 cheers
