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SubjectRe: [PATCH] lib/x86: Optimise csum_partial of buffers that are not multiples of 8 bytes.
On Mon, Dec 13, 2021 at 10:00 AM David Laight <David.Laight@aculab.com> wrote:
>
>
> Add in the trailing bytes first so that there is no need to worry
> about the sum exceeding 64 bits.
>
> Signed-off-by: David Laight <david.laight@aculab.com>
> ---
>
> This ought to be faster - because of all the removed 'adc $0'.
> Guessing how fast x86 code will run is hard!
> There are other ways of handing buffers that are shorter than 8 bytes,
> but I'd rather hope they don't happen in any hot paths.
>
> Note - I've not even compile tested it.
> (But have tested an equivalent change before.)
>
> arch/x86/lib/csum-partial_64.c | 55 ++++++++++++----------------------
> 1 file changed, 19 insertions(+), 36 deletions(-)
>
> diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
> index abf819dd8525..fbcc073fc2b5 100644
> --- a/arch/x86/lib/csum-partial_64.c
> +++ b/arch/x86/lib/csum-partial_64.c
> @@ -37,6 +37,24 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
> u64 temp64 = (__force u64)sum;
> unsigned result;
>
> + if (len & 7) {
> + if (unlikely(len < 8)) {
> + /* Avoid falling off the start of the buffer */
> + if (len & 4) {
> + temp64 += *(u32 *)buff;
> + buff += 4;
> + }
> + if (len & 2) {
> + temp64 += *(u16 *)buff;
> + buff += 2;
> + }
> + if (len & 1)
> + temp64 += *(u8 *)buff;
> + goto reduce_to32;
> + }
> + temp64 += *(u64 *)(buff + len - 8) << (8 - (len & 7)) * 8;

This is reading far away (end of buffer).

Maybe instead read the first bytes and adjust @buff, to allow for
better hardware prefetching ?



> + }
> +
> while (unlikely(len >= 64)) {
> asm("addq 0*8(%[src]),%[res]\n\t"
> "adcq 1*8(%[src]),%[res]\n\t"
> @@ -82,43 +100,8 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
> : "memory");
> buff += 8;
> }
> - if (len & 7) {
> -#ifdef CONFIG_DCACHE_WORD_ACCESS
> - unsigned int shift = (8 - (len & 7)) * 8;
> - unsigned long trail;
> -
> - trail = (load_unaligned_zeropad(buff) << shift) >> shift;
>
> - asm("addq %[trail],%[res]\n\t"
> - "adcq $0,%[res]"
> - : [res] "+r" (temp64)
> - : [trail] "r" (trail));
> -#else
> - if (len & 4) {
> - asm("addq %[val],%[res]\n\t"
> - "adcq $0,%[res]"
> - : [res] "+r" (temp64)
> - : [val] "r" ((u64)*(u32 *)buff)
> - : "memory");
> - buff += 4;
> - }
> - if (len & 2) {
> - asm("addq %[val],%[res]\n\t"
> - "adcq $0,%[res]"
> - : [res] "+r" (temp64)
> - : [val] "r" ((u64)*(u16 *)buff)
> - : "memory");
> - buff += 2;
> - }
> - if (len & 1) {
> - asm("addq %[val],%[res]\n\t"
> - "adcq $0,%[res]"
> - : [res] "+r" (temp64)
> - : [val] "r" ((u64)*(u8 *)buff)
> - : "memory");
> - }
> -#endif
> - }
> +reduce_to32:
> result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
> return (__force __wsum)result;
> }
> --
> 2.17.1
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

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