Re: [dpdk-dev] [dpdk-dev, v3] Implement memcmp using Intel SIMD instrinsics.

[Ravi Kerur <rkerur@gmail.com>]

On Wed, Jan 27, 2016 at 7:08 PM, Zhihong Wang <zhihong.wang@intel.com>
wrote:

> > diff --git a/lib/librte_eal/common/include/arch/x86/rte_memcmp.h b/lib
> > /librte_eal/common/include/arch/x86/rte_memcmp.h
>
> [...]
>
> > +#ifdef __cplusplus
> > +extern "C" {
> > +#endif
> > +
> > +/**
> > + * Compare bytes between two locations. The locations must not overlap.
> > + *
>
> Parameter names should be kept consistent as they are in function body.
>
> > + * @param src_1
> > + *   Pointer to the first source of the data.
> > + * @param src_2
> > + *   Pointer to the second source of the data.
> > + * @param n
> > + *   Number of bytes to compare.
> > + * @return
> > + *   zero if src_1 equal src_2
> > + *   -ve if src_1 less than src_2
> > + *   +ve if src_1 greater than src_2
> > + */
> > +static inline int
> > +rte_memcmp(const void *src_1, const void *src,
> > +             size_t n) __attribute__((always_inline));
> > +
> > +/**
> > + * Find the first different bit for comparison.
> > + */
> > +static inline int
> > +rte_cmpffd (uint32_t x, uint32_t y)
> > +{
> > +     int i;
> > +     int pos = x ^ y;
> > +     for (i = 0; i < 32; i++)
> > +             if (pos & (1<<i))
>
> Coding style check :-)
> BTW, does the bsf instruction provide this check?
>
> > +                     return i;
> > +     return -1;
> > +}
> > +
>
> [...]
>
> > +/**
> > + * Compare 48 bytes between two locations.
> > + * Locations should not overlap.
> > + */
> > +static inline int
> > +rte_cmp48(const void *src_1, const void *src_2)
>
> Guess this is not used.
>

I had left _unused_ with the assumption that it might be needed when actual
performance tests are done on high end servers.

>
> [...]
>
> > +/**
> > + * Compare 256 bytes between two locations.
> > + * Locations should not overlap.
> > + */
> > +static inline int
> > +rte_cmp256(const void *src_1, const void *src_2)
> > +{
> > +     int ret;
> > +
> > +     ret = rte_cmp64((const uint8_t *)src_1 + 0 * 64,
> > +                     (const uint8_t *)src_2 + 0 * 64);
>
> Why not just use rte_cmp128?
>
>
> [...]
>
> > +static inline int
> > +rte_memcmp(const void *_src_1, const void *_src_2, size_t n)
> > +{
> > +     const uint8_t *src_1 = (const uint8_t *)_src_1;
> > +     const uint8_t *src_2 = (const uint8_t *)_src_2;
> > +     int ret = 0;
> > +
> > +     if (n < 16)
> > +             return rte_memcmp_regular(src_1, src_2, n);
> > +
> > +     if (n <= 32) {
> > +             ret = rte_cmp16(src_1, src_2);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             return rte_cmp16(src_1 - 16 + n, src_2 - 16 + n);
> > +     }
> > +
>
> Too many conditions here may harm the overall performance.
> It's a trade-off thing, all about balancing the overhead.
> Just make sure this is tuned based on actual test numbers.
>
>
> > +     if (n <= 48) {
> > +             ret = rte_cmp32(src_1, src_2);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             return rte_cmp16(src_1 - 16 + n, src_2 - 16 + n);
> > +     }
> > +
> > +     if (n <= 64) {
> > +             ret = rte_cmp32(src_1, src_2);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             ret = rte_cmp16(src_1 + 32, src_2 + 32);
> > +
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             return rte_cmp16(src_1 - 16 + n, src_2 - 16 + n);
> > +     }
> > +
> > +     if (n <= 96) {
> > +             ret = rte_cmp64(src_1, src_2);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             ret = rte_cmp16(src_1 + 64, src_2 + 64);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             return rte_cmp16(src_1 - 16 + n, src_2 - 16 + n);
> > +     }
> > +
> > +     if (n <= 128) {
> > +             ret = rte_cmp64(src_1, src_2);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             ret = rte_cmp32(src_1 + 64, src_2 + 64);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             ret = rte_cmp16(src_1 + 96, src_2 + 96);
> > +             if (unlikely(ret != 0))
> > +                     return ret;
> > +
> > +             return rte_cmp16(src_1 - 16 + n, src_2 - 16 + n);
> > +     }
>
> [...]
>
> > +/**
> > + * Compare 48 bytes between two locations.
> > + * Locations should not overlap.
> > + */
> > +static inline int
> > +rte_cmp48(const void *src_1, const void *src_2)
>
> Not used.
>
> > +{
> > +     int ret;
> > +
> > +     ret = rte_cmp16((const uint8_t *)src_1 + 0 * 16,
> > +                     (const uint8_t *)src_2 + 0 * 16);
> > +
> > +     if (unlikely(ret != 0))
> > +             return ret;
> > +
> > +     ret = rte_cmp16((const uint8_t *)src_1 + 1 * 16,
> > +                     (const uint8_t *)src_2 + 1 * 16);
> > +
> > +     if (unlikely(ret != 0))
> > +             return ret;
> > +
> > +     return rte_cmp16((const uint8_t *)src_1 + 2 * 16,
> > +                     (const uint8_t *)src_2 + 2 * 16);
> > +}
> > +
> > +/**
> > + * Compare 64 bytes between two locations.
> > + * Locations should not overlap.
> > + */
> > +static inline int
> > +rte_cmp64(const void *src_1, const void *src_2)
> > +{
> > +     int ret;
> > +
> > +     ret = rte_cmp16((const uint8_t *)src_1 + 0 * 16,
> > +                     (const uint8_t *)src_2 + 0 * 16);
>
> Why not rte_cmp32? And use rte_cmp64 for rte_cmp128, and so on.
> That should make the code looks clearer.
>
>
> It'd be great if you could format this patch into a patch set with several
> little ones. :-)
> Also, the kernel checkpatch is very helpful.
> Good coding style and patch organization make it easy for in-depth reviews.
>
>
Combination of scalar and vector (32/64/128) was done to get optimal
performance numbers. If there is enough interest in this I can work on it
and provide an updated patch set.

Thanks,
Ravi