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    Subject[PATCH 08/13] crc32: Optimize loop counter for x86
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    Date
    Add two changes that improve the performance of x86 systems
    1. replace main loop with incrementing counter
    this change improves the performance of the selftest
    by about 5-6% on Nehalem CPUs. The apparent
    reason is that the compiler can use the loop index
    to perform an indexed memory access. This is
    reported to make the performance of PowerPC CPUs
    to get worse.
    2. replace the rem_len loop with incrementing counter
    this change improves the performance of the selftest,
    which has more than the usual number of occurances,
    by about 1-2% on x86 CPUs. In actual work loads
    the length is most often a multiple of 4 bytes and
    this code does not get executed as often if at all.
    Again this change is reported to make the performance
    of PowerPC get worse.

    From: Bob Pearson <rpearson@systemfabricworks.com>
    Signed-off-by: Bob Pearson <rpearson@systemfabricworks.com>
    [djwong@us.ibm.com: Minor changelog tweaks]
    Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
    ---
    lib/crc32.c | 13 +++++++++++++
    1 files changed, 13 insertions(+), 0 deletions(-)


    diff --git a/lib/crc32.c b/lib/crc32.c
    index 826e163..4eac9c7 100644
    --- a/lib/crc32.c
    +++ b/lib/crc32.c
    @@ -66,6 +66,9 @@ crc32_body(u32 crc, unsigned char const *buf, size_t len, const u32 (*tab)[256])
    # endif
    const u32 *b;
    size_t rem_len;
    +# ifdef CONFIG_X86
    + size_t i;
    +# endif
    const u32 *t0=tab[0], *t1=tab[1], *t2=tab[2], *t3=tab[3];
    const u32 *t4 = tab[4], *t5 = tab[5], *t6 = tab[6], *t7 = tab[7];
    u32 q;
    @@ -86,7 +89,12 @@ crc32_body(u32 crc, unsigned char const *buf, size_t len, const u32 (*tab)[256])
    # endif

    b = (const u32 *)buf;
    +# ifdef CONFIG_X86
    + --b;
    + for (i = 0; i < len; i++) {
    +# else
    for (--b; len; --len) {
    +# endif
    q = crc ^ *++b; /* use pre increment for speed */
    # if CRC_LE_BITS == 32
    crc = DO_CRC4;
    @@ -100,9 +108,14 @@ crc32_body(u32 crc, unsigned char const *buf, size_t len, const u32 (*tab)[256])
    /* And the last few bytes */
    if (len) {
    u8 *p = (u8 *)(b + 1) - 1;
    +# ifdef CONFIG_X86
    + for (i = 0; i < len; i++)
    + DO_CRC(*++p); /* use pre increment for speed */
    +# else
    do {
    DO_CRC(*++p); /* use pre increment for speed */
    } while (--len);
    +# endif
    }
    return crc;
    #undef DO_CRC


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