• Eric Dumazet's avatar
    [TG3]: Avoid an expensive divide. · 6b31a515
    Eric Dumazet authored
    During an oprofile session of linux-2.6.20 on a dual opteron system, I noticed
    an expensive divide was done in tg3_poll().
    
    I am using gcc-4.1.1, so the following comment from drivers/net/tg3.c seems
    over-optimistic :
    
    /* Do not place this n-ring entries value into the tp struct itself,
      * we really want to expose these constants to GCC so that modulo et
      * al.  operations are done with shifts and masks instead of with
      * hw multiply/modulo instructions.  Another solution would be to
      * replace things like '% foo' with '& (foo - 1)'.
      */
    #define TG3_RX_RCB_RING_SIZE(tp)        \
             ((tp->tg3_flags2 & TG3_FLG2_5705_PLUS) ?  512 : 1024)
    
    Assembly code before patch :
    (oprofile results included)
       6434  0.0088 :ffffffff803684b9:       mov    0x6f0(%r15),%eax
        587 8.0e-04 :ffffffff803684c0:       and    $0x40000,%eax
       2170  0.0030 :ffffffff803684c5:       cmp    $0x1,%eax
                    :ffffffff803684c8:       lea    0x1(%r13),%eax
                    :ffffffff803684cc:       sbb    %ecx,%ecx
       2051  0.0028 :ffffffff803684ce:       xor    %edx,%edx
                    :ffffffff803684d0:       and    $0x200,%ecx
         20 2.7e-05 :ffffffff803684d6:       add    $0x200,%ecx
       1986  0.0027 :ffffffff803684dc:       div    %ecx
    103427  0.1410 :ffffffff803684de:       cmp    %edx,0xffffffffffffff7c(%rbp)
    
    Assembly code after the suggested patch :
    
    ffffffff803684b9:           mov    0x6f0(%r15),%eax
    ffffffff803684c0:           and    $0x40000,%eax
    ffffffff803684c5:           cmp    $0x1,%eax
    ffffffff803684c8:           sbb    %eax,%eax
    ffffffff803684ca:           inc    %r13d
    ffffffff803684cd:           and    $0x200,%eax
    ffffffff803684d2:           add    $0x1ff,%eax
    ffffffff803684d7:           and    %eax,%r13d
    ffffffff803684da:           cmp    %r13d,0xffffffffffffff7c(%rbp)
    Signed-off-by: default avatarEric Dumazet <dada1@cosmosbay.com>
    Acked-by: default avatarMichael Chan <mchan@broadcom.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    6b31a515
tg3.c 346 KB