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: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
6b31a515e3