YOSHIFUJI Hideaki / 吉藤英明 a écrit :
In article <[EMAIL PROTECTED]> (at Fri, 21 Dec 2007 07:03:58 +0100), Eric Dumazet
<[EMAIL PROTECTED]> says:
Because tot_len is signed in tcp_v6_send_ack(), tot_len/4 forces compiler
to emit an integer divide, while we can help it to use a right shift,
less expensive.
Are you really sure?
At least, gcc-4.1.2-20061115 (debian) does not make any difference.
And, IMHO, because shift for signed variable is fragile, so we should
avoid using it.
Yes I am sure, but maybe you are on x86_64 ?
gcc-4.2.2 on x86
# objdump --disassemble net/ipv6/tcp_ipv6.o|grep -6 idiv
b2: 66 8b 42 02 mov 0x2(%edx),%ax
b6: ba 04 00 00 00 mov $0x4,%edx
bb: 89 d7 mov %edx,%edi
bd: 66 89 45 00 mov %ax,0x0(%ebp)
c1: 89 d8 mov %ebx,%eax
c3: 99 cltd
c4: f7 ff idiv %edi
c6: 88 c2 mov %al,%dl
c8: 8a 45 0c mov 0xc(%ebp),%al
cb: c1 e2 04 shl $0x4,%edx
ce: 83 e0 0f and $0xf,%eax
d1: 09 d0 or %edx,%eax
d3: 88 45 0c mov %al,0xc(%ebp)
If you think tot_len can be negative, I understand you can be against this
patch. But I am sure it's allways > 0, even if I am a total ipv6 newbie :)
Thank you
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