Re: [PATCH v2] tcp: input header length, prediction, and timestampbugs

From: William Allen Simpson
Date: Tue Jan 19 2010 - 12:35:15 EST

Over the weekend, I've been reviewing the .lst output that Andi
explained, and I've found a few interesting things.

1) The 4.4 compiler isn't very smart about shifts:

static inline unsigned int tcp_header_len_th(const struct tcphdr *th)
return th->doff * 4;
c04ea93e: 0f b6 47 0c movzbl 0xc(%edi),%eax
c04ea942: c0 e8 04 shr $0x4,%al
c04ea945: 0f b6 c0 movzbl %al,%eax
c04ea948: c1 e0 02 shl $0x2,%eax

That could easily be done as shr $0x2 instead.

2) This "fast path" code is under quite a bit of register pressure on
the 32-bit i386. There's a lot of saving and re-loading.

3) Particularly, the seldom used *th and len parameters are saved and
reloaded in the very beginning of the fast path, really wasting time.

4) Since both *th and len are actually indexed loads from *skb (which
the compiler keeps in a register most of the time), doing indexed loads
from the stack (%ebp) is the same, so they shouldn't be sent as
parameters at all!

5) There's already code, added back in 2006 for DMA, that directly
references skb->len instead of the len parameter. Probably lack of
header documentation, so the coder failed to notice:

if (copied_early)
tcp_cleanup_rbuf(sk, skb->len);
c04ead5d: 8b 56 50 mov 0x50(%esi),%edx
c04ead60: 89 d8 mov %ebx,%eax
c04ead62: e8 fc ff ff ff call c04ead63 <tcp_rcv_established+0x633>

Therefore, I'll resubmit this patch, removing the existing len parameter.
And maybe *th, too....
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