On Sun, Jan 28, 2018 at 10:39 PM, Willy Tarreau <w...@1wt.eu> wrote: > Hi, > > [ replaced stable@ and greg@ by netdev@ as my question below is not > relevant to stable ] > > On Mon, Jan 29, 2018 at 02:48:54AM +0100, Daniel Borkmann wrote: >> From: Eric Dumazet <eduma...@google.com> >> >> [ upstream commit 84ccac6e7854ebbfb56d2fc6d5bef9be49bb304c ] >> >> Saves 4 bytes replacing following instructions : >> >> lea rax, [rsi + rdx * 8 + offsetof(...)] >> mov rax, qword ptr [rax] >> cmp rax, 0 >> >> by : >> >> mov rax, [rsi + rdx * 8 + offsetof(...)] >> test rax, rax > > I've just noticed this on stable@. If these 4 bytes matter, why not use > cmpq with an immediate value instead, which saves 2 extra bytes ? : > > - the mov above is 11 bytes total : > > 0: 48 8b 84 d6 78 56 34 mov 0x12345678(%rsi,%rdx,8),%rax > 7: 12 > 8: 48 85 c0 test %rax,%rax > > - the equivalent cmp is only 9 bytes : > > 0: 48 83 bc d6 78 56 34 cmpq $0x0,0x12345678(%rsi,%rdx,8) > 7: 12 00 > > And as a bonus, it doesn't even clobber rax. > > Just my two cents,
Hi Willy Please look more closely at following instructions. We need the value later, not only testing it being zero :)
