Thomas Graf a écrit :
OK. At least my compiler (gcc-3.3.1) does NOT unroll the loop :
Because you don't specify -funroll-loop
I'm using vanilla 2.6.12 : no -funroll-loop in it. Maybe in your tree, not
on 99.9% of 2.6.12 trees.
Are you suggesting everybody should use this compiler flag ?
Something like :
net/sched/Makefile:
CFLAGS_sch_generic.o := -funroll-loops
?
[...]
Please give us the code your compiler produces,
Unrolled version:
pfifo_fast_dequeue:
pushl %esi
xorl %edx, %edx
pushl %ebx
movl 12(%esp), %esi
movl 128(%esi), %eax
leal 128(%esi), %ecx
cmpl %ecx, %eax
je .L132
movl %eax, %edx
movl (%eax), %eax
decl 8(%ecx)
movl $0, 8(%edx)
movl %ecx, 4(%eax)
movl %eax, 128(%esi)
movl $0, 4(%edx)
movl $0, (%edx)
.L132:
testl %edx, %edx
je .L131
movl 96(%edx), %ebx
movl 80(%esi), %eax
decl 40(%esi)
subl %ebx, %eax
movl %eax, 80(%esi)
movl %edx, %eax
.L117:
popl %ebx
popl %esi
ret
.L131:
movl 20(%ecx), %eax
leal 20(%ecx), %edx
xorl %ebx, %ebx
cmpl %edx, %eax
je .L137
movl %eax, %ebx
movl (%eax), %eax
decl 8(%edx)
movl $0, 8(%ebx)
movl %edx, 4(%eax)
movl %eax, 20(%ecx)
movl $0, 4(%ebx)
movl $0, (%ebx)
.L137:
testl %ebx, %ebx
je .L147
.L146:
movl 96(%ebx), %ecx
movl 80(%esi), %eax
decl 40(%esi)
subl %ecx, %eax
movl %eax, 80(%esi)
movl %ebx, %eax
jmp .L117
.L147:
movl 40(%ecx), %eax
leal 40(%ecx), %edx
xorl %ebx, %ebx
cmpl %edx, %eax
je .L142
movl %eax, %ebx
movl (%eax), %eax
decl 8(%edx)
movl $0, 8(%ebx)
movl %edx, 4(%eax)
movl %eax, 40(%ecx)
movl $0, 4(%ebx)
movl $0, (%ebx)
.L142:
xorl %eax, %eax
testl %ebx, %ebx
jne .L146
jmp .L117
OK thanks, but you dont give the code for my version :) shorter and
unrolled as you can see, and with nice predicted branches.
00000fc0 <pfifo_fast_dequeue>:
fc0: 56 push %esi
fc1: 89 c1 mov %eax,%ecx
fc3: 53 push %ebx
fc4: 8d 98 a0 00 00 00 lea 0xa0(%eax),%ebx
fca: 39 98 a0 00 00 00 cmp %ebx,0xa0(%eax)
fd0: 89 da mov %ebx,%edx
fd2: 75 22 jne ff6 <pfifo_fast_dequeue+0x36>
fd4: 8d 90 c4 00 00 00 lea 0xc4(%eax),%edx
fda: 39 90 c4 00 00 00 cmp %edx,0xc4(%eax)
fe0: 89 d3 mov %edx,%ebx
fe2: 75 12 jne ff6 <pfifo_fast_dequeue+0x36>
fe4: 8d 98 e8 00 00 00 lea 0xe8(%eax),%ebx
fea: 31 f6 xor %esi,%esi
fec: 39 98 e8 00 00 00 cmp %ebx,0xe8(%eax)
ff2: 89 da mov %ebx,%edx
ff4: 74 27 je 101d <pfifo_fast_dequeue+0x5d>
ff6: 8b 32 mov (%edx),%esi
ff8: 39 d6 cmp %edx,%esi
ffa: 74 26 je 1022 <pfifo_fast_dequeue+0x62>
ffc: 8b 06 mov (%esi),%eax
ffe: ff 4b 08 decl 0x8(%ebx)
1001: c7 46 08 00 00 00 00 movl $0x0,0x8(%esi)
1008: 89 50 04 mov %edx,0x4(%eax)
100b: 89 02 mov %eax,(%edx)
100d: c7 46 04 00 00 00 00 movl $0x0,0x4(%esi)
1014: c7 06 00 00 00 00 movl $0x0,(%esi)
101a: ff 49 28 decl 0x28(%ecx)
101d: 5b pop %ebx
101e: 89 f0 mov %esi,%eax
1020: 5e pop %esi
1021: c3 ret
1022: ff 49 28 decl 0x28(%ecx)
1025: 31 f6 xor %esi,%esi
1027: eb f4 jmp 101d <pfifo_fast_dequeue+0x5d>
I just noticed that this is a local modification of my own, so in
the vanilla tree it indeed doesn't have any impact on the code
generated.
Still, your patch does not make sense to me. The latest tree
also includes my pfifo_fast changes wich modified the code to
maintain a backlog and made it easy to add more fifos at compile
time. If you want the loop unrolled then let the compiler do it
via -funroll-loop. These kind of optimization seem as uncessary
to me as all the loopback optimizations.
I dont want change compiler flags in my tree and loose this optim when
2.6.13 is released.
I dont know about loopback optimization, I am not involved with this stuff,
maybe you think I'm another guy ?
It seems to me you give unrelated arguments.
I dont know what are your plans, but mine were not to say you are writing bad
code.
Just to give my performance analysis and feedback, I'm sorry if it hurts you.
Eric Dumazet
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