On Fri, Apr 16, 2021 at 10:11 PM Eric Dumazet <eduma...@google.com> wrote:
>
> On Fri, Apr 16, 2021 at 9:44 PM Al Viro <v...@zeniv.linux.org.uk> wrote:
> >
> > On Fri, Apr 16, 2021 at 12:24:13PM -0700, Eric Dumazet wrote:
> > > From: Eric Dumazet <eduma...@google.com>
> > >
> > > We have to loop only to copy u64 values.
> > > After this first loop, we copy at most one u32, one u16 and one byte.
> >
> > Does it actually yield a better code?
> >
>
> Yes, my patch gives a better code, on actual kernel use-case
>
> (net-next tree, look at put_cmsg())
>
> 5ca: 48 89 0f mov %rcx,(%rdi)
> 5cd: 89 77 08 mov %esi,0x8(%rdi)
> 5d0: 89 57 0c mov %edx,0xc(%rdi)
> 5d3: 48 83 c7 10 add $0x10,%rdi
> 5d7: 48 83 c1 f0 add $0xfffffffffffffff0,%rcx
> 5db: 48 83 f9 07 cmp $0x7,%rcx
> 5df: 76 40 jbe 621 <put_cmsg+0x111>
> 5e1: 66 66 66 66 66 66 2e data16 data16 data16 data16 data16 nopw
> %cs:0x0(%rax,%rax,1)
> 5e8: 0f 1f 84 00 00 00 00
> 5ef: 00
> 5f0: 49 8b 10 mov (%r8),%rdx
> 5f3: 48 89 17 mov %rdx,(%rdi)
> 5f6: 48 83 c7 08 add $0x8,%rdi
> 5fa: 49 83 c0 08 add $0x8,%r8
> 5fe: 48 83 c1 f8 add $0xfffffffffffffff8,%rcx
> 602: 48 83 f9 07 cmp $0x7,%rcx
> 606: 77 e8 ja 5f0 <put_cmsg+0xe0>
> 608: eb 17 jmp 621 <put_cmsg+0x111>
> 60a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
> 610: 41 8b 10 mov (%r8),%edx
> 613: 89 17 mov %edx,(%rdi)
> 615: 48 83 c7 04 add $0x4,%rdi
> 619: 49 83 c0 04 add $0x4,%r8
> 61d: 48 83 c1 fc add $0xfffffffffffffffc,%rcx
> 621: 48 83 f9 03 cmp $0x3,%rcx
> 625: 77 e9 ja 610 <put_cmsg+0x100>
> 627: eb 1a jmp 643 <put_cmsg+0x133>
> 629: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
> 630: 41 0f b7 10 movzwl (%r8),%edx
> 634: 66 89 17 mov %dx,(%rdi)
> 637: 48 83 c7 02 add $0x2,%rdi
> 63b: 49 83 c0 02 add $0x2,%r8
> 63f: 48 83 c1 fe add $0xfffffffffffffffe,%rcx
> 643: 48 83 f9 01 cmp $0x1,%rcx
> 647: 77 e7 ja 630 <put_cmsg+0x120>
> 649: eb 15 jmp 660 <put_cmsg+0x150>
> 64b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> 650: 41 0f b6 08 movzbl (%r8),%ecx
> 654: 88 0f mov %cl,(%rdi)
> 656: 48 83 c7 01 add $0x1,%rdi
> 65a: 49 83 c0 01 add $0x1,%r8
> 65e: 31 c9 xor %ecx,%ecx
> 660: 48 85 c9 test %rcx,%rcx
> 663: 75 eb jne 650 <put_cmsg+0x140>
After the change code is now what we would expect (no jmp around)
5db: 48 83 f9 08 cmp $0x8,%rcx
5df: 72 27 jb 608 <put_cmsg+0xf8>
5e1: 66 66 66 66 66 66 2e data16 data16 data16 data16 data16 nopw
%cs:0x0(%rax,%rax,1)
5e8: 0f 1f 84 00 00 00 00
5ef: 00
5f0: 49 8b 10 mov (%r8),%rdx
5f3: 48 89 17 mov %rdx,(%rdi)
5f6: 48 83 c7 08 add $0x8,%rdi
5fa: 49 83 c0 08 add $0x8,%r8
5fe: 48 83 c1 f8 add $0xfffffffffffffff8,%rcx
602: 48 83 f9 08 cmp $0x8,%rcx
606: 73 e8 jae 5f0 <put_cmsg+0xe0>
608: 48 83 f9 04 cmp $0x4,%rcx
60c: 72 11 jb 61f <put_cmsg+0x10f>
60e: 41 8b 10 mov (%r8),%edx
611: 89 17 mov %edx,(%rdi)
613: 48 83 c7 04 add $0x4,%rdi
617: 49 83 c0 04 add $0x4,%r8
61b: 48 83 c1 fc add $0xfffffffffffffffc,%rcx
61f: 48 83 f9 02 cmp $0x2,%rcx
623: 72 13 jb 638 <put_cmsg+0x128>
625: 41 0f b7 10 movzwl (%r8),%edx
629: 66 89 17 mov %dx,(%rdi)
62c: 48 83 c7 02 add $0x2,%rdi
630: 49 83 c0 02 add $0x2,%r8
634: 48 83 c1 fe add $0xfffffffffffffffe,%rcx
638: 48 85 c9 test %rcx,%rcx
63b: 74 05 je 642 <put_cmsg+0x132>
63d: 41 8a 08 mov (%r8),%cl
640: 88 0f mov %cl,(%rdi)
As I said, its minor, I am sure you can come up to something much better !
Thanks !
>
>
> > FWIW, this
> > void bar(unsigned);
> > void foo(unsigned n)
> > {
> > while (n >= 8) {
> > bar(n);
> > n -= 8;
> > }
> > while (n >= 4) {
> > bar(n);
> > n -= 4;
> > }
> > while (n >= 2) {
> > bar(n);
> > n -= 2;
> > }
> > while (n >= 1) {
> > bar(n);
> > n -= 1;
> > }
> > }
> >
> > will compile (with -O2) to
> > pushq %rbp
> > pushq %rbx
> > movl %edi, %ebx
> > subq $8, %rsp
> > cmpl $7, %edi
> > jbe .L2
> > movl %edi, %ebp
> > .L3:
> > movl %ebp, %edi
> > subl $8, %ebp
> > call bar@PLT
> > cmpl $7, %ebp
> > ja .L3
> > andl $7, %ebx
> > .L2:
> > cmpl $3, %ebx
> > jbe .L4
> > movl %ebx, %edi
> > andl $3, %ebx
> > call bar@PLT
> > .L4:
> > cmpl $1, %ebx
> > jbe .L5
> > movl %ebx, %edi
> > andl $1, %ebx
> > call bar@PLT
> > .L5:
> > testl %ebx, %ebx
> > je .L1
> > addq $8, %rsp
> > movl $1, %edi
> > popq %rbx
> > popq %rbp
> > jmp bar@PLT
> > .L1:
> > addq $8, %rsp
> > popq %rbx
> > popq %rbp
> > ret
> >
> > i.e. loop + if + if + if...
