Re: [PATCH] make __section_nr more efficient

[zhouchengming]

On 2016/7/21 5:36, Dave Hansen wrote:
> On 07/19/2016 09:18 PM, Zhou Chengming wrote:
> > When CONFIG_SPARSEMEM_EXTREME is disabled, __section_nr can get
> > the section number with a subtraction directly.
>
> Does this actually *do* anything?
>
> It was a long time ago, but if I remember correctly, the entire loop in
> __section_nr() goes away because root_nr==NR_SECTION_ROOTS, so
> root_nr=1, and the compiler optimizes away the entire subtraction.
>
> So this basically adds an #ifdef and gets us nothing, although it makes
> the situation much more explicit.  Perhaps the comment should say that
> this works *and* is efficient because the compiler can optimize all the
> extreme complexity away.
>
> .

Thanks for your reply. I don't know the compiler will optimize the loop.
But when I see the assembly code of __section_nr, it seems to still have
the loop in it.

My gcc version: gcc version 4.9.0 (GCC)
CONFIG_SPARSEMEM_EXTREME: disabled

Before this patch:

0000000000000000 <__section_nr>:
   0:   55                      push   %rbp
   1:   48 c7 c2 00 00 00 00    mov    $0x0,%rdx
                        4: R_X86_64_32S mem_section
   8:   31 c0                   xor    %eax,%eax
   a:   48 89 e5                mov    %rsp,%rbp
   d:   eb 0d                   jmp    1c <__section_nr+0x1c>
   f:   48 83 c0 01             add    $0x1,%rax
  13:   48 81 fa 00 00 00 00    cmp    $0x0,%rdx
                        16: R_X86_64_32S        mem_section+0x800000
  1a:   74 26                   je     42 <__section_nr+0x42>
  1c:   48 89 d1                mov    %rdx,%rcx
  1f:   ba 10 00 00 00          mov    $0x10,%edx
  24:   48 85 c9                test   %rcx,%rcx
  27:   74 e6                   je     f <__section_nr+0xf>
  29:   48 39 cf                cmp    %rcx,%rdi
  2c:   48 8d 51 10             lea    0x10(%rcx),%rdx
  30:   72 dd                   jb     f <__section_nr+0xf>
  32:   48 39 d7                cmp    %rdx,%rdi
  35:   73 d8                   jae    f <__section_nr+0xf>
  37:   48 29 cf                sub    %rcx,%rdi
  3a:   48 c1 ff 04             sar    $0x4,%rdi
  3e:   01 f8                   add    %edi,%eax
  40:   5d                      pop    %rbp
  41:   c3                      retq
  42:   48 29 cf                sub    %rcx,%rdi
  45:   b8 00 00 08 00          mov    $0x80000,%eax
  4a:   48 c1 ff 04             sar    $0x4,%rdi
  4e:   01 f8                   add    %edi,%eax
  50:   5d                      pop    %rbp
  51:   c3                      retq
  52:   66 66 66 66 66 2e 0f    data32 data32 data32 data32 nopw %cs:0x0(%rax,%rax,1)
  59:   1f 84 00 00 00 00 00

After this patch:

0000000000000000 <__section_nr>:
   0:   55                      push   %rbp
   1:   48 89 f8                mov    %rdi,%rax
   4:   48 2d 00 00 00 00       sub    $0x0,%rax
                        6: R_X86_64_32S mem_section
   a:   48 89 e5                mov    %rsp,%rbp
   d:   48 c1 f8 04             sar    $0x4,%rax
  11:   5d                      pop    %rbp
  12:   c3                      retq
  13:   66 66 66 66 2e 0f 1f    data32 data32 data32 nopw %cs:0x0(%rax,%rax,1)
  1a:   84 00 00 00 00 00


Thanks!