On Wed, Jan 23, 2019 at 10:55:03AM +0100, Michal Hocko wrote:
>On Tue 22-01-19 15:56:28, Wei Yang wrote:
>>
>> I think the answer is yes.
>>
>> * it reduce the code from 6 lines to 3 lines, 50% off
>> * by reducing calculation back and forth, it would be easier for
>> audience to catch what it tries to do
>
>To be honest, I really do not see this sufficient to justify touching
>the code unless the resulting _generated_ code is better/more efficient.
Tried objdump to compare two version.
Base Patched Reduced
Code Size(B) 48 39 18.7%
Instructions 12 10 16.6%
Here is the raw output.
00000000000001be <usemap_size_ywtest1>:
{
1be: e8 00 00 00 00 callq 1c3 <usemap_size_ywtest1+0x5>
zonesize += zone_start_pfn & (pageblock_nr_pages-1);
1c3: 81 e7 ff 01 00 00 and $0x1ff,%edi
{
1c9: 55 push %rbp
return usemapsize / 8;
1ca: 48 ba f8 ff ff ff ff movabs $0x1ffffffffffffff8,%rdx
1d1: ff ff 1f
usemapsize = roundup(zonesize, pageblock_nr_pages);
1d4: 48 8d 84 3e ff 01 00 lea 0x1ff(%rsi,%rdi,1),%rax
1db: 00
usemapsize *= NR_PAGEBLOCK_BITS;
1dc: 48 c1 e8 09 shr $0x9,%rax
usemapsize = roundup(usemapsize, 8 * sizeof(unsigned long));
1e0: 48 8d 04 85 3f 00 00 lea 0x3f(,%rax,4),%rax
1e7: 00
{
1e8: 48 89 e5 mov %rsp,%rbp
}
1eb: 5d pop %rbp
return usemapsize / 8;
1ec: 48 c1 e8 03 shr $0x3,%rax
1f0: 48 21 d0 and %rdx,%rax
}
1f3: c3 retq
00000000000001f4 <usemap_size_ywtest2>:
{
1f4: e8 00 00 00 00 callq 1f9 <usemap_size_ywtest2+0x5>
zonesize += zone_start_pfn & (pageblock_nr_pages-1);
1f9: 81 e7 ff 01 00 00 and $0x1ff,%edi
{
1ff: 55 push %rbp
usemapsize = DIV_ROUND_UP(zonesize, pageblock_nr_pages);
200: 48 8d 84 3e ff 01 00 lea 0x1ff(%rsi,%rdi,1),%rax
207: 00
208: 48 c1 e8 09 shr $0x9,%rax
return BITS_TO_LONGS(usemapsize) * sizeof(unsigned long);
20c: 48 8d 04 85 3f 00 00 lea 0x3f(,%rax,4),%rax
213: 00
{
214: 48 89 e5 mov %rsp,%rbp
}
217: 5d pop %rbp
return BITS_TO_LONGS(usemapsize) * sizeof(unsigned long);
218: 48 c1 e8 06 shr $0x6,%rax
21c: 48 c1 e0 03 shl $0x3,%rax
}
220: c3 retq
--
Wei Yang
Help you, Help me
>--
>Michal Hocko
>SUSE Labs
--
Wei Yang
Help you, Help me
