Andrey M. Borodin <x4...@yandex-team.ru> 于2023年12月14日周四 17:02写道：
>
>
> > On 14 Dec 2023, at 08:12, Amul Sul <sula...@gmail.com> wrote:
> >
> >
> > + int bankno = pageno & ctl->bank_mask;
> >
> > I am a bit uncomfortable seeing it as a mask, why can't it be simply a
> number
> > of banks (num_banks) and get the bank number through modulus op (pageno %
> > num_banks) instead of bitwise & operation (pageno & ctl->bank_mask)
> which is a
> > bit difficult to read compared to modulus op which is quite simple,
> > straightforward and much common practice in hashing.
> >
> > Are there any advantages of using & over % ?
>
use Compiler Explorer[1] tool, '%' has more Assembly instructions than '&'
.
int GetBankno1(int pageno) {
return pageno & 127;
}
int GetBankno2(int pageno) {
return pageno % 127;
}
under clang 13.0
GetBankno1: # @GetBankno1
push rbp
mov rbp, rsp
mov dword ptr [rbp - 4], edi
mov eax, dword ptr [rbp - 4]
and eax, 127
pop rbp
ret
GetBankno2: # @GetBankno2
push rbp
mov rbp, rsp
mov dword ptr [rbp - 4], edi
mov eax, dword ptr [rbp - 4]
mov ecx, 127
cdq
idiv ecx
mov eax, edx
pop rbp
ret
under gcc 13.2
GetBankno1:
push rbp
mov rbp, rsp
mov DWORD PTR [rbp-4], edi
mov eax, DWORD PTR [rbp-4]
and eax, 127
pop rbp
ret
GetBankno2:
push rbp
mov rbp, rsp
mov DWORD PTR [rbp-4], edi
mov eax, DWORD PTR [rbp-4]
movsx rdx, eax
imul rdx, rdx, -2130574327
shr rdx, 32
add edx, eax
mov ecx, edx
sar ecx, 6
cdq
sub ecx, edx
mov edx, ecx
sal edx, 7
sub edx, ecx
sub eax, edx
mov ecx, eax
mov eax, ecx
pop rbp
ret
[1] https://godbolt.org/
The instruction AND is ~20 times faster than IDIV [0]. This is relatively
> hot function, worth sacrificing some readability to save ~ten nanoseconds
> on each check of a status of a transaction.
>
Now that AND is more faster, Can we replace the '% SLRU_MAX_BANKLOCKS'
operation in SimpleLruGetBankLock() with '& 127' :
SimpleLruGetBankLock()
{
int banklockno = (pageno & ctl->bank_mask) % SLRU_MAX_BANKLOCKS;
use '&'
return &(ctl->shared->bank_locks[banklockno].lock);
}
Thoughts?
>
> [0] https://www.agner.org/optimize/instruction_tables.pdf
>
>
