Re: [PATCH] LoongArch: Add Syscall Assembly Implementation

[Xi Ruoyao <xry111@xry111.site>]

On Thu, 2023-03-23 at 20:01 +0800, caiyinyu wrote:
> Without this patch(objdump -d libc.so...):
> 
> 00000000000dd45c <syscall>:
>     dd45c:       02fec063        addi.d          $sp, $sp, -80(0xfb0)
>     dd460:       02c0606c        addi.d          $t0, $sp, 24(0x18)

/* snip */

Yes, I know this will save some cycles.  The problem is during a
syscall, the context switch etc. uses much more time than the
instruction sequence in syscall() function itself.

I wrote a program which calls syscall(SYS_getpid) 1000000 times. 
Without the (unpatched) Glibc syscall(), the time consumed is 1.376s;
with a syscall() in asm (similar to the one in the patch but I changed
the __syscall_error tail call to a "break 0"), the time consumed is
1.203s (0.173s faster).  So we can say it saves 17.3us (about 430
cycles) per syscall.

I doubt if the improvement is really worthy for real life applications:
is there any application invoking syscalls so intensively that the
17.3us will accumulate into something we can feel?

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University