Re: [PATCH] LoongArch: Add Syscall Assembly Implementation

[Xi Ruoyao <xry111@xry111.site>]

General question: is there a notable benefit optimizing syscall with
assembly?  AFAIK nobody will put syscall on a hot path, and the cycles
saved by using the assembly implementation should be negligible
comparing with all the cost of context switch etc.

On Thu, 2023-03-23 at 16:40 +0800, caiyinyu wrote:
> + ENTRY (syscall)
> +        move	t0, a7
> +        move	a7, a0		/* Syscall number -> a0.  */
> +        move	a0, a1		/* shift arg1 - arg6.  */
> +        move	a1, a2
> +        move	a2, a3
> +        move	a3, a4
> +        move	a4, a5
> +        move	a5, a6
> +        move	a6, t0		/* arg7 is saved in t0.  */
> +        syscall 0		/* Do the system call.  */
> +       lu12i.w t0, -1

"li.w t0, -4096" will do the same thing, and be more readable.

And this line seems indented with a tab, while other lines are indented
with 8 spaces.

> +        bltu   t0, a0, L (error)
> +        ret                     /* Return to caller.  */

"ret" is not recognized by GNU assembler <= 2.39, it's better to use
the old-style "jr      ra" for backward compatibility.

> +
> +L (error):
> +        b      __syscall_error

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