Re: [PATCH] LoongArch: Add Syscall Assembly Implementation

[Xi Ruoyao <xry111@xry111.site>]

On Thu, 2023-03-23 at 15:40 -0300, Adhemerval Zanella Netto wrote:
> 
> On 23/03/23 15:26, Xi Ruoyao wrote:
> > 
> > LoongArch has a TARGET_SETUP_INCOMING_VARARGS but it does not use the
> > information from stdarg pass.  I can fix it, but even with the fix GCC
> > would still save 7 registers (now GCC trunk saves 9 registers, the fix
> > would make some improvement but no much).
> > 
> > And the issue seems not trivial to fix.  On x86_64, all of GCC, Clang,
> > and MSVC will save some registers if va_arg is used.  I've not found any
> > compiler which can avoid saving the va_arg GARs unnecessarily yet:
> > 
> > https://godbolt.org/z/n1YqWq9c9
> > 
> > Now to me it seems a bad idea to use va_arg in syscall.c.
> > 
> 
> I think it was the natural way to express kernel communication mechanism
> that indeed takes variadic arguments.   And since it is older than Linux
> (man-pages stated it was from 4BSD), it also mean that you don't bind a
> maximum limit or arguments (although on Linux and BSD does have a pratical
> limit).

Well, my statement is only for Linux and a modern architecture with
enough GARs.

> We can maybe add a implementation that uses named args (which extra
> boilerplate to architectures that accepts 7 arguments instead of usual
> 6); and just enable it if a per-architecture flag is set meaning that
> for that specific ABI the variadic is essentially the same as named
> functions calls.  Something like:
> 
> long int
> syscall (long int number, 
> #if __ASSUME_SYSCALL_NAMED_WORKS
>          long int a0, long int a1, long int a2,
>          long int a3, long int a4, long int a5
> #else
>          ...
> #endif

/* snip */

> It might need some more hacks to hide the syscall prototype.

I've already tried something similar on my box (I named the macro
"SYSCALL_VA_ARG_NOT_NEEDED"); it looks like we don't need to hide the
syscall prototype:  unistd.h is not in misc/syscall.o.d.

The disassemble of misc/syscall.o looks perfect when I defined the macro
in sysdep/unix/sysv/linux/loongarch/sysdep.h:

0000000000000000 <syscall>:
   0:	0015008b 	move        	$a7, $a0
   4:	001500a4 	move        	$a0, $a1
   8:	001500c5 	move        	$a1, $a2
   c:	001500e6 	move        	$a2, $a3
  10:	00150107 	move        	$a3, $a4
  14:	00150128 	move        	$a4, $a5
  18:	00150149 	move        	$a5, $a6
  1c:	002b0000 	syscall     	0x0
  20:	15ffffec 	lu12i.w     	$t0, -1(0xfffff)
  24:	68000984 	bltu        	$t0, $a0, 8(0x8)	# 2c <syscall+0x2c>
  28:	4c000020 	jirl        	$zero, $ra, 0
  2c:	1a00000c 	pcalau12i   	$t0, 0
  30:	00111004 	sub.w       	$a0, $zero, $a0
  34:	28c0018c 	ld.d        	$t0, $t0, 0
  38:	38180984 	stx.w       	$a0, $t0, $tp
  3c:	02bffc04 	addi.w      	$a0, $zero, -1(0xfff)
  40:	4c000020 	jirl        	$zero, $ra, 0

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