On Fri, Jan 05, 2018 at 03:33:55PM -0800, Vineet Gupta wrote:
On 12/13/2017 10:29 PM, Stafford Horne wrote:
Traditionally arc has had a generic syscall implementation of syscall() matchig the common implementation.
During an audit it was found that this implementation seems to duplicate common implementation and is no longer needed.
Signed-off-by: Stafford Horne shorne@gmail.com
libc/sysdeps/linux/arc/Makefile.arch | 2 +- libc/sysdeps/linux/arc/syscall.c | 17 ----------------- 2 files changed, 1 insertion(+), 18 deletions(-)
[snip]
-extern long syscall(long int sysnum, long a, long b, long c, long d, long e, long f);
-long syscall(long int sysnum, long a, long b, long c, long d, long e, long f) -{
- return INLINE_SYSCALL_NCS(sysnum, 6, a, b, c, d, e, f);
-}
varargs generate pathetic code on ARC atleast, it is partly due to the ABI (the args need to be stored on stack)
With current implementation we get
000100ac <syscall>: 100ac: mov_s r8,r0 100ae: mov_s r0,r1 100b0: mov_s r1,r2 100b2: mov_s r2,r3 100b4: mov_s r3,r4 100b6: mov_s r4,r5 100b8: mov_s r5,r6 100ba: trap_s 0 100bc: cmp r0,-1024 100c0: jls [blink] 100c4: st.aw blink,[sp,-4] 100c8: bl 84 ;1011c <__syscall_error> 100cc: ld.ab blink,[sp,4] 100d0: j_s [blink] 100d2: nop_s
With varagrs implementation we get tons of useless stores. So I'd prefer we keep the current implementation for ARC - although granted syscall() may not be the fast path syscall.
Hello Vineet,
Thanks for testing, I did realize varargs might generate slightly less optimal code. But I figured a few thing things
- syscall() is probably not going to be used in any critical code sections. - the compiler could be optimized to make the varargs handling more optimal - we could remove code (but actually ends up with a bigger binary, so not great)
But for now, lets just drop this patch. Thanks for the feedback.
-Stafford
00010398 <syscall>: 10398: sub_s sp,sp,0x1c 1039a: mov_s r8,r0 1039c: st_s r1,[sp,0] 1039e: st_s r2,[sp,0x4] 103a0: st_s r3,[sp,0x8] 103a2: st r4,[sp,12] 103a6: st r5,[sp,16] 103aa: st r6,[sp,20] 103ae: st r7,[sp,24] 103b2: mov_s r0,r1 103b4: mov_s r1,r2 103b6: mov_s r2,r3 103b8: mov_s r3,r4 103ba: mov_s r4,r5 103bc: mov_s r5,r6 103be: trap_s 0 103c0: brhs.nt 0xfffffc00,r0,20 ;103d4 <syscall+0x3c> 103c8: st.aw blink,[sp,-4] 103cc: bl 84 ;10420 <__syscall_error> 103d0: ld.ab blink,[sp,4] 103d4: j_s.d [blink] 103d6: add_s sp,sp,0x1c