Re: Inefficient ia64 system call implementation in glibc

John Worley
2003-09-19T21:46:24Z

H.J. Lu <hjl@lucon.org> write:

> The inline ia64 system call assumes all values passed to kernel are
> signed 64bit. It does sign extension if the incoming arg is not signed
> 64bit. In case of fxstat.c:
>
> int
> __fxstat (int vers, int fd, struct stat *buf)
> {
> return INLINE_SYSCALL (fstat, 2, fd, CHECK_1 (buf));
> }
>
> it leads to
>
> 0000000000000000 <__fxstat>:
> 0: 00 20 39 0c 80 05 [MII] alloc r36=ar.pfs,14,6,0
> 6: f0 e0 01 12 48 a0 mov r15\x1212
> c: 04 08 00 84 mov r37=r1
> 10: 01 38 01 44 00 21 [MII] mov r39=r34
> 16: 60 02 84 2c 00 60 sxt4 r38=r33
> ^^^^^^^^^^^^^
> 1c: 04 00 c4 00 mov r35°;;
> 20: 0a 00 00 00 00 02 [MMI] break.m 0x100000;;
> 26: 10 02 20 00 42 e0 mov r33=r8

The real inefficiency here is the compiler output. Given the
realities of the Itanium 2 implementation, the first two bundles
will require 3 cycles to execute. A better coding would be:

{ .mmi
alloc r36=ar.pfs,14,6,0
mov r15\x1212
mov r35°
}
{ .mmi
mov r37=r1
mov r39=r34
sxt4 r38=r33
} ;;

which will execute in one cycle. The sign extension, although
"unnecessary" doesn't cost any cycles. Admittedly you could use the
mi;;i bundle to pack the break instruction in the second bundle if
you didn't have to sign-extend, but I'd rather see the 3 v. 1 cycle
problem addressed first.

Regards,
John "I worry about this stuff way too much" Worley
john.worley@hp.com