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From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.arch
Subject: Re: skip instructions
Message-ID: <13039@dog.ee.lbl.gov>
Date: 10 May 91 11:26:49 GMT
References: <1182@opus.NMSU.Edu> <1991May05.174307.8952@iecc.cambridge.ma.us> <1991May10.004650.7258@sbcs.sunysb.edu>
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Distribution: comp
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 46
X-Local-Date: Fri, 10 May 91 04:26:49 PDT

In article <1991May10.004650.7258@sbcs.sunysb.edu>
jallen@eeserv1.ic.sunysb.edu (Joseph Allen) writes:
>Also, when are we going to see multiple error return points from OS calls? 
>I.E., if no error goto return point+1, if error goto return point+0 which
>contains a jump to the error handler.

This is a nice idea, but the ret+0/1 is a bit limiting.  On the SPARC,
in which the system call number is passed in %g1 and %g2..%g7 are in
essence `wasted', it might be more reasonable to have the success-return
address passed in %g2.  That is, instead of:

		.globl	_read
	_read:
		mov	SYS_read, %g1	! read
		t	0		! read(%o0, %o1, %o2)
		bcc	1f		! if success, go return
		 .empty			! next instr okay in delay slot
		set	cerror, %g1
		jmp	%g1		! jump cerror
		 nop
	1:	retl			! success: jump to [%o7+8]
		 nop			! nothing to do in delay slot

we would have:

		.globl	_read
	_read:
		mov	SYS_read, %g1	! read
		add	%o7, 8, %g2	! success return
		t	0		! read(%o0, %o1, %o2)
		set	cerror, %g1
		jmp	%g1
		 nop

I will probably do this with a `high bit' in %g1 since it is not compatible
with SunOS (i.e.,
	#define RETFLAG 0x800
	_read: mov SYS_read+RETFLAG, %g1 ! read, return to %g2 on success
or something like that).

Actually, since the return is invariably a `retl' (return to %o7+8) I
might just make it use %o7+8.  This will save one whole instruction on
failure returns (oh boy :-) ).
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 415 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov