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Re: ssnop for mips
- To: cgd at sibyte dot com
- Subject: Re: ssnop for mips
- From: Ralf Baechle <ralf at uni-koblenz dot de>
- Date: Fri, 20 Jul 2001 13:51:09 +0200
- Cc: mrs at windriver dot com, binutils at sources dot redhat dot com, "Kevin D. Kissell" <kevink at mips dot com>
- References: <200107182302.QAA28971@kankakee.wrs.com> <mailpost.995498951.26918@postal.sibyte.com> <yov57kx5fw27.fsf@highland.sibyte.com>
On Wed, Jul 18, 2001 at 08:16:00PM -0700, cgd@sibyte.com wrote:
> mrs@windriver.com ("mike stump") writes:
> > 2001-07-18 Mike Stump <mrs@wrs.com>
> >
> > * mips-opc.c (ssnop): While the documentation makes it seem
> > like this instruction is new for MIPS32, it is not, it is needed
> > on all ISA levels.
>
> for some value of 'needed'. it may have some absolutely different
> 'no-op' meaning on non-MIPS32/MIPS64 processors.
>
> if i recall correctly, that instruction doesn't inhibit multi-issue.
Ssnop is documented as ``breaks superscalar dispatch''. See also the
comments in IRIX <sys/asm.h> which I can't paste here for (C) reasons.
> Personally, I don't think it should be enabled if not MIPS32/MIPS64:
>
> * the behaviour may not be what's expected, and
All CPUs that don't implement ssnop's special semantics will decode it as
sll $zero, $zero, 1 which is obviously harmless.
Ralf