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Subject: Re: [PATCH 3/6] alpha: Unify the not-implemented system call entry
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To:     Al Viro <viro@ZenIV.linux.org.uk>
Cc:     Firoz Khan <firoz.khan@linaro.org>, linux-alpha@vger.kernel.org,
        ink@jurassic.park.msu.ru, mattst88@gmail.com,
        y2038@lists.linaro.org, linux-kernel@vger.kernel.org,
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From:   Richard Henderson <rth@twiddle.net>
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Date:   Fri, 10 Aug 2018 21:10:04 -0700
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On 08/10/2018 07:45 PM, Al Viro wrote:
> BTW, seeing that it's your code - why was unop used in
> alpha_ni_syscall?  I don't remember the rules re pipeline
> stalls; is it that some earlier variants prefer unop to
> nop in such places?  It's not that microoptimizing that
> one makes any difference, but just out of curiosity -
> would something like
> 	lda     $0, -ENOSYS
> 	stq     $sp, 0($sp)	/* sp != 0 */
> 	ret
> do just as well there?

Oh that.  Well, unop is a "load" and pairs with everything
on all cpus, where nop is an integer and doesn't always
pair.  So I got into the habit of using unop for everything.

The extra nop was so that lda + unop // stq + ret paired up
in in two cycles as opposed to lda /stall/ stq // ret in
three cycles on ev4+ev5.  ev6 didn't care.  FWIW.


r~